Glutamyl substrate-induced exposure of a free cysteine residue in the vitamin K-dependent gamma-glutamyl carboxylase is critical for vitamin K epoxidation

Defective γ-Glutamyl Carboxylase Activity and Bleeding in Rambouillet Sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality caused by ineffective hemostasis at parturition. Neonatal-affected lambs presented with inadequate hemostasis at the umbilicus, pale mucus membranes, and markedly prolonged activated clotting time. Affected lambs had consistently prolonged 1-stage prothrombin times and activated partial thromboplastin times that supported a defect in the common pathway or defects in both the intrinsic and extrinsic pathway of the coagulation cascade. Decreased activity of vitamin K-dependent procoagulant factors II, VII, IX, and X in male and female lambs suggested either a defect of the hepatic enzyme γ-glutamyl carboxylase, or vitamin K1 2,3 epoxide reductase. Affected lamb hepatic γ-glutamyl carboxylase activity was markedly decreased compared with that of age- and sex-matched control lambs, while vitamin K1 2,3 epoxide reductase and glucose-6-phosphatase activities were similar between an affected and normal lamb. Subcutaneous vitamin K1 supplementation did not increase vitamin K-dependent procoagulant factor activities in 3 lambs administered vitamin K1 daily. These data confirm defective γ-glutamyl carboxylase activity as the cause of impaired coagulation of sheep in this flock. This flock represents the only viable animal model of hereditarily defective γ-glutamyl carboxylase activity.

Effect of vitamin K-dependent protein precursor propeptide, vitamin K hydroquinone, and glutamate substrate binding on the structure and function of {gamma}-glutamyl carboxylase

The γ-glutamyl carboxylase utilizes four substrates to catalyze carboxylation of certain glutamic acid residues in vitamin K-dependent proteins. How the enzyme brings the substrates together to promote catalysis is an important question in understanding the structure and function of this enzyme. The propeptide is the primary binding site of the vitamin K-dependent proteins to carboxylase. It is also an effector of carboxylase activity. We tested the hypothesis that binding of substrates causes changes to the carboxylase and in turn to the substrate-enzyme interactions. In addition we investigated how the sequences of the propeptides affected the substrate-enzyme interaction. To study these questions we employed fluorescently labeled propeptides to measure affinity for the carboxylase. We also measured the ability of several propeptides to increase carboxylase catalytic activity. Finally we determined the effect of substrates: vitamin K hydroquinone, the pentapeptide FLEEL, and NaHCO(3), on the stability of the propeptide-carboxylase complexes. We found a wide variation in the propeptide affinities for carboxylase. In contrast, the propeptides tested had similar effects on carboxylase catalytic activity. FLEEL and vitamin K hydroquinone both stabilized the propeptide-carboxylase complex. The two together had a greater effect than either alone. We conclude that the effect of propeptide and substrates on carboxylase controls the order of substrate binding in such a way as to ensure efficient, specific carboxylation.

Chemical strategies for generating protein biochips

Protein biochips are at the heart of many medical and bioanalytical applications. Increasing interest has been focused on surface activation and subsequent functionalization strategies for immobilizing these biomolecules. Different approaches using covalent and noncovalent chemistry are reviewed; particular emphasis is placed on the chemical specificity of protein attachment and on retention of protein function. Strategies for creating protein patterns (as opposed to protein arrays) are also outlined. An outlook on promising and challenging future directions for protein biochip research and applications is also offered.

Theoretical Prediction of the Hydride Affinities of Various p- and o-Quinones in DMSO

The hydride affinities of 80 various p- and o-quinones in DMSO solution were predicted by using B3LYP/6-311++G (2df,p)//B3LYP/6-31+G* and MP2/6-311++G**//B3LYP/6-31+G* methods, combined with the PCM cluster continuum model for the first time. The results show that the hydride affinity scale of the 80 quinones in DMSO ranges from -47.4 kcal/mol for 9,10-anthraquinone to -124.5 kcal/mol for 3,4,5,6-tetracyano-1,2-quinone. Such a long scale of the hydride affinities (-47.4 to -124.5 kcal/mol) indicates that the 80 quinones can form a large and useful library of organic oxidants, which can provide various organic hydride acceptors that the hydride affinities are known for chemists to choose in organic syntheses. By examining the effect of substituent on the hydride affinities of quinones, it is found that the hydride affinities of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma: DeltaGH-(Q) approximately -16.0Sigmasigmai - 70.5 (kcal/mol) for p-quinones and DeltaGH-(Q) approximately -16.2Sigmasigmai - 81.5 (kcal/mol) for o-quinones only if the substituents have no large electrostatic inductive effect and large ortho-effect. Study of the effect of the aromatic properties of quinone on the hydride affinities showed that the larger the aromatic system of quinone is, the smaller the hydride affinity of the quinone is, and the decrease of the hydride affinities is linearly to take place with the increase of the number of benzene rings in the molecule of quinones, from which the hydride affinities of aromatic quinones with multiple benzene rings can be predicted. By comparing the hydride affinities of p-quinones and the corresponding o-quinones, it is found that the hydride affinities of o-quinones are generally larger than those of the corresponding p-quinones by ca. 11 kcal/mol. Analyzing the effect of solvent on the hydride affinities of quinones showed that the effects of solvent (DMSO) on the hydride affinities of quinones are mainly dependent on the electrostatic interaction of the charged hydroquinone anions (QH-) with solvent (DMSO). All the information disclosed in this work should provide some valuable clues to chemists to choose suitable quinones or hydroquinones as efficient hydride acceptors or donors in organic syntheses and to predict the thermodynamics of hydride exchange between quinones and hydroquinones in DMSO solution.

Truncated gamma-glutamyl carboxylase in rambouillet sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality due to ineffective hemostasis at parturition. Decreased activities of coagulation factors II, VII, IX, and X, and severely reduced hepatic gamma-glutamyl carboxylase activity with adequate vitamin K 2,3 epoxide reductase activity was determined.(1,)(21) Parenteral vitamin K(1) supplementation did not improve vitamin K-dependent coagulation factor activities in 3 affected lambs. Affected lamb gamma-glutamyl carboxylase deoxyribonucleic acid was sequenced, and 4 single nucleotide polymorphisms (SNPs 2-5) of the gamma-glutamyl carboxylase gene were identified. Single nucleotide polymorphism-4 results in an arginine to stop codon (UGA) substitution, which prematurely terminates the peptide at residue 686 (R686Stop). This genotype (GATT/GATT) has a strong association with the coagulopathy observed in clinically affected lambs, P < 0.001. The frequency of SNP-3 in exon 11 (R486H) within the MARC 1.1 database is high in the US sheep population overall. Gamma-glutamyl carboxylase activity in hepatic microsomes from a SNP-3 homozygous lamb lacking the SNP-4 mutation (GACC/GACC) was similar to control sheep homozygous for arginine at 486 and also lacking SNP-4 (TGCC/TGCC), indicating that the R486H does not measurably impact gamma-glutamyl carboxylase activity. The remaining two SNPs (2 and 5) are located within non-coding intron sequences. These 4 SNPs allowed for determining the genotype associated with the observed fatal coagulopathy. Screening for the premature truncation (SNP-4) based on the presence of a Bbv I restriction site in clinically normal lambs but not in the homozygous affected lambs allows for detection of the heterozygous state (GATT/GACC), because carrier animals are clinically normal.

High-affinity chelator thiols for switchable and oriented immobilization of histidine-tagged proteins: a generic platform for protein chip technologies

Protein micro-/nanoarrays are becoming increasingly important in systematic approaches for the exploration of protein-protein interactions and dynamic protein networks, so there is a high demand for specific, generic, stable, uniform, and locally addressable protein immobilization on solid supports. Here we present multivalent metal-chelating thiols that are suitable for stable binding of histidine-tagged proteins on biocompatible self-assembled monolayers (SAMs). The architectures and physicochemical properties of these SAMs have been probed by various surface-sensitive techniques such as contact angle goniometry, ellipsometry, and infrared reflection-absorption spectroscopy. The specific molecular organization of proteins and protein complexes was demonstrated by surface plasmon resonance, confocal laser scanning, and atomic force microscopy. In contrast to the mono-NTA/His6 tag interaction, which has major drawbacks because of its low affinity and fast dissociation, drastically improved stability of protein binding by these multivalent chelator surfaces was observed. The immobilized histidine-tagged proteins are uniformly oriented and retain their function. At the same time, proteins can be removed from the chip surface under mild conditions (switchability). This new platform for switchable and oriented immobilization should assist proteome-wide wide analyses of protein-protein interactions as well as structural and single-molecule studies.

Chemical Modification of Cysteine Residues Is a Misleading Indicator of Their Status as Active Site Residues in the Vitamin K-dependent γ-Glutamyl Carboxylation Reaction

The enzymatic activity of the vitamin K-dependent proteins requires the post-translational conversion of specific glutamic acids to gamma-carboxy-glutamic acid by the integral membrane enzyme, gamma-glutamyl carboxylase. Whether or not cysteine residues are important for carboxylase activity has been the subject of a number of studies. In the present study we used carboxylase with point mutations at cysteines, chemical modification, and mass spectrometry to examine this question. Mutation of any of the free cysteine residues to alanine or serine had little effect on carboxylase activity, although C343A mutant carboxylase had only 38% activity compared with that of wild type. In contrast, treatment with either thiol-reactive reagent 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid, disodium salt, or sodium tetrathionate, caused complete loss of activity. We identified the residues modified, using matrix-assisted laser desorption/ionization time of flight mass spectrometry, as Cys(323) and Cys(343). According to our results, these residues are on the cytoplasmic side of the microsomal membrane, whereas catalytic residues are expected to be on the lumenal side of the membrane. Carboxylase was partially protected from chemical modification by factor IXs propeptide. Although all mutant carboxylases bound propeptide with normal affinity, chemical modification caused a >100-fold decrease in carboxylase affinity for the consensus propeptide. We conclude that cysteine residues are not directly involved in carboxylase catalysis, but chemical modification of Cys(323) and Cys(343) may disrupt the three-dimensional structure, resulting in inactivation.

A new model for vitamin K-dependent carboxylation: the catalytic base that deprotonates vitamin K hydroquinone is not Cys but an activated amine

Vitamin K-dependent (VKD) proteins require carboxylation for diverse functions that include hemostasis, apoptosis, and Ca(2+) homeostasis, yet the mechanism of carboxylation is not well understood. Combined biochemical and chemical studies have led to a long-standing model in which a carboxylase Cys catalytic base deprotonates vitamin K hydroquinone (KH(2)), leading to KH(2) oxygenation and Glu carboxylation. We previously identified human carboxylase Cys-99 and Cys-450 as catalytic base candidates: Both were modified by N-ethylmaleimide (NEM) and Ser-substituted mutants retained partial activity, suggesting that the catalytic base is activated for increased basicity. Mutants with Cys-99 or Cys-450 substituted by Ala, which cannot ionize to function as a catalytic base, were therefore analyzed. Both single and double mutants had activity, indicating that Cys-99 and Cys-450 do not deprotonate KH(2). [(14)C]NEM modification of C99A/C450A revealed one additional reactive group; however, Ser-substituted mutants of each of the eight remaining Cys retained substantial activity. To unequivocally test, then, whether any Cys or Cys combination acts as the catalytic base, a mutant with all 10 Cys substituted by Ala was generated. This mutant showed 7% wild-type activity that depended on factor IX coexpression, indicating a VKD protein effect on carboxylase maturation. NEM and diethyl pyrocarbonate inhibition suggested that the catalytic base is an activated His. These results change the paradigm for VKD protein carboxylation. The identity of the catalytic base is critical to understanding carboxylase mechanism and this work will therefore impact both reinterpretation of previous studies and future ones that define how this important enzyme functions.

A novel method to assay proteins in blood plasma after intravenous injection of plasmid DNA

Gene therapy is expected to lead to new and useful methods to treat diseases. The development of assays to quantitate gene-therapy-derived proteins circulating in blood will be essential to investigate the effects and side effects of the introduced proteins. The purpose of this study is to evaluate whether a protein circulating at trace concentrations in blood can be measured by tagging a peptide corresponding to glucagon residues 19-29 onto its C-terminal end. We constructed plasmids encoding chimeric proteins and transferred them into rats by hydrodynamics-based delivery. When plasmids encoding human IL8-glucagon 19-29 chimeric protein were injected into rats to evaluate the accuracy of this method, there was a high correlation between chimeric proteins measured by an enzyme-linked immunosorbent assay for human IL8 and one by a radioimmunoassay for glucagon. Furthermore, when plasmids coding rat IFN gamma receptor IgG-Fc glucagon 19-29 chimeric protein were injected to evaluate the time course of chimeric proteins in blood plasma, we could calculate the concentrations in blood from 10 microl plasma samples using glucagon 19-29 tag as follows: 2815+/-2318 ng/ml after 4 hours (mean+/-s.D.), 6061+/-2789 ng/ml after 8 hours, 5752+/-2270 ng/ml after 12 hours, 2870+/-1062 ng/ml after one day, 1440+/-334 ng/ml after three days, 1120+/-433 ng/ml after seven days, and 281+/-162 ng/ml after 16 days. Blood sugar levels which might have been increased by glucagon did not increase even at peak chime- ric protein concentrations. These results demonstrate a useful and convenient method to assay gene therapy products circulating in blood using a glucagon 19-29 tagging vector.

Fabrication of histidine-tagged fusion protein arrays for surface plasmon resonance imaging studies of protein-protein and protein-DNA interactions

The creation and characterization of histidine-tagged fusion protein arrays using nitrilotriacetic acid (NTA) capture probes on gold thin films for the study of protein-protein and protein-DNA interactions is described. Self-assembled monolayers of 11-mercaptoundecylamine were reacted with the heterobifunctional linker N-succinimidyl S-acetylthiopropionate (SATP) to create reactive sulfhydryl-terminated surfaces. NTA capture agents were immobilized by reacting maleimide-NTA molecules with the sulfhydryl surface. The SATP and NTA attachment chemistry was confirmed with Fourier transform infrared reflection absorption spectroscopy. Oriented protein arrays were fabricated using a two-step process: (i) patterned NTA monolayers were first formed through a single serpentine poly(dimethylsiloxane) microchannel; (ii) a second set of parallel microchannels was then used to immobilize multiple His-tagged proteins onto this pattern at discrete locations. SPR imaging measurements were employed to characterize the immobilization and specificity of His-tagged fusion proteins to the NTA surface. SPR imaging measurements were also used with the His-tagged fusion protein arrays to study multiple antibody-antigen binding interactions and to monitor the sequence-specific interaction of double-stranded DNA with TATA box-binding protein. In addition, His-tagged fusion protein arrays created on gold surfaces were also used to monitor antibody binding with fluorescence microscopy in a sandwich assay format.

Determination of disulfide bond assignment of human vitamin K-dependent gamma-glutamyl carboxylase by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Vitamin K-dependent gamma-glutamyl carboxylase is a 758 amino acid integral membrane glycoprotein that catalyzes the post-translational conversion of certain protein glutamate residues to gamma-carboxyglutamate. Carboxylase has ten cysteine residues, but their form (sulfhydryl or disulfide) is largely unknown. Pudota et al. in Pudota, B. N., Miyagi, M., Hallgren, K. W., West, K. A., Crabb, J. W., Misono, K. S., and Berkner, K. L. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 13033-13038 reported that Cys-99 and Cys-450 are the carboxylase active site residues. We determined the form of all cysteines in carboxylase using in-gel protease digestion and matrix-assisted laser desorption/ionization mass spectrometry. The spectrum of non-reduced, trypsin-digested carboxylase revealed a peak at m/z 1991.9. Only this peak disappeared in the spectrum of the reduced sample. This peak's m/z is consistent with the mass of peptide 92-100 (Cys-99) disulfide-linked with peptide 446-453 (Cys-450). To confirm its identity, the m/z 1991.9 peak was isolated by a timed ion selector as the precursor ion for further MS analysis. The fragmentation pattern exhibited two groups of triplet ions characteristic of the symmetric and asymmetric cleavage of disulfide-linked tryptic peptides containing Cys-99 and Cys-450. Mutation of either Cys-99 or Cys-450 caused loss of enzymatic activity. We created a carboxylase variant with both C598A and C700A, leaving Cys-450 as the only remaining cysteine residue in the 60-kDa fragment created by limited trypsin digestion. Analysis of this fully active mutant enzyme showed a 30- and the 60-kDa fragment were joined under non-reducing conditions, thus confirming Cys-450 participates in a disulfide bond. Our results indicate that Cys-99 and Cys-450 form the only disulfide bond in carboxylase.

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.

Characterization and optimization of peptide arrays for the study of epitope-antibody interactions using surface plasmon resonance imaging

The characterization of peptide arrays on gold surfaces designed for the study of peptide-antibody interactions using surface plasmon resonance (SPR) imaging is described. A two-step process was used to prepare the peptide arrays: (i) a set of parallel microchannels was used to deliver chemical reagents to covalently attach peptide probes to the surface by a thiol-disulfide exchange reaction; (ii) a second microchannel with a wraparound design was used as a small-volume flow cell (5 microL) to introduce antibody solutions to the peptide surface. As a demonstration, the interactions of the FLAG epitope tag and monoclonal anti-FLAG M2 were monitored by SPR imaging using a peptide array. This peptide-antibody pair was studied because of its importance as a means to purify fusion proteins. The surface coverage of the FLAG peptide was precisely controlled by creating the peptide arrays on mixed monolayers of alkanethiols containing an amine-terminated surface and an inert alkanethiol. The mole fraction of peptide epitopes was also controlled by reacting solutions containing FLAG peptide and the non-interacting peptide HA or cysteine. By studying variants based on the FLAG binding motif, it was possible to distinguish peptides differing by a single amino acid substitution using SPR imaging. In addition, quantitative analysis of the signal was accomplished using the peptide array to simultaneously determine the binding constants of the antibody-peptide interactions for four peptides. The binding constant, K(ads), for the FLAG peptide was measured and found to be 1.5 x 10(8) M(-1) while variants made by the substitution of alanine for residues based on the binding motif had binding constants of 2.8 x 10(7), 5.0 x 10(6), and 2.0 x 10(6) M(-1).

Expression and regulation of thioredoxin reductases and other selenoproteins in bone

The expression of thioredoxin reductases and other selenoproteins in cells of the bone microenvironment may represent an important means of regulation of bone resorption and remodeling in health and disease. Selenoproteins and their substrates may influence intracellular and extracellular redox-dependent signaling, transcription factor activity, posttranslational modification of proteins, and general or compartmentalized scavenging from ROIs. However, the evaluation of their biological role in bone and their potential in terms of therapeutic approaches is just beginning.

Identification of the vitamin K-dependent carboxylase active site: Cys-99 and Cys-450 are required for both epoxidation and carboxylation

The vitamin K-dependent carboxylase modifies and renders active vitamin K-dependent proteins involved in hemostasis, cell growth control, and calcium homeostasis. Using a novel mechanism, the carboxylase transduces the free energy of vitamin K hydroquinone (KH(2)) oxygenation to convert glutamate into a carbanion intermediate, which subsequently attacks CO(2), generating the gamma-carboxylated glutamate product. How the carboxylase effects this conversion is poorly understood because the active site has not been identified. Dowd and colleagues [Dowd, P., Hershline, R., Ham, S. W. & Naganathan, S. (1995) Science 269, 1684-1691] have proposed that a weak base (cysteine) produces a strong base (oxygenated KH(2)) capable of generating the carbanion. To define the active site and test this model, we identified the amino acids that participate in these reactions. N-ethyl maleimide inhibited epoxidation and carboxylation, and both activities were equally protected by KH(2) preincubation. Amino acid analysis of (14)C- N-ethyl maleimide-modified human carboxylase revealed 1.8-2.3 reactive residues and a specific activity of 7 x 10(8) cpm/hr per mg. Tryptic digestion and liquid chromatography electrospray mass spectrometry identified Cys-99 and Cys-450 as active site residues. Mutation to serine reduced both epoxidation and carboxylation, to 0. 2% (Cys-99) or 1% (Cys-450), and increased the K(m)s for a glutamyl substrate 6- to 8-fold. Retention of some activity indicates a mechanism for enhancing cysteine/serine nucleophilicity, a property shared by many active site thiol enzymes. These studies, which represent a breakthrough in defining the carboxylase active site, suggest a revised model in which the glutamyl substrate indirectly coordinates at least one thiol, forming a catalytic complex that ionizes a thiol to initiate KH(2) oxygenation.