Enhancing protein C interaction with thrombin results in a clot-activated anticoagulant

Thrombin has dual trypsin-like and chymotrypsin-like specificity

BACKGROUND

The residue at the site of activation of protein C is Arg in all species except the ray-finned fish, where it is Trp. This feature raises the question of whether thrombin is the physiological activator of protein C across vertebrates.

OBJECTIVES

To establish if thrombin can cleave at Trp residues.

METHODS

The activity of wild-type thrombin and mutant D189S was tested with a library of chromogenic substrates and toward wild-type protein C and mutants carrying substitutions at the site of cleavage.

RESULTS

Thrombin has trypsin-like and chymotrypsin-like specificity and cleaves substrates at Arg or Trp residues. Cleavage at Arg is preferred, but cleavage at Trp is significant and comparable with that of chymotrypsin. The D189S mutant of thrombin has broad specificity and cleaves at basic and aromatic residues without significant preference. Thrombin also cleaves natural substrates at Arg or Trp residues, showing activity toward protein C across vertebrates, including the ray-finned fish. The rate of activation of protein C in the ray-finned fish is affected by the sequence preceding Trp at the scissile bond.

CONCLUSION

The results provide a possible solution for the paradoxical presence of a Trp residue at the site of cleavage of protein C in ray-finned fish and support thrombin as the physiological activator of protein C in all vertebrates. The dual trypsin-like and chymotrypsin-like specificity of thrombin suggests that the spectrum of physiological substrates of this enzyme is broader currently assumed.

Zymogen and activated protein C have similar structural architecture

Activated protein C is a trypsin-like protease with anticoagulant and cytoprotective properties that is generated by thrombin from the zymogen precursor protein C in a reaction greatly accelerated by the cofactor thrombomodulin. The molecular details of this activation remain elusive due to the lack of structural information. We now fill this gap by providing information on the overall structural organization of these proteins using single molecule FRET and small angle X-ray scattering. Under physiological conditions, both zymogen and protease adopt a conformation with all domains vertically aligned along an axis 76 Å long and maximal particle size of 120 Å. This conformation is stabilized by binding of Ca2+ to the Gla domain and is affected minimally by interaction with thrombin. Hence, the zymogen protein C likely interacts with the thrombin-thrombomodulin complex through a rigid body association that produces a protease with essentially the same structural architecture. This scenario stands in contrast to an analogous reaction in the coagulation cascade where conversion of the zymogen prothrombin to the protease meizothrombin by the prothrombinase complex is linked to a large conformational transition of the entire protein. The presence of rigid epidermal growth factor domains in protein C as opposed to kringles in prothrombin likely accounts for the different conformational plasticity of the two zymogens. The new structural features reported here for protein C have general relevance to vitamin K-dependent clotting factors containing epidermal growth factor domains, such as factors VII, IX, and X.

Role of the activation peptide in the mechanism of protein C activation

Protein C is a natural anticoagulant activated by thrombin in a reaction accelerated by the cofactor thrombomodulin. The zymogen to protease conversion of protein C involves removal of a short activation peptide that, relative to the analogous sequence present in other vitamin K-dependent proteins, contains a disproportionately high number of acidic residues. Through a combination of bioinformatic, mutagenesis and kinetic approaches we demonstrate that the peculiar clustering of acidic residues increases the intrinsic disorder propensity of the activation peptide and adversely affects the rate of activation. Charge neutralization of the acidic residues in the activation peptide through Ala mutagenesis results in a mutant activated by thrombin significantly faster than wild type. Importantly, the mutant is also activated effectively by other coagulation factors, suggesting that the acidic cluster serves a protective role against unwanted proteolysis by endogenous proteases. We have also identified an important H-bond between residues T176 and Y226 that is critical to transduce the inhibitory effect of Ca2+ and the stimulatory effect of thrombomodulin on the rate of zymogen activation. These findings offer new insights on the role of the activation peptide in the function of protein C.

Liberating R169 promotes anticoagulant protein C

In this issue of Blood, Pozzi et al demonstrate that removing an anionic cage promotes exposure of R169 thereby generating a protein C (PC) that is far more readily activated.

Exposure of R169 controls protein C activation and autoactivation

Protein C is activated by thrombin with a value of k(cat)/K(m) = 0.11mM(-1)s(-1) that increases 1700-fold in the presence of the cofactor thrombomodulin. The molecular origin of this effect triggering an important feedback loop in the coagulation cascade remains elusive. Acidic residues in the activation domain of protein C are thought to electrostatically clash with the active site of thrombin. However, functional and structural data reported here support an alternative scenario. The thrombin precursor prethrombin-2 has R15 at the site of activation in ionic interaction with E14e, D14l, and E18, instead of being exposed to solvent for proteolytic attack. Residues E160, D167, and D172 around the site of activation at R169 of protein C occupy the same positions as E14e, D14l, and E18 in prethrombin-2. Caging of R169 by E160, D167, and D172 is responsible for much of the poor activity of thrombin toward protein C. The E160A/D167A/D172A mutant is activated by thrombin 63-fold faster than wild-type in the absence of thrombomodulin and, over a slower time scale, spontaneously converts to activated protein C. These findings establish a new paradigm for cofactor-assisted reactions in the coagulation cascade.

Crystal structure of thrombin bound to the uncleaved extracellular fragment of PAR1

Abundant structural information exists on how thrombin recognizes ligands at the active site or at exosites separate from the active site region, but remarkably little is known about how thrombin recognizes substrates that bridge both the active site and exosite I. The case of the protease-activated receptor PAR1 is particularly relevant in view of the plethora of biological effects associated with its activation by thrombin. Here, we present the 1.8 A resolution structure of thrombin S195A in complex with a 30-residue long uncleaved extracellular fragment of PAR1 that documents for the first time a productive binding mode bridging the active site and exosite I. The structure reveals two unexpected features of the thrombin-PAR1 interaction. The acidic P3 residue of PAR1, Asp(39), does not hinder binding to the active site and actually makes favorable interactions with Gly(219) of thrombin. The tethered ligand domain shows a considerable degree of disorder even when bound to thrombin. The results fill a significant gap in our understanding of the molecular mechanisms of recognition by thrombin in ways that are relevant to other physiological substrates.

Modulation of sepsis outcome with variants of activated protein C

Activated protein C (aPC) is the key effector protease of the natural protein C anticoagulant pathway and exerts anticoagulant, as well as anti-inflammatory activity. This dual mode of action has been thought to underlie the therapeutic efficacy of recombinant aPC in the treatment of patients suffering from severe forms of sepsis. The development and characterization of recombinant variants of aPC with altered bioactivity profiles has generated an opportunity to test this concept by dissecting the roles of aPC's anticoagulant and cell-signaling functions in the treatment of sepsis. Animal studies suggest that aPC variants with near-normal signaling function, but with greatly diminished anticoagulant potential may exhibit a substantially improved risk-to-benefit ratio in sepsis therapy.

Activated protein C in sepsis: the promise of nonanticoagulant activated protein C

PURPOSE OF REVIEW

To discuss the potential use of recombinant activated protein C (aPC) variants with altered bioactivity in sepsis therapy.

RECENT FINDINGS

Since the initial Protein C Worldwide Evaluation in Severe Sepsis trial demonstrating efficacy of aPC therapy to reduce mortality of severe sepsis, follow-up studies have failed to resolve concerns about the low overall risk-to-benefit ratio of this therapy and suggest that it might only be effective in severely ill patients with the most aggravated forms of coagulopathy. New studies begin to shed light on the potential mechanisms of how aPC therapy may alter sepsis outcome, and how recombinant aPC variants with altered bioactivities may improve the efficacy and safety of this therapy.

SUMMARY

aPC variants with selectively diminished antithrombotic activity, but normal cytoprotective potential, may allow more efficient dosing without increasing adverse bleeding effects and therefore provide a safer and possibly more efficient alternative to normal aPC. Critical questions about the precise mechanisms by which aPC therapy reduces mortality remain to be resolved in order to identify patients most likely to benefit from it and to reevaluate potential efficacy of aPC therapy in children and patients with less than severe sepsis.

Thrombin

Thrombin is a Na+-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na+ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. The anticoagulant function of thrombin is under the allosteric control of the cofactor thrombomodulin. Much has been learned on the mechanism of Na+ binding and recognition of natural substrates by thrombin. Recent structural advances have shed light on the remarkable molecular plasticity of this enzyme and the molecular underpinnings of thrombin allostery mediated by binding to exosite I and the Na+ site. This review summarizes our current understanding of the molecular basis of thrombin function and allosteric regulation. The basic information emerging from recent structural, mutagenesis and kinetic investigation of this important enzyme is that thrombin exists in three forms, E*, E and E:Na+, that interconvert under the influence of ligand binding to distinct domains. The transition between the Na+ -free slow from E and the Na+ -bound fast form E:Na+ involves the structure of the enzyme as a whole, and so does the interconversion between the two Na+ -free forms E* and E. E* is most likely an inactive form of thrombin, unable to interact with Na + and substrate. The complexity of thrombin function and regulation has gained this enzyme pre-eminence as the prototypic allosteric serine protease. Thrombin is now looked upon as a model system for the quantitative analysis of biologically important enzymes.

Activated protein C protects against diabetic nephropathy by inhibiting endothelial and podocyte apoptosis

Data providing direct evidence for a causative link between endothelial dysfunction, microvascular disease and diabetic end-organ damage are scarce. Here we show that activated protein C (APC) formation, which is regulated by endothelial thrombomodulin, is reduced in diabetic mice and causally linked to nephropathy. Thrombomodulin-dependent APC formation mediates cytoprotection in diabetic nephropathy by inhibiting glomerular apoptosis. APC prevents glucose-induced apoptosis in endothelial cells and podocytes, the cellular components of the glomerular filtration barrier. APC modulates the mitochondrial apoptosis pathway via the protease-activated receptor PAR-1 and the endothelial protein C receptor EPCR in glucose-stressed cells. These experiments establish a new pathway, in which hyperglycemia impairs endothelial thrombomodulin-dependent APC formation. Loss of thrombomodulin-dependent APC formation interrupts cross-talk between the vascular compartment and podocytes, causing glomerular apoptosis and diabetic nephropathy. Conversely, maintaining high APC levels during long-term diabetes protects against diabetic nephropathy.

Thrombomodulin changes the molecular surface of interaction and the rate of complex formation between thrombin and protein C

The interaction of thrombin with protein C triggers a key down-regulatory process of the coagulation cascade. Using a panel of 77 Ala mutants, we have mapped the epitope of thrombin recognizing protein C in the absence or presence of the cofactor thrombomodulin. Residues around the Na(+) site (Thr-172, Lys-224, Tyr-225, and Gly-226), the aryl binding site (Tyr-60a), the primary specificity pocket (Asp-189), and the oxyanion hole (Gly-193) hold most of the favorable contributions to protein C recognition by thrombin, whereas a patch of residues in the 30-loop (Arg-35 and Pro-37) and 60-loop (Phe-60h) regions produces unfavorable contributions to binding. The shape of the epitope changes drastically in the presence of thrombomodulin. The unfavorable contributions to binding disappear and the number of residues promoting the thrombin-protein C interaction is reduced to Tyr-60a and Asp-189. Kinetic studies of protein C activation as a function of temperature reveal that thrombomodulin increases >1,000-fold the rate of diffusion of protein C into the thrombin active site and lowers the activation barrier for this process by 4 kcal/mol. We propose that the mechanism of thrombomodulin action is to kinetically facilitate the productive encounter of thrombin and protein C and to allosterically change the conformation of the activation peptide of protein C for optimal presentation to the thrombin active site.

Regulation of blood coagulation

The protein C anticoagulant pathway converts the coagulation signal generated by thrombin into an anticoagulant response through the activation of protein C by the thrombin-thrombomodulin (TM) complex. The activated protein C (APC) thus formed interacts with protein S to inactivate two critical coagulation cofactors, factors Va and VIIIa, thereby dampening further thrombin generation. The proposed mechanisms by which TM switches the specificity of thrombin include conformational changes in thrombin, blocking access of normal substrates to thrombin and providing a binding site for protein C. The function of protein S appears to be to alter the cleavage site preferences of APC in factor Va, probably by changing the distance of the active site of APC relative to the membrane surface. The clinical relevance of this pathway is now established through the identification of deficient individuals with severe thrombotic complications and through the analysis of families with partial deficiencies in these components and an increased thrombotic tendency. One possible reason that even partial deficiencies are a thrombotic risk is that the function of the pathway can be down-regulated by inflammatory mediators. For instance, clinical studies have shown that the extent to which protein C levels decrease in patients with septic shock is predictive of a negative outcome. Initial clinical studies suggest that supplementation with protein C may be useful in the treatment of acute inflammatory diseases such as sepsis.

Vascular protease receptors: integrating haemostasis and endothelial cell functions

The endothelium plays a crucial dynamic role as a protective interface between blood and the underlying tissues during the haemostatic process, which maintains blood flow in the circulation and prevents life-threatening blood loss. Following vessel wall injury with initial platelet adhesion and aggregation to exposed subendothelial extracellular matrix, the initiation, amplification, and control of haemostasis depend on structurally unrelated membrane-associated receptors for blood coagulation proteases including tissue factor, G-protein-coupled protease-activatable receptors, thrombomodulin, and protein C receptor, respectively. In addition to their regulatory role in haemostasis, the respective (pro-)enzyme ligands such as Factors VIIa and Xa, thrombin or protein C mediate specific signalling pathways in vascular cells related to migration, proliferation or adhesion. The functional importance of these receptors beyond haemostasis has been manifested by various lethal and pathological phenotypes in knock-out mice. These protease receptors thereby provide important molecular links in the vascular system and serve to integrate haemostasis with endothelial cell functions which are relevant for the (patho-)physiological responses to injury or inflammatory challenges.

Transgenic animal bioreactors in biotechnology and production of blood proteins

The regulatory elements of genes used to target the tissue-specific expression of heterologous human proteins have been studied in vitro and in transgenic mice. Hybrid genes exhibiting the desired performance have been introduced into large animals. Complex proteins like protein C, factor IX, factor VIII, fibrinogen and hemoglobin, in addition to simpler proteins like alpha 1-antitrypsin, antithrombin III, albumin and tissue plasminogen activator have been produced in transgenic livestock. The amount of functional protein secreted when the transgene is expressed at high levels may be limited by the required posttranslational modifications in host tissues. This can be overcome by engineering the transgenic bioreactor to express the appropriate modifying enzymes. Genetically engineered livestock are thus rapidly becoming a choice for the production of recombinant human blood proteins.

Allosteric modulation of the activity of thrombin

Substrates containing a P3 aspartic residue are in general cleaved poorly by thrombin. This may be partly due to an unfavourable interaction between the P3 aspartate and Glu192 in the active site of thrombin. In Protein C activation and perhaps also thrombin receptor cleavage, binding of ligands at the anion-binding exosite of thrombin seems to improve the activity of thrombin with substrates containing a P3 aspartate. To investigate the importance of Glu192 and exosite-binding in modulating thrombin's interactions with a P3 aspartate, peptidyl chloromethanes based on the sequence of the thrombin receptor (containing a P3 aspartate) have been synthesized and the kinetics of their inactivation of alpha-thrombin and the mutant Glu192-->Gln determined. The values of the inactivation rate constant (ki) for the chloromethanes containing a P3 aspartate were about two-fold higher with the Glu192-->Gln mutant. A peptide based on the sequence of hirudin (rhir52 65), which binds to the anion-binding exosite of thrombin, was an allosteric modulator of the amidolytic activity of the Glu192-->Gln mutant; a 5-fold decrease in the K(m) value for the substrate D-Phe-pipecolyl-Arg-p-nitroanilide was observed in the presence of saturating concentrations of rhir52-65. This exosite-binding peptide also increased the ki values of chloromethanes containing a P3 aspartate with both alpha-thrombin and the Glu192-->Gln mutant. However, the increases in the ki values were greater with the Glu192-->Gln mutant (5-fold compared with 2-fold for alpha-thrombin). Thus exosite binding does not seem to mitigate putative unfavourable interactions between Glu192 and the P3 aspartate. Moreover, increases in the ki caused by exosite binding were not unique to chloromethanes containing a P3 aspartate; increases of the same magnitude were also observed when the P3 position was occupied by the favourable D-phenylalanine in place of the unfavourable aspartate. The results obtained were consistent with exosite binding's causing changes in the conformation of the S2 and/or S1 site of thrombin.

Antithrombotic Efficacy in the Guinea Pig of a Derivative of Human Protein C With Enhanced Activation by Thrombin

Conversion by α-thrombin of the zymogen human protein C (HPC) to activated protein C (aPC) is an important physiologic feedback control mechanism for the coagulation cascade. Although activation of HPC by thrombomodulin-bound thrombin is relatively rapid, activation by free thrombin occurs at a significantly slower rate. Previously, we generated a “hyper-activatable” derivative of HPC (FLIN-Q3) with an increased activation rate by free α-thrombin in vitro. In this study, the antithrombotic efficacy of FLIN-Q3 was compared with both native zymogen and aPC in an arteriovenous shunt model of thrombosis in the guinea pig. Recombinant proteins were infused 15 minutes before and throughout a 15-minute period while blood was circulated from carotid to jugular through tubing that enclosed a thread on which fibrin was deposited. Parallel dose-dependent antithrombotic responses were observed. Under these non–steady-state conditions, the calculated infusion doses associated with a 50% reduction of thrombus mass were 2.7, 24, and 250 mg/kg/h for aPC, FLIN-Q3, and HPC, respectively. Thrombus weight correlated inversely with plasma concentration of aPC, measured amidolytically, from either direct infusion of aPC or that generated from the zymogens in the animal, and similarly correlated inversely with anticoagulant activity measured by whole blood aPTT. Neither zymogen form showed significant aPC activity before shunt circulation, suggesting a requirement for exposure to thrombin. After the infusion was discontinued for 15 minutes, a second period of thrombus formation in the shunt demonstrated the ability of zymogen forms of PC, unlike aPC, to provide “on-demand” anticoagulant responses to repeated thrombotic stimuli. Thus, a “hyper-activatable” PC molecule such as FLIN-Q3 may represent a superior form of anticoagulant therapy than either the native zymogen or aPC.

Nonenzymatic anticoagulant activity of the mutant serine protease Ser360Ala-activated protein C mediated by factor Va

The human plasma serine protease, activated protein C (APC), primarily exerts its anticoagulant function by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. A recombinant active site Ser 360 to Ala mutation of protein C was prepared, and the mutant protein was expressed in human 293 kidney cells and purified. The activation peptide of the mutant protein C zymogen was cleaved by a snake venom activator, Protac C, but the "activated" S360A APC did not have amidolytic activity. However, it did exhibit significant anticoagulant activity both in clotting assays and in a purified protein assay system that measured prothrombinase activity. The S360A APC was compared to plasma-derived and wild-type recombinant APC. The anticoagulant activity of the mutant, but not native APC, was resistant to diisopropyl fluorophosphate, whereas all APCs were inhibited by monoclonal antibodies against APC. In contrast to native APC, S360A APC was not inactivated by serine protease inhibitors in plasma and did not bind to the highly reactive mutant protease inhibitor M358R alpha 1 antitrypsin. Since plasma serpins provide the major mechanism for inactivating APC in vivo, this suggests that S360A APC would have a long half-life in vivo, with potential therapeutic advantages. S360A APC rapidly inhibited factor Va in a nonenzymatic manner since it apparently did not proteolyze factor Va. These data suggest that native APC may exhibit rapid nonenzymatic anticoagulant activity followed by enzymatic irreversible proteolysis of factor Va. The results of clotting assays and prothrombinase assays showed that S360A APC could not inhibit the variant Gln 506-FVa compared with normal Arg 506-FVa, suggesting that the active site of S360A APC binds to FVa at or near Arg 506.

Mutation of protease domain residues Lys37-39 in human protein C inhibits activation by the thrombomodulin-thrombin complex without affecting activation by free thrombin

Activated protein C (aPC) is an important feedback regulator of the clotting cascade. In vivo, the conversion of protein C (PC) from its zymogen to activated form is mediated primarily by thrombin bound to thrombomodulin (TM), an endothelial cell surface protein. Molecular modeling suggests residues Lys37-Lys38-Lys39 of protein C's serine protease domain reside in a surface-exposed loop (variable region 1) whose high concentration of positive charge might be involved in protein-protein interactions. In this study, we have examined the role of the conserved tribasic Lys37-39 charge center in human protein C activation. This sequence was changed to acidic by substitution with Asp37-Glu38-Asp39 (DED) and Glu37-Glu38-Glu39 (EEE), or to neutrality by substitution with Gly37-Gly38-Gly39 (GGG). These mutant PCs, expressed and purified from recombinant human 293 cells, appeared normal with regard to intracellular processing, ability to be secreted, and formation of a viable active site for tripeptidyl-p-nitroanilide substrate cleavage. For activation by free thrombin, wild-type (wt) and mutant PCs displayed equivalent activation rates, as well as identical calcium-dependent inhibition of such activation. Activation of wt-PC with a soluble TM-thrombin complex yielded a 2,000-fold faster rate compared with that by free thrombin at the same (physiological) calcium level. In contrast, the acidic mutants DED and EEE exhibited virtually no TM-mediated increase in activation rate, while the neutral mutant GGG was somewhat intermediate with a 30-fold stimulation of activation rate. These reductions in activation rate were independent of the presence of chondroitin sulfate on TM. Our observations represent the first identification of residues whose mutation essentially uncouples activation by the TM-thrombin complex without affecting activation by free thrombin. Further, our results suggest that VR1 residues within the zymogen form of a serine protease can be important for recognition by physiological activators.

Enhanced protein C activation and inhibition of fibrinogen cleavage by a thrombin modulator

A modulator of the enzymatic activity of human thrombin, designated LY254603, was identified that enhances the thrombin-catalyzed generation of the anticoagulant factor activated protein C, yet inhibits thrombin-dependent fibrinogen clotting. By means of mutant substrates, it was shown that LY254603 mediates the change in enzymatic substrate specificity through an alteration in thrombin's S3 substrate recognition site, a mechanism that appeared to be independent of allosteric changes induced by either sodium ions or by thrombomodulin. This compound may represent the prototype of a class of agents that specifically modulates the balance between thrombin's procoagulant and anticoagulant functions.

Bioproduction of human enzymes in transgenic tobacco

Transgenic plants have significant potential in the bioproduction of complex human therapeutic proteins due to ease of genetic manipulation, lack of potential contamination with human pathogens, conservation of eukaryotic cell machinery mediating protein modification, and low cost of biomass production. Tobacco has been used as our initial transgenic system because Agrobacterium-mediated transformation is highly efficient, prolific seed production greatly facilitates biomass scale-up, and development of new "health-positive" uses for tobacco has significant regional support. We have targeted bioproduction of complex recombinant human proteins with commercial potential as human pharmaceuticals. Human protein C (hPC), a highly processed serum protease of the coagulation/anticoagulation cascade, was produced at low levels in transgenic tobacco leaves. Analogous to its processing in mammalian systems, tobacco-synthesized hPC appears to undergo multiple proteolytic cleavages, disulfide bond formation, and N-linked glycosylation. Although tobacco-derived hPC has not yet been tested for all posttranslational modifications or for enzymatic (anticlotting) activity, these results are promising and suggest considerable conservation of protein processing machinery between plants and animals. CropTech researchers have also produced the human lysosomal enzyme glucocerebrosidase (hGC) in transgenic tobacco. This glycoprotein has significant commercial potential as replacement therapy in patients with Gaucher's disease. Regular intravenous administration of modified glucocerebrosidase, derived from human placentae or CHO cells, has proven highly effective in reducing disease manifestations in patients with Gaucher's disease. However, the enzyme is expensive (dubbed the "world's most expensive drug" by the media), making it a dramatic model for evaluating the potential of plants to provide a safe, low-cost source of bioactive human enzymes. Transgenic tobacco plants were generated that contained the human glucocerebrosidase cDNA under the control of an inducible plant promoter. hGC expression was demonstrated in plant extracts by enzyme activity assay and immunologic cross-reactivity with anti-hGC antibodies. Tobacco-synthesized hGC comigrates with human placental-derived hGC during electrophoretic separations, is glycosylated, and, most significantly, is enzymatically active. Although expression levels vary depending on transformant and induction protocol, hGC production of > 1 mg/g fresh weight of leaf tissue has been attained in crude extracts. Our studies provide strong support for the utilization of tobacco for high-level production of active hGC for purification and eventual therapeutic use at potentially much reduced costs. Furthermore, this technology should be directly adaptable to the production of a variety of other complex human proteins of biologic and pharmaceutical interest.

Antithrombotic effects of activated protein C and protein S in a rabbit model of microarterial thrombosis

The antithrombotic properties of activated protein C (APC) and protein S were investigated in a rabbit model of microarterial thrombosis. The study focused on the ability of intact and thrombin-cleaved bovine protein S to potentiate the biological effects of bovine APC in vivo. Segments of the central arteries of the ears were subjected to arteriotomy, deep-vessel wall trauma, and arteriotomy suture. Five minutes before vascular reperfusion, groups of rabbits were infused with boluses of 0.1 mg/kg bovine APC alone or combined with different doses of intact (0.5, 0.1, or 0.05 mg/kg) or thrombin-cleaved (0.5 mg/kg) protein S. APC in combination with the two higher doses of protein S produced a potent antithrombotic response, as judged by assessment of vessel patency rates, while only the group receiving APC+0.5 mg/kg protein S showed significant reduction of thrombus weights as well. The biological effect depended on the active site in APC, as the antithrombotic effect was lost on pretreatment of APC with the serine protease inhibitor D-phenylalanyl-L-prolyl-L-arginine chloromethylketone. The potentiation of the APC response by protein S depended on the structural integrity of the protein, and cleavage of the thrombin-sensitive region in protein S by thrombin resulted in a loss of biological response. No hemorrhagic side effects were noted by the APC-protein S combination, and the anticoagulant response was mild, even to the highest doses of APC and protein S.(ABSTRACT TRUNCATED AT 250 WORDS)

Human thrombin variable region 1, including E39, is involved in interactions with alpha 1-antitrypsin M358R and protein C

We used an antithrombin autoantibody (IgG D), the epitope of which encompasses ABE1 and amino acids located within variable region 1, to study thrombin interactions with R358 alpha 1-AT and protein C. IgG D inhibited the thrombin interaction with R358 alpha 1-AT, while hirugen had no effect, indicating that the interaction of R358 alpha 1-AT with thrombin may involve the VR1 subsite. We also obtained evidence that VR1 may be involved in the activation of protein C by thrombin in the absence of thrombomodulin. Moreover, IgG D attenuated the inhibitory effect of calcium ions during protein C activation by thrombin, probably by masking E39 within the VR1 site.

Identification of a region in protein C involved in thrombomodulin-stimulated activation by thrombin: potential repulsion at anion-binding site I in thrombin

During coagulation human protein C is activated by thrombin; however, this cleavage reaction is slow unless thrombin is complexed with a cofactor, thrombomodulin. Near the thrombin cleavage site in protein C is a cluster of basic residues, at positions P5' (Lys-174), P8' (Arg-177) and P9' (Arg-178). We have explored the role of this basic cluster in the activation of protein C by thrombin, and by thrombin-thrombomodulin complex, by substitution of glutamic acid at each position to generate the acidic protein C derivative P'-EEE. The activation rate of P'-EEE by free alpha-thrombin was approx. 12-fold faster than that observed for wild-type (wt) human protein C zymogen (HPC) in the presence of calcium, but unchanged in the absence of calcium. While the thrombin-catalysed activation of wt-HPC was stimulated approx. 300-fold by thrombomodulin, we observed no effect of thrombomodulin on thrombin-catalysed activation of the P'-EEE derivative. Using synthetic peptides that bind to anion-binding site I of thrombin (thrombin-receptor sequence 52-66 and hirudin sequence 54-65 SO4 Tyr), we found that the rate of thrombin-catalysed activation of wt-HPC in the presence of calcium could be increased severalfold in a dose-dependent manner. However, the enhanced rate of thrombin-catalysed activation of P'-EEE could be progressively reduced to wt-HPC levels with increasing concentrations of both synthetic peptides. Our data suggest that the P' basic cluster in protein C reduces interaction with free alpha-thrombin through electrostatic repulsion with anion-binding site I, a site that is masked when thrombomodulin binds thrombin. Further, the lack of thrombomodulin cofactor activity with thrombin-catalysed activation of P'-EEE suggests that the basic cluster in protein C forms a contact site with thrombomodulin.

Calcium inhibition of the activation of protein C by thrombin. Role of the P3 and P3' residues

Protein C, a precursor to a natural plasma anticoagulant, and the platelet thrombin receptor, involved in cell activation, both require proteolytic cleavage to be activated. In humans, the sequences adjacent to the scissile bond of protein C, DPR/LID and the thrombin receptor DPR/SFL are similar. Previous studies with Asp-->Gly mutants indicated that both the P3 and the P3' Asp residues make either peptides or protein C a poor substrate for free thrombin, but thrombin interaction with thrombomodulin overcomes these inhibitory interactions. Similar mechanisms are probably operative in the thrombin receptor. In rodents, the P3 Asp residue of the human thrombin receptor is replaced by Asn and in protein C, the P3' residue is Asn. To determine the functional significance of these Asp-->Asn substitutions, the Asp in the P3 or P3' position of human protein C was changed to Asn. The resultant mutants were still resistant to activation by thrombin, and still required Ca2+ for activation by thrombin-thrombomodulin complex. We conclude that, unlike activation of the Asp-->Gly mutants by thrombin, activation of the P3 and P3' Asp-->Asn mutants is still potently inhibited by physiological Ca2+. Furthermore, even though the charge has been deleted, thrombomodulin acceleration is retained.

Structural basis for type I and type II deficiencies of antithrombotic plasma protein C: patterns revealed by three-dimensional molecular modelling of mutations of the protease domain

Familial deficiency of protein C is associated with inherited thrombophilia. To explore how specific missense mutations might cause observed clinical phenotypes, know protein C missense mutations were mapped onto three-dimensional homology models of the protein C protease domain, and the implications for domain folding and structure were evaluated. Most Type I missense mutations either replaced internal hydrophobic residues (I201T, L223F, A259V, A267T, A346T, A346V, G376D) or nearby interacting residues (I403M, T298M, Q184H), thus disrupting the packing of internal hydrophobic side chains, or changed hydrophilic residues, thus disrupting ion pairs (N256D, R178W). Mutations (P168L, R169W) at the activation site destabilized the region containing the activation peptide structure. Most Type II mutations involved solvent-exposed residues and were clustered either in a positively charged region (R147W, R157Q, R229Q, R352W) or were located in or near the active site region (S252N, D359N, G381S, G391S, H211Q). The cluster of arginines 147, 157, 229, and 352 may identify a functionally important exosite. Identification of the spatial relationships of natural mutations in the protein C model is helpful for understanding manifestations of protein C deficiency and for identification of novel, functionally important molecular features and exosites.

Charge reversal at the P3' position in protein C optimally enhances thrombin affinity and activation rate

We have examined the properties of several human protein C (HPC) derivatives with substitutions for acidic residues near the thrombin cleavage site, including changing the P3' Asp to Asn (D172N), Gly (D172G), Ala (D172A), or Lys (D172K). The rate of thrombin-catalyzed activation of D172N, D172G, and D172A was increased 4-9-fold compared to wild-type HPC, primarily due to a reduction in the inhibitory effect of calcium and a resulting increase in affinity for free alpha-thrombin. There was no significant increase in activation rate or affinity with these 3 derivatives in the absence of calcium, confirming that P3' Asp affects calcium dependency in the native protein C molecule. With charge reversal at P3' (D172K), there was a 30-fold increase in activation rate in the presence of calcium, but unlike the other derivatives, there was a substantial effect (5-fold) on the activation rate and affinity for free alpha-thrombin in the absence of calcium. Thus, protein C affinity for thrombin appears to be influenced by a combination of calcium-dependent and -independent effects of the acidic P3' residue.

Models of the serine protease domain of the human antithrombotic plasma factor activated protein C and its zymogen

Three-dimensional structural analysis of physiologically important serine proteases is useful in identifying functional features relevant to the expression of their activities and specificities. The human serine protease anticoagulant protein C is currently the object of many genetic site-directed mutagenesis studies. Analyzing relationships between its structure and function and between naturally occurring mutations and their corresponding clinical phenotypes would be greatly assisted by a 3-dimensional structure of the enzyme. To this end, molecular models of the protease domain of protein C have been produced using computational techniques based on known crystal structures of homologous enzymes and on protein C functional information. The resultant models corresponding to different stages along the processing pathway of protein C were analyzed for structural and electrostatic differences arising during the process of protein C maturation and activation. The most satisfactory models included a calcium ion bound to residues homologous to those that ligate calcium in the trypsin structure. Inspection of the surface features of the models allowed identification of residues putatively involved in specific functional interactions. In particular, analysis of the electrostatic potential surface of the model delineated a positively charged region likely to represent a novel substrate recognition exosite. To assist with future mutational studies, binding of an octapeptide representing a protein C cleavage site of its substrate factor Va to the enzyme's active site region was modeled and analyzed.