Decreased serum undercarboxylated osteocalcin is associated with cognitive impairment in male patients with type 2 diabetes

BACKGROUND

Basic and clinical researches have suggested that type 2 diabetes (T2DM) is associated with cognitive impairment, and diabetes mellitus increases the risk of cognitive impairment and dementia. Recently, some reports found that undercarboxylated osteocalcin (ucOC) could affect brain functions, and decreased in patients with T2DM. We aimed to investigate the association of serum ucOC with cognitive impairment in T2DM patients.

METHODS

A total of 196 male T2DM patients without medications known to affect bone metabolism or history of bone fracture, aged ≥18years were recruited and divided into impaired cognition group and normal cognition group. We use the scores of Minimum Mental State Examination (MMSE) to evaluate the subjects' cognitive function. Detailed cognitive performance was also evaluated by the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Serum ucOC was measured by Enzyme-Linked Immunosorbent Assay (ELISA) kit.

RESULTS

Compared to male T2DM patients with normal cognition, the mean osteocalcin concentrations were significantly lower in male T2DM patients with impaired cognition (P<0.05). RBANS total and all indexes scores were also lower in patients with impaired cognition (all P<0.05). After adjusted effects of confounding factors, serum ucOC was positively correlated with a variety indexes of RBANS except visuospatial/constructional.

CONCLUSIONS

The serum ucOC is positively correlated with RBANS scores in male T2DM patients. It suggests that serum ucOC may be involved in the development and progression of cognitive dysfunction in T2DM patients.

Lipid specificity of the membrane binding domain of coagulation factor X

Essentials Membrane-binding GLA domains of coagulation factors are essential for proper clot formation. Factor X (FX) is specific to phosphatidylserine (PS) lipids through unknown atomic-level interactions. Molecular dynamics simulations were used to develop the first membrane-bound model of FX-GLA. PS binding modes of FX-GLA were described, and potential PS-specific binding sites identified.

SUMMARY

Background Factor X (FX) binds to cell membranes in a highly phospholipid-dependent manner and, in complex with tissue factor and factor VIIa (FVIIa), initiates the clotting cascade. Experimental information concerning the membrane-bound structure of FX with atomic resolution has remained elusive because of the fluid nature of cellular membranes. FX is known to bind preferentially to phosphatidylserine (PS). Objectives To develop the first membrane-bound model of the FX-GLA domain to PS at atomic level, and to identify PS-specific binding sites of the FX-GLA domain. Methods Molecular dynamics (MD) simulations were performed to develop an atomic-level model for the FX-GLA domain bound to PS bilayers. We utilized a membrane representation with enhanced lipid mobility, termed the highly mobile membrane mimetic (HMMM), permitting spontaneous membrane binding and insertion by FX-GLA in multiple 100-ns simulations. In 14 independent simulations, FX-GLA bound spontaneously to the membrane. The resulting membrane-bound models were converted from HMMM to conventional membrane and simulated for an additional 100 ns. Results The final membrane-bound FX-GLA model allowed for detailed characterization of the orientation, insertion depth and lipid interactions of the domain, providing insight into the molecular basis of its PS specificity. All binding simulations converged to the same configuration despite differing initial orientations. Conclusions Analysis of interactions between residues in FX-GLA and lipid-charged groups allowed for potential PS-specific binding sites to be identified. This new structural and dynamic information provides an additional step towards a full understanding of the role of atomic-level lipid-protein interactions in regulating the critical and complex clotting cascade.

Gla-rich protein, a new player in tissue calcification?

A novel γ-carboxyglutamate (Gla)-containing protein, named Gla-rich protein (GRP) after its high content in Gla residues or upper zone of growth plate and cartilage matrix associated protein after its preferential expression by cartilage chondrocyte, was recently identified in sturgeon, mice, and humans through independent studies. GRP is the most densely γ-carboxylated protein identified to date and its structure has been remarkably conserved throughout vertebrate evolution but is apparently absent from bird genomes. Several transcript and genomic variants affecting key protein features or regulatory elements were described and 2 paralogs were identified in the teleost fish genome. In the skeleton, most relevant levels of GRP gene expression were observed in cartilaginous tissues and associated with chondrocytes, suggesting a role in chondrogenesis. But GRP expression was also detected in bone cells, indicative of a more widespread role for the protein throughout skeletal formation. Although the molecular function of GRP is yet unknown, the high content of Gla residues and its accumulation at sites of pathological calcification in different human pathologies affecting skin or the vascular system and in breast cancer tumors suggest that GRP may function as a modulator of calcium availability. Because of its association with fibrillar collagens, GRP could also be involved in the organization and/or stabilization of cartilage matrix. Although transgenic mice did not reveal obvious phenotypic alterations in skeletal development or structure, zebrafish morphants lack craniofacial cartilage and exhibit limited calcification, suggesting a role for GRP during skeletal development, but additional functional data are required to understand its function.

The efficacy of activated protein C in murine endotoxemia is dependent on integrin CD11b

Activated protein C (APC), the only FDA-approved biotherapeutic drug for sepsis, possesses anticoagulant, antiinflammatory, and barrier-protective activities. However, the mechanisms underlying its anti-inflammatory functions are not well defined. Here, we report that the antiinflammatory activity of APC on macrophages is dependent on integrin CD11b/CD18, but not on endothelial protein C receptor (EPCR). We showed that CD11b/CD18 bound APC within specialized membrane microdomains/lipid rafts and facilitated APC cleavage and activation of protease-activated receptor-1 (PAR1), leading to enhanced production of sphingosine-1-phosphate (S1P) and suppression of the proinflammatory response of activated macrophages. Deletion of the gamma-carboxyglutamic acid domain of APC, a region critical for its anticoagulant activity and EPCR-dependent barrier protection, had no effect on its antiinflammatory function. Genetic inactivation of CD11b, PAR1, or sphingosine kinase-1, but not EPCR, abolished the ability of APC to suppress the macrophage inflammatory response in vitro. Using an LPS-induced mouse model of lethal endotoxemia, we showed that APC administration reduced the mortality of wild-type mice, but not CD11b-deficient mice. These data establish what we believe to be a novel mechanism underlying the antiinflammatory activity of APC in the setting of endotoxemia and provide clear evidence that the antiinflammatory function of APC is distinct from its barrier-protective function and anticoagulant activities.

Proline and gamma-carboxylated glutamate residues in matrix Gla protein are critical for binding of bone morphogenetic protein-4

Arterial calcification is ubiquitous in vascular disease and is, in part, prevented by matrix Gla protein (MGP). MGP binds calcium ions through gamma-carboxylated glutamates (Gla residues) and inhibits bone morphogenetic protein (BMP)-2/-4. We hypothesized that a conserved proline (Pro)64 is essential for BMP inhibition. We further hypothesized that calcium binding by the Gla residues is a prerequisite for BMP inhibition. Site-directed mutagenesis was used to modify Pro64 and the Gla residues, and the effect on BMP-4 activity, and binding of BMP-4 and calcium was tested using luciferase reporter gene assays, coimmunoprecipitation, crosslinking, and calcium quantification. The results showed that Pro64 was critical for binding and inhibition of BMP-4 but not for calcium binding. The Gla residues were also required for BMP-4 binding but flexibility existed. As long as 1 Gla residue remained on each side of Pro64, the ability to bind and inhibit BMP-4 was preserved. Chelation of calcium ions by EDTA or warfarin treatment of cells led to loss of ability of MGP to bind BMP-4. Our results also showed that phenylalanine could replace Pro64 without loss of function and that zebrafish MGP, which lacks upstream Gla residues, did not function as a BMP inhibitor. The effect of MGP mutagenesis on vascular calcification was determined in calcifying vascular cells. Only MGP proteins with preserved ability to bind and inhibit BMP-4 prevented osteogenic differentiation and calcification. Together, our results suggest that BMP and calcium binding in MGP are independent but functionally intertwined processes and that the BMP binding is essential for prevention of vascular calcification.

Post-translational modifications regulate matrix Gla protein function: importance for inhibition of vascular smooth muscle cell calcification

BACKGROUND

Matrix Gla protein (MGP) is a small vitamin K-dependent protein containing five gamma-carboxyglutamic acid (Gla) residues that are believed to be important in binding Ca(2+), calcium crystals and bone morphogenetic protein. In addition, MGP contains phosphorylated serine residues that may further regulate its activity. In vivo, MGP has been shown to be a potent inhibitor of vascular calcification; however, the precise molecular mechanism underlying the function of MGP is not yet fully understood.

METHODS AND RESULTS

We investigated the effects of MGP in human vascular smooth muscle cell (VSMC) monolayers that undergo calcification after exposure to an increase in Ca(2+) concentration. Increased calcium salt deposition was found in cells treated with the vitamin K antagonist warfarin as compared to controls, whereas cells treated with vitamin K(1) showed decreased calcification as compared to controls. With conformation-specific antibodies, it was confirmed that warfarin treatment of VSMCs resulted in uncarboxylated (Gla-deficient) MGP. To specifically test the effects of MGP on VSMC calcification, we used full-length synthetic MGP and MGP-derived peptides representing various domains in MGP. Full length MGP, the gamma-carboxylated motif (Gla) (amino acids 35-54) and the phosphorylated serine motif (amino acids 3-15) inhibited calcification. Furthermore, we showed that the peptides were not taken up by VSMCs but bound to the cell surface and to vesicle-like structures.

CONCLUSIONS

These data demonstrate that both gamma-glutamyl carboxylation and serine phosphorylation of MGP contribute to its function as a calcification inhibitor and that MGP may inhibit calcification via binding to VSMC-derived vesicles.

Vascular calcification and uremia: what do we know?

In the last decade, the nephrology community has focused its attention on the main cause of morbidity and mortality in chronic renal failure patients: cardiovascular disease. In addition, recent studies pointed out that vascular calcification (VC) is a major cause of cardiovascular disease in the dialysis population. Interestingly, the pathogenesis of VC and soft tissue calcification in chronic kidney disease (CKD) has been extensively investigated. Nowadays we know that VC is associated not only with passive calcium phosphate deposition, but also with an active, cell-mediated process. To better understand the pathogenesis of VC in CKD, numerous regulatory proteins have been studied, because of their ability to inhibit mineral deposition in the vessels. We here examine the state of the art of those substances recognized as regulatory key factors in preventing VC in uremic conditions, such as fetuin A (alpha2-Heremans-Schmid glycoprotein), matrix gamma-carboxyglutamic acid protein, pyrophosphate, osteoprotegerin and bone morphogenetic protein. We conclude that at present it is too early to introduce these novel markers into clinical practice.

A vasopressin/oxytocin-related conopeptide with gamma-carboxyglutamate at position 8

Vasopressins and oxytocins are homologous, ubiquitous and multifunctional peptides present in animals. Conopressins are vasopressin/oxytocin-related peptides that have been found in the venom of cone snails, a genus of marine predatory molluscs that envenom their prey with a complex mixture of neuroactive peptides. In the present paper, we report the purification and characterization of a unique conopressin isolated from the venom of Conus villepinii, a vermivorous cone snail species from the western Atlantic Ocean. This novel peptide, designated gamma-conopressin-vil, has the sequence CLIQDCPgammaG* (gamma is gamma-carboxyglutamate and * is C-terminal amidation). The unique feature of this vasopressin/oxytocin-like peptide is that the eighth residue is gamma-carboxyglutamate instead of a neutral or basic residue; therefore it could not be directly classified into either the vasopressin or the oxytocin peptide families. Nano-NMR spectroscopy of the peptide isolated directly from the cone snails revealed that the native gamma-conopressin-vil undergoes structural changes in the presence of calcium. This suggests that the peptide binds calcium, and the calcium-binding process is mediated by the gamma-carboxyglutamate residue. However, the negatively charged residues in the sequence of gamma-conopressin-vil may mediate calcium binding by a novel mechanism not observed in other peptides of this family.

Role of vitamin K on biochemical markers, bone mineral density, and fracture risk

Osteoporosis is a multifactorial chronic disease that may become even more prevalent and more of a public health problem in the decades to come. Recent research has indicated that a number of macro- and micronutrients are involved in the development of bone health. In the past decade it became evident that vitamin K played a significant role in human health beyond its well-established function in blood clotting. In fact, among the proteins known or suspected to be involved in bone and vascular biology there are several members of the vitamin K dependent or gamma-carboxyglutamic acid protein family. Based on the current evidence from epidemiologic and intervention studies, there are insufficient data to recommend a routine supplementation of vitamin K for optimal bone health. New experimental and placebo-controlled studies in humans should clarify our understanding of the role vitamin K plays in improving bone health.

From gamma-carboxy-glutamate to protein C

The story I shall recount started in 1969, when I was given the opportunity to join the Department of Clinical Chemistry at the University Hospital in Malmö. I had just finished medical school at the university in the neighboring town of Lund. Parallel to pursuing my medical studies I had spent some time in the Department of Biochemistry. I did not know much about biochemistry, but it was enough for me to realize that I wanted to do laboratory research rather than developing a clinical career. I was happy to accept an offer to start working in the laboratory, particularly as the head of the department, Professor Carl-Bertil Laurell, had an excellent reputation. As it turned out, I came to spend almost all of my professional life in the laboratory.

A single gamma-carboxyglutamic acid residue in a novel cysteine-rich secretory protein without propeptide

Gamma-glutamyl carboxylase catalyzes the modification of specific glutamyl residues to gamma-carboxyglutamyl (Gla) residues in precursor proteins that possess the appropriate gamma-carboxylation recognition signal within the propeptide region. We describe the immunopurification and first biochemical characterization of an invertebrate high molecular weight Gla-containing protein with homologues in mammals. The protein, named GlaCrisp, was isolated from the venom of the marine cone snail Conus marmoreus. GlaCrisp gave intense signals in Western blot experiments employing the Gla-specific antibody M3B, and the presence of Gla was chemically confirmed by amino acid analysis after alkaline hydrolysis. Characterization of a full-length cDNA clone encoding GlaCrisp deduced a precursor containing an N-terminal signal peptide but, unlike other Gla-containing proteins, no apparent propeptide. The predicted mature protein of 265 amino acid residues showed considerable sequence similarity to the widely distributed cysteine-rich secretory protein family and closest similarity (65% identity) to the recently described substrate-specific protease Tex31. In addition, two cDNA clones encoding the precursors of two isoforms of GlaCrisp were identified. The predicted precursor isoforms differed at three amino acid positions (-6, 9, and 25). Analysis by Edman degradation and nanoelectrospray ionization mass spectrometry, before and after methyl esterfication, identified a Gla residue at amino acid position 9 in GlaCrisp. This is the first example of a Gla-containing protein without an obvious gamma-carboxylation recognition site. The results define a new class of Gla proteins and support the notion that gamma-carboxylation of glutamyl residues is phylogenetically older than blood coagulation and the vertebrate lineage.

Vitamin K-dependent proteins in Ciona intestinalis, a basal chordate lacking a blood coagulation cascade

We have isolated and sequenced several cDNAs derived from the sea squirt Ciona intestinalis that encode vitamin K-dependent proteins. Four of these encode gamma-carboxyglutamic acid (Gla) domain-containing proteins, which we have named Ci-Gla1 through Ci-Gla4. Two additional cDNAs encode the apparent orthologs of gamma-glutamyl carboxylase and vitamin K epoxide reductase. Ci-Gla1 undergoes gamma-glutamyl carboxylation when expressed in CHO cells and is homologous to Gla-RTK, a putative receptor tyrosine kinase previously identified in a related ascidian. The remaining three Gla domain proteins are similar to proteins that participate in fundamental developmental processes, complement regulation, and blood coagulation. These proteins are generally expressed at low levels throughout development and exhibit either relatively constant expression (Ci-Gla1, gamma-glutamyl carboxylase, and vitamin K epoxide reductase) or spatiotemporal regulation (Ci-Gla2, -3, and -4). These results demonstrate the evolutionary emergence of the vitamin K-dependent Gla domain before the divergence of vertebrates and urochordates and suggest novel functions for Gla domain proteins distinct from their roles in vertebrate hemostasis. In addition, these findings highlight the usefulness of C. intestinalis as a model organism for investigating vitamin K-dependent physiological phenomena, which may be conserved among the chordate subphyla.

[Protective effects of vitamin K against osteoporosis and its pleiotropic actions]

Vitamin K is a nutrient originally identified as an essential factor for blood coagulation. Recently, vitamin K has emerged as a potential protector against osteoporosis and hepatocarcinoma. Accumulated evidence indicates that subclinical non-hemostatic vitamin K deficiency in extrahepatic tissues, particularly in bone, exists widely in the otherwise healthy adult population. Both vitamin K(1) and K(2) have been shown to exert protective effects against osteoporosis. Moreover, therapeutic potential of vitamin K(2) as an anti-hepatoma drug has been recently highlighted. Most of the new biological functions of vitamin K in bone and hepatoma cells are considered to be attributable to promotion of gamma-carboxylation of glutamic acid residues in vitamin K-dependent proteins, which is shared by both vitamins K(1) and K(2). In contrast, vitamin K(2)-specific, gamma-carboxylation-unrelated functions have also been demonstrated. These functions include stimulation of steroid and xenobiotic receptor (SXR)-mediated transcription and anti-oxidant property. Thus, biological differences between vitamins K(1) and K(2), and a potential involvement of gamma-carboxylation-independent actions in the new roles of vitamin K remain open issues. Molecular bases of coagulation-unrelated pleiotropic actions of vitamin K and its implications in human health deserve further investigations.

Precursors of Novel Gla-Containing Conotoxins Contain a Carboxy-Terminal Recognition Site That Directs γ-Carboxylation,

Vitamin K-dependent gamma-glutamyl carboxylase catalyzes the conversion of glutamyl residues to gamma-carboxyglutamate. Its substrates include vertebrate proteins involved in blood coagulation, bone mineralization, and signal transduction and invertebrate ion channel blockers known as conotoxins. Substrate recognition involves a recognition element, the gamma-carboxylation recognition site, typically located within a cleavable propeptide preceding the targeted glutamyl residues. We have purified two novel gamma-carboxyglutamate-containing conotoxins, Gla-TxX and Gla-TxXI, from the venom of Conus textile. Their cDNA-deduced precursors have a signal peptide but no apparent propeptide. Instead, they contain a C-terminal extension that directs gamma-carboxylation but is not found on the mature conotoxin. A synthetic 13-residue "postpeptide" from the Gla-TxXI precursor reduced the K(m) for the reaction of the Conus gamma-carboxylase with peptide substrates, including FLEEL and conantokin-G, by up to 440-fold, regardless of whether it was positioned at the N- or C-terminal end of the mature toxin. Comparison of the postpeptides to propeptides from other conotoxins suggested some common elements, and amino acid substitutions of these residues perturbed gamma-carboxylation of the Gla-TxXI peptide. The demonstration of a functional and transferable C-terminal postpeptide in these conotoxins indicates the presence of the gamma-carboxylation recognition site within the postpeptide and defines a novel precursor structure for vitamin K-dependent polypeptides. It also provides the first formal evidence to prove that gamma-carboxylation occurs as a post-translational rather than a cotranslational process.

The γ-carboxyglutamic acid domain of anticoagulant protein S is involved in activated protein C cofactor activity, independently of phospholipid binding

We expressed 2 chimeras between human protein S (PS) and human prothrombin (FII) in which the prothrombin γ-carboxyglutamic acid (Gla) domain replaced the PS Gla domain in native PS (GlaFII-PS) or in PS deleted of the thrombin-sensitive region (TSR) (GlaFII-ΔTSR-PS). Neither PS/FII chimera had activated protein C (APC) cofactor activity in plasma clotting assays or purified systems, but both bound efficiently to phospholipids. This pointed to a direct involvement of the PS Gla domain in APC cofactor activity through molecular interaction with APC. Using computational methods, we identified 2 opposite faces of solvent-exposed residues on the PS Gla domain (designated faces 1 and 2) as potentially involved in this interaction. Their importance was supported by functional characterization of a PS mutant in which the face 1 and face 2 PS residues were reintroduced into GlaFII-PS, leading to significant APC cofactor activity, likely through restored interaction with APC. Furthermore, by characterizing PS mutants in which PS face 1 and PS face 2 were individually replaced by the corresponding prothrombin faces, we found that face 1 was necessary for efficient phospholipid binding but that face 2 residues were not strictly required for phospholipid binding and were involved in the interaction with APC.

The First γ-Carboxyglutamic Acid-containing Contryphan

Contryphans constitute a group of conopeptides that are known to contain an unusual density of post-translational modifications including tryptophan bromination, amidation of the C-terminal residue, leucine, and tryptophan isomerization, and proline hydroxylation. Here we report the identification and characterization of a new member of this family, glacontryphan-M from the venom of Conus marmoreus. This is the first known example of a contryphan peptide carrying glutamyl residues that have been post-translationally carboxylated to gamma-carboxyglutamyl (Gla) residues. The amino acid sequence of glacontryphan-M was determined using automated Edman degradation and electrospray ionization mass spectrometry. The amino acid sequence of the peptide is: Asn-Gla-Ser-Gla-Cys-Pro-D-Trp-His-Pro-Trp-Cys. As with most other contryphans, glacontryphan-M is amidated at the C terminus and maintains the five-residue intercysteine loop. The occurrence of a D-tryptophan residue was confirmed by chemical synthesis and HPLC elution profiles. Using fluorescence spectroscopy we demonstrated that the Gla-containing peptide binds calcium with a K(D) of 0.63 mM. Cloning of the full-length cDNA encoding glacontryphan-M revealed that the primary translation product carries an N-terminal signal/propeptide sequence that is homologous to earlier reported contryphan signal/propeptide sequences up to 10 amino acids preceding the toxin region. Electrophysiological experiments, carried out on mouse pancreatic B-cells, showed that glacontryphan-M blocks L-type voltage-gated calcium ion channel activity in a calcium-dependent manner. Glacontryphan-M is the first contryphan reported to modulate the activity of L-type calcium ion channels.

Isolation and characterization of three novel Gla-containing Conus marmoreus venom peptides, one with a novel cysteine pattern

One defining characteristic of Conus venom peptides is the high frequency of posttranslational modifications found. We report the discovery and initial characterization of three novel gamma-carboxyglutamic acid (Gla)-containing conotoxins, Gla-MrII, Gla-MrIII, and Gla-MrIV, isolated from the venom of the mollusc-hunting cone snail Conus marmoreus. Peptide Gla-MrII, a 50 amino acid residue peptide, carries eight cysteine residues arranged in a novel cysteine pattern, and five gamma-carboxyglutamic acid residues. The monoisotopic molecular mass was determined by electrospray ionization mass spectrometry to 5860.23 Da, consistent with the peptide having the cysteine residues disulphide-bonded and having a free acid C-terminus. Peptides Gla-MrIII and Gla-MrIV each contain two gamma-carboxyglutamic acid residues and share little sequence similarity to previously identified conotoxins. Both peptides contain four cysteine residues that are positioned in the linear sequence in a manner reminiscent of conotoxins belonging to cysteine scaffold superfamily T (scaffold T-1). Determination of the monoisotopic molecular masses revealed that Gla-MrIII is amidated at its C-terminus while Gla-MrIV has a free C-terminal acid.

Glucocorticoid effects on vitamin K-dependent carboxylase activity and matrix Gla protein expression in rat lung

The role of glucocorticoids in the regulation of vitamin K-dependent carboxylase activity was investigated in fetal and adult lung. Glucocorticoid deficiency induced by adrenalectomy (ADX) stimulated adult lung growth and reduced carboxylation in a tissue-specific manner. Type II epithelial cells were enriched in carboxylase activity, where ADX-induced downregulation was retained in freshly isolated cells. Carboxylase activity in fetal type II cells was one-half that found in fetal fibroblasts isolated from the same lungs, and both populations increased activity with time in culture. Both carboxylase activity and formation of gamma-carboxyglutamate (Gla)-containing proteins were stimulated by dexamethasone (Dex) in fetal type II cells. Matrix Gla protein (MGP), a vitamin K-dependent protein known to be synthesized in type II cells, was also found in fetal fibroblasts, where its expression was stimulated by Dex. These combined results suggested an important role for glucocorticoids and MGP in the developing lung, where both epithelial and mesenchymal cells coordinate precise control of branching morphogenesis. We investigated MGP expression and its regulation by Dex in the fetal lung explant model. MGP mRNA and protein were increased in parallel with the formation of highly branched lungs, and this increase was stimulated twofold by Dex at each day of culture. Dex-treated explants were characterized by large, dilated, conducting airways and a peripheral rim of highly branched saccules compared with uniformly branched controls. We propose that glucocorticoids are important regulators of vitamin K function in the developing and adult lung.

Role of the Gla and first epidermal growth factor-like domains of factor X in the prothrombinase and tissue factor-factor VIIa complexes

Factor X (FX) has high structure homology with other proteins of blood coagulation such as factor IX (FIX) and factor VII (FVII). These proteins present at their amino-terminal extremity a gamma-carboxyglutamic acid containing domain (Gla domain), followed by two epidermal growth factor-like (EGF1 and EGF2) domains, an activation peptide, and a serine protease domain. After vascular damage, the tissue factor-FVIIa (TF-FVIIa) complex activates both FX and FIX. FXa interacts stoichiometrically with tissue pathway inhibitor (TFPI), regulating TF-FVIIa activity by forming the TF-FVIIa-TFPI-FXa quaternary complex. Conversely, FXa boosts coagulation by its association with its cofactor, factor Va (FVa). To investigate the contribution of the Gla and EGF1 domains of FX in these complexes, FX chimeras were produced in which FIX Gla and EGF1 domains substituted the corresponding domains of FX. The affinity of the two chimeras, FX/FIX(Gla) and FX/FIX(EGF1), for the TF-FVIIa complex was markedly reduced compared with that of wild-type-FX (wt-FX) independently of the presence of phospholipids. Furthermore, the association rate constants of preformed FX/FIX(Gla)-TFPI and FX/FIX(EGF1)-TFPI complexes with TF-FVIIa were, respectively, 10- and 5-fold slower than that of wt-FXa-TFPI complex. Finally, the apparent affinity of FVa was 2-fold higher for the chimeras than for wt-FX in the presence of phospholipids and equal in their absence. These data demonstrate that FX Gla and EGF1 domains contain residues, which interact with TF-FVIIa exosites contributing to the formation of the TF-FVIIa-FX and TF-FVIIa-TFPI-FXa complexes. On the opposite, FXa Gla and EGF1 domains are not directly involved in FVa binding.

Mutagenesis of the gamma-carboxyglutamic acid domain of human factor VII to generate maximum enhancement of the membrane contact site

Site-directed mutagenesis of the 40 N-terminal residues (gamma-carboxyglutamic acid domain) of blood clotting factor VII was carried out to identify sites that improve membrane affinity. Improvements and degree of change included P10Q (2-fold), K32E (13-fold), and insertion of Tyr at position 4 (2-fold). Two other beneficial changes, D33F (2-fold) and A34E (1.5-fold), may exert their impact via influence of K32E. The modification D33E (5.2-fold) also resulted in substantial improvement. The combined mutant with highest affinity, (Y4)P10Q/K32E/D33F/A34E, showed 150-296-fold enhancement over wild-type factor VIIa, depending on the assay used. Undercarboxylation of Glu residues at positions 33 and 34 may result in an underestimate of the true contributions of gamma-carboxyglutamic acid at these positions. Except for the Tyr(4) mutant, all other beneficial mutations were located on the same surface of the protein, suggesting a possible membrane contact region. An initial screening assay was developed that provided faithful evaluation of mutants in crude mixtures. Overall, the results suggest features of membrane binding by vitamin K-dependent proteins and provide reagents that may prove useful for research and therapy.

Identification of a gene encoding a typical gamma-carboxyglutamic acid domain in the tunicate Halocynthia roretzi

We report the identification of a gene capable of encoding a novel Gla (gamma-carboxyglutamic acid) protein from the tunicate Halocynthia roretzi, a primitive member of the phylum Chordata. We call this new hypothetical protein Gla-RTK; it has a Gla domain typical of human vitamin K-dependent coagulation factors, a transmembrane domain, and a receptor tyrosine kinase domain. The receptor tyrosine kinase domain is very similar to the ARK (adhesion-related kinase) family of receptor tyrosine kinases. The ARK family includes Axl, Tyro3, and c-Mer. This gene also encodes a propeptide that binds to the human gamma-glutamyl carboxylase within a range of affinities observed for mammalian propeptides. The cDNA for this putative protein is found distributed throughout the oocyte and embryo but the cDNA is apparently not transcribed except during oogenesis. One of the most interesting aspects of this hypothetical protein is that its Gla domain is highly homologous to the Gla domain of Gas6, a ligand for Axl, while its receptor tyrosine kinase domain is highly homologous to Axl.

Expression and characterization of recombinant vitamin K-dependent gamma-glutamyl carboxylase from an invertebrate, Conus textile

The marine snail Conus is the sole invertebrate wherein both the vitamin K-dependent carboxylase and its product, gamma-carboxyglutamic acid, have been identified. To examine its biosynthesis of gamma-carboxyglutamic acid, we studied the carboxylase from Conus venom ducts. The carboxylase cDNA from Conus textile has an ORF that encodes a 811-amino-acid protein which exhibits sequence similarity to the vertebrate carboxylases, with 41% identity and approximately 60% sequence similarity to the bovine carboxylase. Expression of this cDNA in COS cells or insect cells yielded vitamin K-dependent carboxylase activity and vitamin K-dependent epoxidase activity. The recombinant carboxylase has a molecular mass of approximately 130 kDa. The recombinant Conus carboxylase carboxylated Phe-Leu-Glu-Glu-Leu and the 28-residue peptides based on residues -18 to +10 of human proprothrombin and proFactor IX with Km values of 420 micro m, 1.7 micro m and 6 micro m, respectively; the Km for vitamin K is 52 micro m. The Km values for peptides based on the sequence of the conotoxin epsilon-TxIX and two precursor analogs containing 12 or 29 amino acids of the propeptide region are 565 micro m, 75 micro m and 74 micro m, respectively. The recombinant Conus carboxylase, in the absence of endogenous substrates, is stimulated up to fivefold by vertebrate propeptides but not by Conus propeptides. These results suggest two propeptide-binding sites in the carboxylase, one that binds the Conus and vertebrate propeptides and is required for substrate binding, and the other that binds only the vertebrate propeptide and is required for enzyme stimulation. The marked functional and structural similarities between the Conus carboxylase and vertebrate vitamin K-dependent gamma-carboxylases argue for conservation of a vitamin K-dependent carboxylase across animal species and the importance of gamma-carboxyglutamic acid synthesis in diverse biological systems.

Gas6 binding to photoreceptor outer segments requires gamma-carboxyglutamic acid (Gla) and Ca(2+) and is required for OS phagocytosis by RPE cells in vitro

The phagocytosis of photoreceptor outer segments by retinal pigment epithelial (RPE) cells plays a critical role in preserving normal retinal function. Recently the receptor protein tyrosine kinase (RTK) Mer, has been shown to be necessary for this cellular process to take place. Gas6, the ligand for the Mer RTK, can specifically and selectively stimulate the phagocytosis of photoreceptor outer segments (OS) by normal cultured rat RPE cells, as we have previously reported. The Gas6 protein has been shown to associate with plasma membrane phosphatidylserine by its amino-terminal portion, while its carboxyl-terminal portion can bind and activate Mer and its related RTKs, Axl and Tyro-3. Given the capability of Gas6 to interact with more than one molecule, we have performed a series of experiments to further dissect the interactions of Gas6 with the OS and RPE and to determine the specific calcium requirements necessary for Gas6 to exert its stimulatory effect on phagocytosis. These experiments show that Gas6 must bind to OS before the stimulation of OS ingestion can occur and that this binding requires a Ca(2+) concentration of 500-600 microM. The same Ca(2+) concentration is required for the Gas6 mediated stimulation of OS ingestion. We further demonstrate that in order to bind to OS and to stimulate OS phagocytosis, Gas6 requires gamma-carboxylation in a vitamin K-dependent reaction. By analogy with other systems, we propose that Ca(2+) mediates the linkage between the gamma-carboxyglutamic acid (Gla)-rich N-terminal region of Gas6 with phospholipids, presumably phosphatidylserine, in the plasma membrane of the OS. Only after this binding has occurred can Gas6 interact with receptor molecule(s) on the surface of the RPE, and activate RPE cell signaling pathways leading to OS ingestion. These studies further underscore the importance of Gas6 in the phagocytic function of the RPE and open new avenues of investigation to understand the molecular events mediated and triggered by Gas6, and its interaction with the OS and RPE.

Vitamin K-dependent protein S localizing complement regulator C4b-binding protein to the surface of apoptotic cells

Apoptosis is characterized by a lack of inflammatory reaction in surrounding tissues, suggesting local control of complement activation. During the initial stage of apoptosis, cells expose negatively charged phospholipid phosphatidylserine on their surfaces. The vitamin K-dependent protein S has a high affinity for this type of phospholipid. In human plasma, 60-70% of protein S circulates in complex with C4b-binding protein (C4BP). The reason why protein S and C4BP form a high-affinity complex in plasma is not known. However, C4BP is an important regulator of the classical pathway of the complement system where it acts as a cofactor in degradation of complement protein C4b. Using Jurkat cells as a model system for apoptosis, we now show protein S to bind to apoptotic cells. We further demonstrate protein S-mediated binding of C4BP to apoptotic cells. Binding of the C4BP-protein S complex to apoptotic cells was calcium-dependent and could be blocked with Abs directed against the phospholipid-binding domain in protein S. Annexin V, which binds to exposed phosphatidylserine on the apoptotic cell surface, could inhibit the binding of protein S. The C4BP that was bound via protein S to the apoptotic cells was able to interact with the complement protein C4b, supporting a physiological role of the C4BP/protein S complex in regulation of complement on the surface of apoptotic cells.

Implication of protein S thrombin-sensitive region with membrane binding via conformational changes in the gamma-carboxyglutamic acid-rich domain

In the vitamin K-dependent protein family, only protein S (PS) contains a thrombin-sensitive region (TSR), located between the domain containing the gamma-carboxyglutamic acid and the first epidermal growth factor-like domain. To better define the role of TSR in the PS molecule, we expressed a recombinant human PS (rHPS) and its analogue lacking TSR (rTSR-less), and prepared factor Xa- and thrombin-cleaved rHPS. A peptide reproducing TSR (TSR-peptide) was also synthesized in an attempt to obtain direct evidence of the domain involvement in PS anticoagulant activity. In a coagulation assay, both rTSR-less and factor Xa-cleaved PS were devoid of activated protein C cofactor activity. The TSR-peptide did not inhibit rHPS activity, showing that TSR must be embedded in the native protein to promote interaction with activated protein C. The binding of rHPS to activated platelets and to phospholipid vesicles was not modified after factor Xa- or thrombin-mediated TSR cleavage, whereas the binding of rTSR-less was markedly reduced. This suggested a role for TSR in conferring to PS a strong affinity for phospholipid membranes. TSR-peptide did not directly bind to activated platelets or compete with rHPS for phospholipid binding. The results of the present study show that TSR may not interact directly with membranes, but probably constrains the gamma-carboxyglutamic acid-rich domain in a conformation allowing optimal interaction with phospholipids.

Matrix gamma-carboxyglutamic acid protein is a key regulator of PTH-mediated inhibition of mineralization in MC3T3-E1 osteoblast-like cells

As part of its overall function as a major regulator of calcium homeostasis, PTH stimulates bone resorption and inhibits osteoblast-mediated biomineralization. To determine the basis for the inhibitory actions of this hormone, we compared the time course of PTH-dependent inhibition of mineralization in MC3T3-E1 osteoblast-like cells with changes in mRNA levels for several extracellular matrix proteins previously associated either with induction or inhibition of mineralization. Mineralizing activity was rapidly lost in PTH-treated cells ( approximately 30% inhibition after 3 h, 50% inhibition at 6 h). Of the proteins examined, changes in matrix gamma-carboxyglutamic acid protein were best correlated with PTH-dependent inhibition of mineralization. Matrix gamma-carboxyglutamic acid protein mRNA was rapidly induced 3 h after PTH treatment, with a 6- to 8-fold induction seen after 6 h. Local in vivo injection of PTH over the calvaria of mice also induced a 2-fold increase in matrix gamma-carboxyglutamic acid protein mRNA. Warfarin, an inhibitor of matrix gamma-carboxyglutamic acid protein gamma-carboxylation, reversed the effects of PTH on mineralization in MC3T3-E1 cells, whereas vitamin K enhanced PTH activity, as would be expected if a gamma-carboxyglutamic acid-containing protein were required for PTH activity. Levels of the other mRNAs examined were not well correlated with the observed changes in mineralization. Osteopontin, an in vitro inhibitor of mineralization, was induced approximately 4-fold 12 h after PTH addition. Bone sialoprotein mRNA, which encodes an extracellular matrix component most frequently associated with mineral induction, was inhibited by 50% after 12 h of PTH treatment. Osteocalcin mRNA, encoding the other known gamma-carboxyglutamic acid protein in bone, was also inhibited by PTH, but, again, with a significantly slower time course than was seen for mineral inhibition. Taken together, these results show that the rapid inhibition of osteoblast mineralization induced by in vitro PTH treatment is at least in part explained by induction of matrix gamma-carboxyglutamic acid protein.

Synthesis of gamma-carboxylated polypeptides by alpha-cells of the pancreatic islets

gamma-Carboxylated proteins were detected in normal human pancreas by immunohistochemistry with a monoclonal antibody (M3B) specific for gamma-carboxyglutamyl residues. Staining appeared to be localized to the glucagon-secreting alpha-cells in the islets of Langerhans. Consistent with this, sections from a glucagonoma were stained much more intensely with the M3B antibody than those from an insulinoma. A murine alpha-cell line (alphaTC1 Clone 9) was cultured and gamma-carboxylated polypeptides, identified immunologically as prothrombin, protein S and (tentatively) Gas6, were isolated from the intracellular compartment by chromatography on an M3B-coupled resin. As in liver, prothrombin is synthesized by alpha-cells as a gamma-carboxylated zymogen that can be cleaved by ecarin to form an active serine protease that is inhibited by hirudin. The pancreas thus appears to be a novel site of synthesis for certain vitamin K-dependent proteins.

The tissue factor region that interacts with factor Xa in the activation of factor VII

Tissue factor is the cell membrane-anchored cofactor for factor VIIa and triggers the coagulation reactions. The initial step is the conversion of factor VII to factor VIIa which, in vitro, is efficiently catalyzed by low concentrations of factor Xa. To identify the tissue factor region that interacts with the activator factor Xa during this process, we evaluated a panel of soluble tissue factor (1-219) mutants for their ability to support factor Xa-mediated activation of factor VII. The tissue factor residues identified as most important for this interaction (Tyr157, Lys159, Ser163, Gly164, Lys165, Lys166, and Tyr185) were identical to those found to be important for the interaction of substrate factor X with the tissue factor.factor VIIa complex. The residues form a continuous surface-exposed patch with an area of about 500 A(2), which appears to be located outside the tissue factor-factor VII contact zone. In agreement, the two monoclonal antibodies 5G6 and D3H44-F(ab')(2), whose epitopes overlap with this identified region, inhibited the rates of factor VII activation by 86% and 95%, respectively. These antibodies also strongly inhibited the conversion of (125)I-labeled factor VII when cell membrane-expressed, full-length tissue factor (1-263) was employed. Together the results suggest the usage of a common surface region of tissue factor in its dual role-as a cofactor for factor Xa-mediated factor VII activation and as a cofactor for factor VIIa-mediated factor X activation. The finding that factor Xa and factor X may engage in similar, if not identical, molecular interactions with tissue factor further indicates that factor Xa and factor X are similarly oriented toward their respective interaction partners in the ternary catalytic complexes.

Enhanced gamma-carboxylation of recombinant factor X using a chimeric construct containing the prothrombin propeptide

Factor Xa is the serine protease component of prothrombinase, the enzymatic complex responsible for thrombin generation. Production of recombinant factor X/Xa has proven to be difficult because of inefficient gamma-carboxylation, a critical post-translational modification. The affinities of the vitamin K-dependent propeptides for the gamma-carboxylase vary over 2 logs, with the propeptide of factor X having the highest affinity followed by the propeptides of factor VII, protein S, factor IX, protein C, and prothrombin [Stanley, T. B. (1999) J. Biol. Chem. 274, 16940-16944]. On the basis of this observation, it was hypothesized that exchanging the propeptide of factor X with one that binds the gamma-carboxylase with a reduced affinity would enhance gamma-carboxylation by allowing greater substrate turnover. A chimeric cDNA consisting of the human prothrombin signal sequence and propeptide followed by mature human factor X was generated and stably transfected into HEK 293 cells, and modified factor X was purified from conditioned medium. The results indicate that on average 85% of the total factor X produced with the prothrombin propeptide was fully gamma-carboxylated, representing a substantial improvement over a system that employs the native factor X propeptide, with which on average only 32% of the protein is fully gamma-carboxylated. These results indicate that the affinity of the gamma-carboxylase for the propeptide greatly influences the extent of gamma-carboxylation. It was also observed that regardless of which propeptide sequence is directing gamma-carboxylation (factor X or prothrombin), two pools of factor X are secreted; one is uncarboxylated and a second is fully gamma-carboxylated, supporting the notion that the gamma-carboxylase is a processive enzyme.

Effect of gamma-glutamyl carboxylation of renal microsomes on calcium oxalate monohydrate crystal binding in hyperoxaluria

AIM

The gamma-carboxy glutamic acid (gla) containing proteins have been shown recently to have a role in calcium oxalate urolithiasis. Therefore, the effect of carboxylation on calcium oxalate monohydrate (COM) binding was studied in experimental hyperoxaluria.

METHODS

Urolithiasis was induced in experimental rats by oral feeding 1% ethylene glycol (EG). The microsomes were carboxylated in presence of reduced vitamin K and COM binding activity was determined.

RESULTS

Microsomal proteins adsorbed maximally with COM crystals. On carboxylation of the renal microsomal proteins, a significant increase in the COM crystal adsorption by 2.5-fold (p < 0.001) was observed in the hyperoxaluric condition. Further cyboxylated microsomes of EG-treated rats showed significant binding with calcium oxalate.

CONCLUSION

This study presents an increased COM adsorption after the carboxylation of renal microsomes of hyperoxaluric rats.

Identification and purification of vitamin K-dependent proteins and peptides with monoclonal antibodies specific for gamma -carboxyglutamyl (Gla) residues

Novel monoclonal antibodies that specifically recognize gamma-carboxyglutamyl (Gla) residues in proteins and peptides have been produced. As demonstrated by Western blot and time-resolved immunofluorescence assays the antibodies are pan-specific for most or all of the Gla-containing proteins tested (factors VII, IX, and X, prothrombin, protein C, protein S, growth arrest-specific protein 6, bone Gla protein, conantokin G from a cone snail, and factor Xa-like proteins from snake venom). Only the Gla-containing light chain of the two-chain proteins was bound. Decarboxylation destroyed the epitope(s) on prothrombin fragment 1, and Ca(2+) strongly inhibited binding to prothrombin. In Western blot, immunofluorescence, and surface plasmon resonance assays the antibodies bound peptides conjugated to bovine serum albumin that contained either a single Gla or a tandem pair of Gla residues. Binding was maintained when the sequence surrounding the Gla residue(s) was altered. Replacement of Gla with glutamic acid resulted in a complete loss of the epitope. The utility of the antibodies was demonstrated in immunochemical methods for detecting Gla-containing proteins and in the immunopurification of a factor Xa-like protein from tiger snake venom. The amino acid sequences of the Gla domain and portions of the heavy chain of the snake protein were determined.

Matrix GLA protein is a developmental regulator of chondrocyte mineralization and, when constitutively expressed, blocks endochondral and intramembranous ossification in the limb

Matrix GLA protein (MGP), a gamma-carboxyglutamic acid (GLA)-rich, vitamin K-dependent and apatite-binding protein, is a regulator of hypertrophic cartilage mineralization during development. However, MGP is produced by both hypertrophic and immature chondrocytes, suggesting that MGP's role in mineralization is cell stage-dependent, and that MGP may have other roles in immature cells. It is also unclear whether MGP regulates the quantity of mineral or mineral nature and quality as well. To address these issues, we determined the effects of manipulations of MGP synthesis and expression in (a) immature and hypertrophic chondrocyte cultures and (b) the chick limb bud in vivo. The two chondrocyte cultures displayed comparable levels of MGP gene expression. Yet, treatment with warfarin, a gamma-carboxylase inhibitor and vitamin K antagonist, triggered mineralization in hypertrophic but not immature cultures. Warfarin effects on mineralization were highly selective, were accompanied by no appreciable changes in MGP expression, alkaline phosphatase activity, or cell number, and were counteracted by vitamin K cotreatment. Scanning electron microscopy, x-ray microanalysis, and Fourier-transform infrared spectroscopy revealed that mineral forming in control and warfarin-treated hypertrophic cell cultures was similar and represented stoichiometric apatite. Virally driven MGP overexpression in cultured chondrocytes greatly decreased mineralization. Surprisingly, MGP overexpression in the developing limb not only inhibited cartilage mineralization, but also delayed chondrocyte maturation and blocked endochondral ossification and formation of a diaphyseal intramembranous bone collar. The results show that MGP is a powerful but developmentally regulated inhibitor of cartilage mineralization, controls mineral quantity but not type, and appears to have a previously unsuspected role in regulating chondrocyte maturation and ossification processes.

Localization of labile posttranslational modifications by electron capture dissociation: the case of gamma-carboxyglutamic acid

Tandem mass spectrometry (MS/MS) of 28 residue peptides harboring gamma-carboxylated glutamic acid residues, a posttranslational modification of several proenzymes of the blood coagulation cascade, using either collisions or infrared photons results in complete ejection of the gamma-CO2 moieties (-44 Da) before cleavage of peptide-backbone bonds. However, MS/MS using electron capture dissociation (ECD) in a Fourier transform mass spectrometer cleaves backbone bonds without ejecting CO2, allowing direct localization of this labile modification. Sulfated side chains are also retained in ECD backbone fragmentations of a 21-mer peptide, although CAD causes extensive SO3 loss. ECD thus is a unique complement to conventional methods for MS/MS, causing less undesirable loss of side-chain functionalities as well as more desirable backbone cleavages.

Enhancement of vitamin-K-dependent protein function by modification of the gamma-carboxyglutamic acid domain: studies of protein C and factor VII

Vitamin-K-dependent proteins are found in both the pro- and anti-coagulation cascades, and their use in coagulation therapies is expanding rapidly. The vitamin-K-dependent, gamma-carboxyglutamic acid (Gla)-containing regions of proteins in this family are homologous and are responsible for membrane association. Site-directed mutations that enhance the membrane affinity of protein C, an anticoagulant, and of factor VII, a procoagulant, have been identified. These protein C and Factor VII mutants show enhanced activity in many assays, offering opportunities to study the role of membrane in blood clotting reactions and proteins that may have greater therapeutic value.

Membrane association with multiple calcium ions: vitamin-K-dependent proteins, annexins and pentraxins

Peripheral membrane association with high calcium stoichiometry is shared by three families of proteins: annexins, pentraxins and vitamin-K-dependent proteins. Recent crystal structure determinations, biophysical studies of membrane binding and analyses of protein electrostatic properties offer striking and different concepts for membrane association by each of these protein families.

Calcium binding mode of gamma-carboxyglutamic acids in conantokins

Conantokin-T (con-T) and conantokin-G (con-G) are two highly homologous peptide toxins found in Conus venom. The former is a 21-residue peptide with four gamma-carboxyglutamic acid (Gla) residues (at positions 3, 4, 10 and 14), while the latter is a 17-residue peptide with five gamma-carboxyglutamic acid residues (at positions 3, 4, 7, 10 and 14). Despite the apparent similarity in number and relative positions of the gamma-carboxyglutamic acid residues, (113)Cd-NMR studies indicated a distinct metal binding behavior for con-G and con-T. There appears to be four binding sites in con-G in contrast to one metal binding site in con-T. To elucidate the mode of calcium binding by the gamma-carboxyglutamic acid residues in these conantokins, we designed various analogous peptides with their gamma-carboxyglutamic acid replaced by other amino acid residues. (113)Cd-NMR experiments on conantokin analogues reveal that the major difference in the number of metal binding sites between con-G and con-T is due to the residue at position 7. We also performed molecular simulations to calculate the relative binding free energies of several potential binding sites. Based on our theoretical and experimental results, we propose a 'four-site' binding model for conantokin-G and a 'single-site' binding model for conantokin-T.

Binding of cations to individual gamma-carboxyglutamate residues of conantokin-G and conantokin-T

Conantokin-G (con-G) and conantokin-T (con-T) are naturally occurring gamma-carboxyglutamate (Gla)-containing peptides that interact with multivalent cations in functionally relevant manners. Selective 13C-enrichment of Cgamma and Cdelta in each of the Gla residues has allowed metal binding affinities to be measured at individual side chains. Con-T possesses two metal binding sites, one with high affinity at Gla10/Gla14 and another with weak binding at Gla3/Gla4. Con-G contains two sites of comparable low affinity for Ca2+. Analysis of the 13C line-widths of con-G in the presence of Mg2+ allowed the order of metal binding to be determined, with Gla10/Gla14 loading before the Gla3/Gla4/Gla7 cluster. While the variant peptide, apo-con-T[Lys7Gla], was shown to have a very low alpha-helical content, this peptide binds a second metal with much greater affinity than wild-type con-T. This provides additional evidence that Gla7 in con-G is primarily responsible for destabilizing the apo-form, but is an important ligand for metal chelation. The residue-specific alpha-helical stabilities of con-G and con-T in their metal-free and metal-loaded states were estimated by determining rates of proton exchange from backbone peptide bond amides with deuterium atoms from 2H20-containing solvents. For both peptides, the lifetimes of protons on several peptide bond amides increased as metals of higher affinity were bound to the peptides, with the longest half-lives found in the region of the alpha-helical turn stabilized by the Gla10/Gla14 metal coordination site. We propose that Gla10 and Gla14 constitute the primary tight metal ion binding site in both peptides. This detailed analysis with physiologically relevant metal cations is crucial for deciphering the roles of critical amino acids in the bioactivity of the conantokin peptides.

Expression of the gene encoding the matrix gla protein by mature osteoblasts in human fracture non-unions

BACKGROUND

Osteoblast phenotypic abnormality, namely the expression of collagen type III, has been shown previously in fracture non-union woven bone.

AIMS

To investigate osteoblasts from fracture non-unions for evidence of gene expression of non-collagenous bone matrix proteins that have been implicated in mineralisation, namely matrix gla protein (MGP), osteonectin, osteopontin, and osteocalcin. MGP is a consistent component of bone matrix, but there are no reports of osteoblasts in the skeleton expressing the gene for MGP, and the site of synthesis of skeletal MGP (perhaps the liver) has yet to be determined.

METHODS

Biopsies from normally healing human fractures and non-unions were examined by means of in situ hybridisation, using 35S labelled probes and autoradiography to disclose levels of gene expression.

RESULTS

In normally healing fractures, mature osteoblasts on woven bone were negative for MGP mRNA, but positive for osteonectin, osteopontin, and osteocalcin mRNA molecules. In non-unions, osteoblasts displayed a novel phenotype: they were positive for MGP mRNA, in addition to osteonectin, osteopontin, and osteocalcin mRNA molecules.

CONCLUSIONS

Mature osteoblasts in slowly healing fractures have an unusual phenotype: they express the gene encoding MGP, which indicates that control of osteoblast gene expression in non-unions is likely to be abnormal. This might be of importance in the pathogenesis of non-uniting human fractures, and is of current interest given the emerging status of MGP as an inhibitor of mineralisation.

Importance of factor VIIa Gla-domain residue Arg-36 for recognition of the macromolecular substrate factor X Gla-domain

Macromolecular substrate docking with coagulation enzyme-cofactor complexes involves multiple contacts distant from the enzyme's catalytic cleft. Here we characterize the binding of the Gla-domain of macromolecular substrate coagulation factor X to the complex of tissue factor (TF) and VIIa. Site-directed mutagenesis of charged residue side chains in the VIIa Gla-domain identified Arg-36 as being important for macromolecular substrate docking. Ala substitution for Arg-36 resulted in an increased KM and a decreased rate of X activation. X with a truncated Gla-domain was activated by mutant and wild-type VIIa at indistinguishable rates, demonstrating that Arg-36 interactions require a properly folded Gla-domain of the macromolecular substrate. VIIa Arg-36 was also required for effective docking of the X Gla-domain in the absence of phospholipid, demonstrating that the Gla-domain of VIIa participates in protein-protein interactions with X. In the absence of TF, the mutant VIIa had essentially normal function, indicating that the cofactor positions VIIa's Gla-domain for optimal macromolecular substrate docking. Computational docking suggests multiple charge complementary contacts of the X Gla-domain with TF.VIIa. A prominent interaction is made by the functionally important X residue Gla-14 with the center of the extended docking site created by residues in the carboxyl module of TF and the contiguous VIIa Gla-domain. These data demonstrate the functional importance of interactions of the Gla-domains of enzyme and substrate, and begin to elucidate the molecular details of the ternary TF.VIIa.X complex.

Renal excretion of gamma-carboxyglutamic acid and metabolic rate in 3-18 years old humans

The modified amino acid gamma-carboxyglutamic acid (Gla) occurs in several proteins such as prothrombin, blood coagulation factors VII, IX and X, proteins C, S and Z as well as matrix Gla protein and osteocalcin. The amount of Gla excreted in urine is a common indicator of the whole-body degradation of these proteins. We have determined the renal excretion rates of Gla in 3, 6, 10, 14 and 18 years old male and female human subjects (n = 14 per age group and sex) and calculated the respective resting metabolic rates (RMR) on the basis of the body weights using published formulas. We found high correlations between the excretion rates of Gla (mumol/d/kg body weight) and the RMR (kJ/d/kg body weight) in the females (n = 70) of r = 0.70 (y = 0.003x + 0.29) and in the males (n = 70) of r = 0.70 (y = 0.0038x + 0.27) and in all subjects (n = 140) of r = 0.69 (y = 0.0035x + 0.27); p < 0.01. We postulate that in children and adolescents a causal relationship exists between the whole-body degradation rate of Gla containing proteins and the metabolic rate.

Coagulation factor X-binding protein from Deinagkistrodon acutus venom is a Gla domain-binding protein

Factor IX/factor X-binding protein (IX/X-bp) is an anticoagulant isolated from the venom of Trimeresurus flavoviridis (habu snake) and binds predominantly to factor IX. In this study, we isolated IX/X-bp-like proteins from the venom of Deinagkistrodon acutus (hundred pace snake) with binding characteristics different from those of IX/X-bp. The complete amino acid sequence and binding characteristics of the main anticoagulant protein, named X-bp, were investigated. The concentrations of X-bp at half-maximal binding to solid-phase factors X and IX were 0.4 and 3 nM, respectively. The binding of X-bp to solid-phase factor X was inhibited by 50% by 6- and 9-fold excess concentrations of factor X and Gla domain (GD) peptide 1-44, respectively, but was not influenced by GD peptide 1-41 and Gla domainless factor X. X-bp bound two Ca2+ ions per molecule with Kd values of 16 +/- 0.7 (mean +/- SE, n = 6) and 103 +/- 10 microM. X-bp was a heterodimer of C-type lectin-like subunits. The 16 kDa chain (A chain) consisted of 129 amino acid residues and was 68% identical to the sequence of the A chain of IX/X-bp. The 15 kDa chain (B chain) consisted of 123 amino acid residues and was 87% identical to IX/X-bp. Three-dimensional model construction from the known fold of IX/X-bp showed that amino acid residues different from those of IX/X-bp are mostly on the molecular surface. Some of these are concentrated on a part of the concave surface which is considered to be the coagulation factor-binding site, presumably acting as a discriminator for ligand binding. These results indicated that X-bp isolated from D. acutus venom was a GD-binding protein, and the C-terminal region of GD peptide was critical for folding of the peptide.

Role of the propeptide and gamma-glutamic acid domain of factor IX for in vitro carboxylation by the vitamin K-dependent carboxylase

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the processive carboxylation of specific glutamates in a number of proteins related to blood coagulation and bone. To address the independent importance of the propeptide, gamma-carboxyglutamic acid (Gla) domain and elements beyond the Gla domain of factor IX in vitamin K-dependent carboxylation, we have examined the kinetics of carboxylation of peptides containing (1) propeptide and Gla domain, (2) the Gla domain alone, (3) uncarboxylated bone Gla protein, (4) propeptide followed by the entire uncarboxylated factor IX molecule, and (5) the factor IX propeptide followed by a non-Gla domain sequence. Our studies indicate that peptides with a covalently linked propeptide have Km values similar to the physiological substrate of the carboxylase. In contrast, the Gla domain of factor IX has a >/=230-fold higher Km for the carboxylase than the corresponding peptide with a covalently linked propeptide. This contrasts with bone Gla protein, another vitamin K-dependent protein, which appears not to require a covalently linked propeptide for high-affinity binding to the carboxylase. Analysis of the carboxylation products of a propeptide/non-Gla domain substrate indicate that it is carboxylated multiple times in a processive manner. These studies show that the perceived binding affinity of the carboxylase substrate and processivity is conferred by the propeptide without requiring the conserved Gla domain sequences and that factor IX and bone Gla protein may have distinct mechanisms of interacting with the carboxylase.

Conformational changes in conantokin-G induced upon binding of calcium and magnesium as revealed by NMR structural analysis

The apo- and metal-bound solution conformations of synthetic conantokin-G (con-G, G1Egamma gammaL5Q gamma NQgamma 10LIRgamma K15SN-CONH2, gamma = gamma-carboxyglutamic acid), an antagonist of N-methyl-D-aspartate receptor-derived neuronal ion channels, have been examined by one- and two-dimensional 1H NMR at neutral pH. A complete structure for the Mg2+-loaded peptide was defined by use of distance geometry calculations and was found to exist as an alpha-helix that spans the entire peptide. The alpha-helical nature of Mg2+/con-G was also supported by the small values (<5.5 Hz) of the 3JHNalpha coupling constants measured for amino acid residues 3-5, 8, 9, and 11-16, and the small values (<4 ppb/K) of the temperature coefficients observed for the alphaNH protons of residues 5-17. This conformation contrasted with that obtained for apo-con-G, which was nearly structureless in solution. Docking of Mg2+ into con-G was accomplished by use of the genetic algorithm/molecular dynamics simulation method, employing the NMR-derived Mg2+-loaded structure for initial coordinates in the midpoint calculations. For the 3 Mg2+/con-G model, it was found that binding of one Mg2+ ion is stabilized by oxygen atoms from three gamma-carboxylates of Gla3, Gla4, and Gla7; another Mg2+ is coordinated by two oxygen atoms, one from each of the gamma-carboxylates of Gla7; and a third metal ion through three donor oxygen atoms of gamma-carboxylates from Gla10 and Gla14. As shown from direct metal binding measurements to mutant con-G peptides, these latter two Gla residues probably stabilized the tightest binding Mg2+ ion. Circular dichroism studies of these same peptide variants demonstrated that all Gla residues contribute to the adoption of the Mg2+-dependent alpha-helical conformation in con-G. The data obtained in this investigation provide a molecular basis for the large conformational alteration observed in apo-con-G as a result of divalent cation binding and allow assessment of the roles of individual Gla residues in defining certain of the structure-function properties of con-G.

The roles of individual gamma-carboxyglutamate residues in the solution structure and cation-dependent properties of conantokin-T

The solution structure of the Ca2+-loaded conantokin-T (con-T), a gamma-carboxyglutamate (Gla)-containing 21-residue peptide (NH2-G1EgammagammaY5QKMLgamma10NLRgammaA15EVKKN20A-CONH2,gam ma = Gla), has been elucidated by use of distance geometry calculations with experimental distances derived from two-dimensional 1H NMR spectroscopy. An end-to-end alpha-helix was the dominant conformation in solution, similar to that of apo-con-T, except that reorientation of several side chains occurred in the Ca2+-coordinated complex. The most notable examples of this were those of Gla10 and Gla14, which were more optimally positioned for complexation with Ca2+. In addition to the stabilization offered to the alpha-helix by Ca2+ binding, hydrophobic clustering of the side chains of Tyr5, Met8, Leu9, and Leu12, and ionic interactions between Lys7 and Gla3/Gla10 and between Arg13 and Gla14, along with hydrogen bonding between Gln6 and Gla10, were among the side chain interactions likely playing a significant role in maintenance of the alpha-helical conformation. Docking of Ca2+ in the con-T structure was accomplished using genetic algorithm-molecular dynamics simulation approaches. The results showed that one Ca2+ ion is most likely coordinated by four side chain oxygen atoms, two each from Gla10 and Gla14. Another bound Ca2+ ion has as its donor sites three oxygen atoms, two from Gla3 and one from Gln6. To examine the functional roles of the individual Gla residues, a series of variant peptides have been synthesized with Ala substituted for each Gla residue, and several properties of the resulting variants have been examined. The data obtained demonstrated the importance of Gla10 and Gla14 in stabilizing binding of the highest affinity Ca2+ site and in governing the conformational change induced by Ca2+. The critical nature of Gla3 and Gla4 in inhibition of the spermine-induced potentiation of the binding of MK-801 to open ion channels of the N-methyl-D-aspartate receptor was established, as well as the role of Gla4 in stabilizing the apo-con-T alpha-helical conformation.