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.

Transmembrane domain interactions and residue proline 378 are essential for proper structure, especially disulfide bond formation, in the human vitamin K-dependent gamma-glutamyl carboxylase

We used recombinant techniques to create a two-chain form (residues 1-345 and residues 346-758) of the vitamin K-dependent gamma-glutamyl carboxylase, a glycoprotein located in the endoplasmic reticulum containing five transmembrane domains. The two-chain carboxylase had carboxylase and epoxidase activities similar to those of one-chain carboxylase. In addition, it had normal affinity for the propeptide of factor IX. We employed this molecule to investigate formation of the one disulfide bond in carboxylase, the transmembrane structure of carboxylase, and the potential interactions among the carboxylase's transmembrane domains. Our results indicate that the two peptides of the two-chain carboxylase are joined by a disulfide bond. Proline 378 is important for the structure necessary for disulfide formation. Results with the P378L carboxylase indicate that noncovalent bonds maintain the two-chain structure even when the disulfide bond is disrupted. As we had previously proposed, the fifth transmembrane domain of carboxylase is the last and only transmembrane domain in the C-terminal peptide of the two-chain carboxylase. We show that the noncovalent association between the two chains of carboxylase involves an interaction between the fifth transmembrane domain and the second transmembrane domain. Results of a homology model of transmembrane domains 2 and 5 suggest that not only do these two domains associate but that transmembrane domain 2 may interact with another transmembrane domain. This latter interaction may be mediated at least in part by a motif of glycine residues in the second transmembrane domain.

Identification of the N-linked glycosylation sites of vitamin K-dependent carboxylase and effect of glycosylation on carboxylase function

The vitamin K-dependent carboxylase is an integral membrane protein which is required for the post-translational modification of a variety of vitamin K-dependent proteins. Previous studies have suggested carboxylase is a glycoprotein with N-linked glycosylation sites. In this study, we identify the N-glycosylation sites of carboxylase by mass spectrometric peptide mapping analyses combined with site-directed mutagenesis. Our mass spectrometric results show that the N-linked glycosylation in carboxylase occurs at positions N459, N550, N605, and N627. Eliminating these glycosylation sites by changing asparagine to glutamine caused the mutant carboxylase to migrate faster on SDS-PAGE gels, adding further evidence that these sites are glycosylated. In addition, the mutation studies identified N525, a site that cannot be recovered by mass spectroscopy analysis, as a glycosylation site. Furthermore, the potential glycosylation site at N570 is glycosylated only if all five natural glycosylation sites are simultaneously mutated. Removal of the oligosaccharides by glycosidase from wild-type carboxylase or by elimination of the functional glycosylation sites by site-directed mutagenesis did not affect either the carboxylation or epoxidation activity when the small FLEEL pentapeptide was used as a substrate, suggesting that N-linked glycosylation is not required for the enzymatic function of carboxylase. In contrast, when site N570 and the five natural glycosylation sites were mutated simultaneously, the resulting carboxylase protein was degraded. Our results suggest that N-linked glycosylation is not essential for carboxylase enzymatic activity but is important for protein folding and stability.

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.

Binding of the Factor IX γ-Carboxyglutamic Acid Domain to the Vitamin K-dependent γ-Glutamyl Carboxylase Active Site Induces an Allosteric Effect That May Ensure Processive Carboxylation and Regulate the Release of Carboxylated Product

Propeptides of the vitamin K-dependent proteins bind to an exosite on gamma-glutamyl carboxylase; while they are bound, multiple glutamic acids in the gamma-carboxyglutamic acid (Gla) domain are carboxylated. The role of the propeptides has been studied extensively; however, the role of the Gla domain in substrate binding is less well understood. We used kinetic and fluorescence techniques to investigate the interactions of the carboxylase with a substrate containing the propeptide and Gla domain of factor IX (FIXproGla41). In addition, we characterized the effect of the Gla domain and carboxylation on propeptide and substrate binding. For the propeptide of factor IX (proFIX18), FIXproGla41, and carboxylated FIXproGla41, the Kd values were 50, 2.5, and 19.7 nM and the koff values were 273 x 10(-5), 9 x 10(-5), and 37 x 10(-5) s(-1), respectively. The koff of proFIX18 is reduced 3-fold by FLEEL and 9-fold by the Gla domain (residues 1-46) of FIX. The pre-steady state rate constants for carboxylation of FIXproGla41 was 0.02 s(-1) in enzyme excess and 0.016 s(-1) in substrate excess. The steady state rate in substrate excess is 4.5 x 10(-4) s(-1). These results demonstrate the following. 1) The pre-steady state carboxylation rate constant of FIXproGla41 is significantly slower than that of FLEEL. 2) The Gla domain plays an allosteric role in substrate-enzyme interactions. 3) Carboxylation reduces the allosteric effect. 4) The similarity between the steady state carboxylation rate constant and product dissociation rate constant suggests that product release is rate-limiting. 5) The increased dissociation rate after carboxylation contributes to the release of product.

A conserved region of human vitamin K-dependent carboxylase between residues 393 and 404 is important for its interaction with the glutamate substrate

Certain individuals with combined deficiencies of vitamin K-dependent proteins have a mutation, L394R, in their gamma-glutamyl carboxylase causing impaired glutamate binding. The sequence surrounding Leu394 is similar in all known carboxylases, suggesting that the region is functionally important. To test this hypothesis we made the following mutant enzymes: W390A, Y395A, S398A, W399A, and H404A. We purified the enzymes and corrected the activity measurements for active enzyme concentration. Carboxylases W390A, S398A, and H404A had activities similar to that of wild type; however, Y395A and W399A had lower activities than did wild type. In the following descriptions we include our previously reported results for L394R. Kinetic studies with the substrate FLEEL, revealed Km values of 0.5 (wild type), 6.5 (L394R), 15 (Y395A), and 24 (W399A) mm. The kcat values relative to wild type were 51% (L394R), 1% (Y395A), and 2% (W399A). The kcat/Km values were 24-fold (L394R) and >2000-fold lower for Y395A and W399A than for wild-type carboxylase. Inhibition of FLEEL carboxylation by the competitive inhibitor, Boc-mEEV, gave Ki values of 0.013 (wild type), 1.4 (L394R), 2.1 (Y395A), and >5 (W399A) mm. The Y395A propeptide affinity was similar to that of wild type, but those of L394R and W399A were 16-22-fold less than that of wild type. Results of kinetic studies with a propeptide-containing substrate were consistent with results of propeptide binding and FLEEL kinetics. Although propeptide and vitamin K binding in some mutants were affected, our data provide compelling evidence that glutamate recognition is the primary function of the conserved region around Leu394.

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.

The putative vitamin K-dependent gamma-glutamyl carboxylase internal propeptide appears to be the propeptide binding site

The vitamin K-dependent gamma-glutamyl carboxylase binds an 18-amino acid sequence usually attached as a propeptide to its substrates. Price and Williamson (Protein Sci. (1993) 2, 1997-1998) noticed that residues 495-513 of the carboxylase shares similarity with the propeptide. They suggested that this internal propeptide could bind intramolecularly to the propeptide binding site of carboxylase, thereby preventing carboxylation of substrates lacking a propeptide recognition sequence. To test Price's hypothesis, we created nine mutant enzyme species that have single or double mutations within this putative internal propeptide. The apparent K(d) values of these mutant enzymes for human factor IX propeptide varied from 0.5- to 287-fold when compared with that of wild type enzyme. These results are consistent with the internal propeptide hypothesis but could also be explained by these residues participating in propeptide binding site per se. To distinguish between the two alternative hypotheses, we measured the dissociation rates of propeptides from each of the mutant enzymes. Changes in an internal propeptide should not affect the dissociation rates, but changes to a propeptide binding site may affect the dissociation rate. We found that dissociation rates varied in a manner consistent with the apparent K(d) values measured above. Furthermore, kinetic studies using propeptide-containing substrates demonstrated a correlation between the affinity for propeptide and V(max). Taken together, our results indicated that these mutations affected the propeptide binding site rather than a competitive inhibitory internal propeptide sequence. These results agree with our previous observations, indicating that residues in this region are involved in propeptide binding.

Circulating and binding characteristics of wild-type factor IX and certain Gla domain mutants in vivo

Residue K5 in factor IX gamma-carboxyglutamic acid (Gla) domain participates in binding endothelial cells/collagen IV. We injected recombinant factor IX containing mutations at residue 5 (K5A, K5R) into factor IX-deficient mice and compared their behavior with that of wild-type factor IX. The plasma concentration of factor IX that binds to endothelial cells/collagen IV (recombinant wild type and K5R) was consistently lower than that of the one that does not bind (K5A). Mice treated with wild type or K5R had 79% of the injected factor IX in the liver after 2 minutes, whereas 17% remained in circulation. In mice injected with K5A, 59% of the injected factor IX was found in liver and 31% was found in plasma. When we blocked the liver circulation before factor IX injection, 74% of K5A and 64% of K5R remained in the blood. When we treated the mouse with EDTA after injecting exogenous factor IX, the blood levels of factor IX that bind to endothelial cells/collagen IV increased, presumably because of release from endothelial cell/collagen IV binding sites. In contrast, the levels of the mutants that do not bind were unaffected by EDTA. In immunohistochemical studies, factor IX appears on the endothelial surfaces of mouse arteries after factor IX injection and of human arteries from surgical specimens. Thus, we have demonstrated that factor IX binds in vivo to endothelial cell-collagen IV surfaces. Our results suggest that factor IX Gla-domain mediated binding to endothelial cells/collagen IV plays a role in controlling factor IX concentration in the blood.

Residues Y179 and H101 of a hydrophobic patch of factor VII are involved in activation by factor Xa

We studied factor Xa activation of human factor VII in hopes of identifying factor VII residues, not adjacent to the cleavage site, involved in this interaction. We made eight factor VIIs with single mutations (N100A, H101A, D102Q, L144A, R147A, Y179A, D186A, and F256A) and two factor VIIs with multiple mutations [MM3 (L144A/R147A/D186A) and MM4 (N100A/H101A/Y179A/F256A)]. Residues in MM3 have previously been identified as affecting factor X activation, and the residues of MM4 are located at a hydrophobic patch of factor VII on the opposite side of the catalytic domain from those in MM3. Only H101A, Y179A, and MM4 were activated significantly more slowly than the wild type. Results of our kinetic analyses showed that the catalytic efficiency of factor Xa for activation of factor VII was 176- and 234-fold higher than that for H101A andY179A, respectively. All the mutants with measurable activity had affinities for tissue factor similar to those of the wild type. The activated hydrophobic patch residues, except N100A, which is adjacent to one of the catalytic residues, had normal activities toward both a small peptide substrate and factor X. The rest of the activated mutants (except D102Q with no activity) had reduced activities toward the small substrate (except R147A) and factor X. We conclude that factor VII activation by factor Xa and factor VIIa's catalytic interaction with factor X involve different regions in the catalytic domain, and residues H101 and Y179, part of an aromatic hydrophobic patch, are specifically involved in factor Xa activation of factor VII.

Factor VIIa's first epidermal growth factor-like domain's role in catalytic activity

Factor VIIa-tissue factor complex formation initiates the extrinsic blood coagulation pathway. We investigated factor VIIa's first epidermal growth factor-like (egf1) domain's role in the catalytic activity increase caused when factor VIIa binds tissue factor. Starting with a factor VIIa with factor IX's egf1 domain (factor VII(IXegf1)a), we made 4 proteins with egf1 residues changed to those in factor VIIa, including E51A, D64Q, FG74-75PA, and K79R. We measured each enzyme's affinity for tissue factor and determined the enzymes' kinetic constants with and without tissue factor. The Kd for factor VII(IXegf1)a binding to tissue factor was 60-200-fold higher than that of factor VIIa depending on the assay employed. Only factor VII(IXegf1)a with the K79R (K79Ra) mutation, among all the mutants, had an effect on binding with a Kd 3-8-fold lower than that of factor VII(IXegf1)a. In kinetic analyses with a small peptide substrate, in the absence of tissue factor, factor VIIa, factor VII(IXegf1)a, and K79Ra had similar kcat's and Km's. With tissue factor, due to a kcat decrease, factor VII(IXegf1)a's catalytic efficiency (kcat/Km) was 2-fold lower than factor VIIa's. K79Ra's catalytic efficiency was intermediate between those of factor VIIa and factor VII(IXegf1)a. With factor X as substrate, in the absence of tissue factor, K79Ra and factor VII(IXegf1)a had catalytic efficiencies 1.5-fold and 2-fold lower than that of factor VIIa. In contrast, with tissue factor and with factor X as substrate, due to higher Km's, factor VII(IXegf1)a and K79Ra had only 9% and 33% of factor VIIa's catalytic efficiency. Our results suggest the egf1 domain's role in tissue factor binding involves critical alignment of tissue factor with factor VIIa's catalytic domain. Proper alignment in turn promotes optimal catalytic activities.

Changing residue 338 in human factor IX from arginine to alanine causes an increase in catalytic activity

This study was designed to identify functionally important factor IX (FIX) residues. Using recombinant techniques and cell culture, we produced a mutant FIX with arginine at 338 changed to alanine (R338A-FIX). This molecule had approximately 3 times greater clotting activity than that of wild type FIX (wt-FIX) in the activated partial thromboplastin assay. R338A-FIX reacted normally with a panel of three FIX specific monoclonal antibodies and migrated on sodium dodecyl sulfate-polyacrylamide gels indistinguishably from wt-FIX. Using functional assays, we determined that R338A-FIXa's Kd for factor VIIIa (FVIIIa) was similar to that of wt-FIXa. Our kinetic analysis, using factor X as substrate, indicated that the mutation's major effects were a 3-fold increase in kcat and a 2-fold decrease in Km both manifested only in the presence of FVIIIa. R338A-FIXa's increased catalytic efficiency did not result from ablation of a thrombin sensitive site, reported to occur at arginine 338, since in our assays the thrombin inhibitor, hirudin, had no effect on activity of either wt-FIXa or R338A-FIXa. R338A-FIXa and wt-FIXa had equal activity, with or without FVIIIa, toward the synthetic substrate, methylsulfonyl-D-cyclohexylglycyl-arginine-p-nitroanilide. Interestingly, R338A-FIXa had reduced affinity for heparin. Therefore, we propose that R338A-FIXa's increased activity is not due to an allosteric effect on the active site, but that the Arg-338 residue is part of an exosite that binds both factor X and the mucopolysaccharide, heparin.

A coagulation factor IX-deficient mouse model for human hemophilia B

Coagulation factor IX deficiency causes hemophilia B in humans. We have used gene targeting to develop a coagulation factor IX-deficient (factor IX-knockout) mouse strain. Mouse embryonic stem (ES) cells were targeted by a socket-containing vector that replaces the promoter through exon 3 of the factor IX gene by neoDeltaHPRT, which is a functional neo gene plus a partially deleted hypoxanthine phosphoribosyl transferase minigene. Chimeric mice generated using these socket-containing ES cells transmitted the targeted factor IX gene to their female offspring. Male offspring from these females were characterized and shown to exhibit a phenotype similar to hemophilia B. This factor IX-deficient mouse strain will be useful for studying gene therapy methods and structure-function relationships of recombinant factor IX proteins in vivo.

The roles of factor VII's structural domains in tissue factor binding

Factor VIIa binds to tissue factor in one of the initial steps of blood clotting. In order to determine the role of the various domains of the factor VII molecule in this interaction, we made several chimeric factor VII proteins using recombinant DNA techniques. The molecules have factor IX domains substituted into factor VII and vice versa. The domains exchanged were the 4-carboxyglutamic acid plus aromatic stack domain (gla), the first epidermal growth factor-like domain (Egf-1), the second epidermal growth factor-like domain (Egf-2), and the catalytic domain. Using tissue factor-coated microtiter wells, competition binding studies with 125I-labeled factor VIIa indicated factor VIIa's Kd is 4.2 nM. Employing the same microtiter plate assay, koff and kon were determined and yielded a Kd of 1.5 nM. The results of competitive binding experiments and activation assays using chimeric proteins indicated the interaction between factor VIIa and tissue factor involves direct contact between tissue factor and factor VIIa's Egf-1 domain and catalytic domain. On the other hand, the gla and Egf-2 domains, while necessary for optimal binding, may merely impart structure to the rest of the molecule. However, either one or both of the latter domains might contribute a relatively small amount of energy to direct binding.

The role of the epidermal growth factor-1 and hydrophobic stack domains of human factor IX in binding to endothelial cells

To determine the function and specificity in factor IX of the first epidermal growth factor (EGF)-like domain and the eight-amino acid hydrophobic stack encoded by exon C (residues 39-46), these domains were replaced by the corresponding polypeptide regions of factor X and chimeric proteins were produced in human embryo kidney cells. Both chimeras were activated by factor XIa at a rate similar to plasma factor IX and exhibited calcium-dependent fluorescence quenching similar to plasma factor IX. Both chimeras competed equally for binding to the endothelial cell receptor. Our findings make it unlikely that the first EGF-like domain or the hydrophobic stack of factor IX are responsible for the specific binding of factor IX to its endothelial cell receptor.

Binding of alpha 2-macroglobulin-thrombin complexes and methylamine-treated alpha 2-macroglobulin to human blood monocytes

The binding of alpha 2-macroglobulin (alpha 2M) to human peripheral blood monocytes was investigated. Monocytes, the precursors of tissue macrophages, were isolated from fresh blood by centrifugal elutriation or density gradient centrifugation. Binding studies were performed using 125I-labeled alpha 2M. Cells and bound ligand were separated from free ligand by rapid vacuum filtration. Nonlinear least-squares analysis of data obtained in direct binding studies at 0 degrees C showed that monocytes bound the alpha 2M-thrombin complex with a Kd of 3.0 +/- 0.9 nM and the monocyte had 1545 +/- 153 sites/cell. Thrombin alone did not compete for the site. Binding was divalent cation dependent. Direct binding studies also demonstrated that monocytes bound methylamine-treated alpha 2M in a manner similar to alpha 2M-thrombin. Competitive binding studies showed that alpha 2M-thrombin and methylamine-treated alpha 2M bound to the same sites on the monocyte. In contrast, native alpha 2M did not compete with alpha 2M-thrombin for the site. Studies done at 37 degrees C suggested that after binding, the monocyte internalized and degraded alpha 2M-thrombin and excreted the degradation products. Receptor turnover and degradation of alpha 2M-thrombin complexes were blocked in monocytes treated with chloroquine, an inhibitor of lysosomal function. Our results indicate that human monocytes have a divalent cation dependent, high-affinity binding site for alpha 2M-thrombin and methylamine-treated alpha 2M which may function to clear alpha 2M-proteinase complexes from the circulation.

Homo- and heterodimer formation with prothrombin and prothrombin fragment 1 in the presence of calcium ions

The purpose of the current study is to present further evidence for prothrombin self-association as assessed by chemical crosslinking. When the self-association (evaluated by covalent crosslinking with dithiobis(succinimidylpropionate) of prothrombin or fragment 1 was evaluated at the same molar concentration of protein, similar rates of dimer formation were observed for either protein. When prothrombin and fragment 1 were incubated together with the crosslinking reagent and calcium ions, a heterodimer consisting of prothrombin and fragment 1 was observed in addition to prothrombin dimer and fragment 1 dimer. Similar experiments with prethrombin 1 showed neither significant self-association nor effect on prothrombin self-association. Comparison of the formation of prothrombin fragment 1 heterodimer formation with the effect of fragment 1 on prothrombin activation by factor Xa suggests that the anticoagulant activity of fragment 1 is not solely a result of the formation of a heterodimer between prothrombin and fragment 1.

Structural and functional characterization of the inhibition of urokinase by alpha 2-macroglobulin

We have investigated the interaction of alpha 2-macroglobulin (alpha 2M) with the serine proteinase urokinase, an activator of plasminogen. Urokinase formed sodium dodecyl sulfate stable complexes with purified alpha 2M and with alpha 2M in plasma. These complexes could be visualized after polyacrylamide gel electrophoresis by protein blots using 125I-labeled anti-urokinase antibody or by fibrin autography, a measure of fibrinolytic activity. According to gel electrophoretic analyses under reducing conditions, urokinase cleaved alpha 2M subunits and formed apparently covalent complexes with alpha 2M. Urokinase cleaved only about 60% of the alpha 2M subunits maximally at a mole ratio of 2:1 (urokinase: alpha 2M). Binding of urokinase to alpha 2M protected the urokinase active site from inhibition by antithrombin III-heparin and inhibited, to a significant extent, plasminogen activation by urokinase. Reaction of urokinase with alpha 2M caused an increase in intrinsic protein fluorescence and, thus, induced the conformational change in alpha 2M that is characteristic of its interactions with active proteinases. Our results indicate that both in plasma and in a purified system the alpha 2M-urokinase reaction is functionally significant.

Activation of normal and abnormal human factor IX with trypsin

Human factor IX is activated to factor IXa beta when factor XIa cleaves two peptide bonds, Arg 145-Ala 146 and Arg 180-Val 181, to release an activation peptide. In factor IX Chapel Hill (IXCH), isolated from a hemophilia B patient with a mild bleeding disorder, the arginine 145 residue has been replaced with a histidine. Thus factor IXCH is activated by factor XIa by cleaving only at the Arg 180-Val 181 bond, leaving the activation peptide attached, and resulting in an activated species, factor IXa alpha CH, that, like normal factor IXa alpha, is only 20% as active as factor IXa beta. It is reported that both factor IX and factor IXCH could be activated by trypsin to forms of factor IXa beta and factor IXa beta CH that had clotting activities identical to factor XIa-activated factor IX. Amino-terminal amino acid sequence analysis showed that trypsin cleaved factor IX at the same bonds as did factor XIa; factor IXCH was cleaved at the Arg 180-Val 181 bond, as normal, and was cleaved near the histidine 145, at the Lys 142-Leu 143 bond, releasing a slightly larger activation peptide than from normal factor IXa beta. Metal ions had no effect on the rate of activation of factor IX by trypsin; however, metal ions had a profound effect on the rate at which further incubation with trypsin inactivated factor IXa. Calcium and manganese protected factor IXa from inactivation by trypsin more effectively than magnesium, which was more effective than no metal ion. It is concluded that trypsin can activate normal factor IX and factor IXCH to fully active IXa beta forms.

Structural and functional characteristics of activated human factor IX after chemical modification of gamma-carboxyglutamic acid residues

Activated human factor IX (factor IXa) was treated under mildly acidic conditions with a mixture of formaldehyde and morpholine. This reagent has been shown to react preferentially with gamma-carboxyglutamyl (Gla) residues and to convert these residues to gamma-methyleneglutamyl residues (Wright, S.F., Bourne, C.D., Hoke, R.A., Koehler, K.A., and Hiskey, R.G. (1984) Anal. Biochem. 139, 82-90). The modified enzyme was evaluated for coagulant activity and calcium-dependent fluorescence quenching. [14C]Formaldehyde was employed to allow quantitation of the modification and to facilitate localization of the modified residues in the primary structure of factor IXa. In the presence of the [14C]formaldehyde/morpholine reagent, factor IXa rapidly lost coagulant activity, which corresponded to incorporation of radiolabel. Examination of the relationship between protein modification (radiolabel incorporation) and the loss of coagulant activity suggested that modification of 1 mol of Gla/mol of factor IXa results in complete loss of factor IXa coagulant activity. Primary structure analysis of the radioactivity labeled factor IXa suggested that modification of any one of 11 Gla residues was responsible for the loss of coagulant activity. In the presence of calcium, modified factor IXa exhibited a smaller Gla-dependent decrease in protein fluorescence than native factor IXa, but the Gla-independent fluorescence change was the same for both proteins. It therefore appears that the Gla domain of factor IXa must be completely intact for the enzyme to undergo a functionally important calcium-dependent conformational change necessary for coagulant activity.

Inactivation of human blood coagulation factor X by chemical modification of gamma-carboxyglutamic acid residues

The inactivation of human factor X by incubation with a reagent known to chemically modify gamma-carboxyglutamic acid to gamma-methylene glutamic acid was studied. Incubation of factor X at pH 5.0 with a preincubated formaldehyde/morpholine mixture (0.9 M/1.0 M) resulted in a progressive decrease in factor X coagulant activity. In the presence of calcium (20 mM) the rate of factor X inactivation was decreased -3-fold. By using [14]C-formaldehyde, modified-factor X (less than 5% residual activity) was found to contain 7 mols of [14]C per mol of protein. Modified-factor X was not activated by Russell's viper venom in the presence of calcium, suggesting that the loss of coagulant activity was related to the inability of modified-factor X to be activated.

Bovine alpha- and beta-thrombin. Reduced fibrinogen-clotting activity of beta-thrombin is not a consequence of reduced affinity for fibrinogen

beta-Thrombin, a product of the limited proteolysis of alpha-thrombin, is characterized by greatly reduced fibrinogen-clotting activity as compared to alpha-thrombin but with unchanged activity toward ester substrates. The present study was designed to elucidate the basis for the changes in the catalytic activity resulting from the conversion of bovine alpha-thrombin to bovine beta-thrombin. Fibrinogen was utilized as a competitive inhibitor in the hydrolysis of a peptide nitroanilide substrate by bovine alpha- and beta-thrombin. The Ki values obtained for fibrinogen in these experiments were similar for alpha- and beta-thrombin (about 10 microM). Similar values for Ki were obtained when fibrinogen was used to inhibit the inactivation of bovine alpha- and beta-thrombin by diisopropylphosphorofluoridate. These experiments suggested that the conversion of bovine alpha- to beta-thrombin does not affect the fibrinogen-binding site on thrombin. Differences in the reactivity of functional groups at the active site were then explored. beta-Thrombin was observed to undergo modification at the active site histidine at a slower rate than that of alpha-thrombin when reacted with either tosyllysyl chloromethyl ketone or diethyl pyrocarbonate. It is suggested that the difference in the fibrinogen-clotting activity of these two forms of thrombin can result from changes in the reactivity of the active site histidine residue.

Characterization of thrombin binding to alpha 2-macroglobulin

The formation and structural characteristics of the human alpha 2-macroglobulin (alpha 2M)-thrombin complex were studied by intrinsic protein fluorescence, sulfhydryl group titration, electrophoresis in denaturing and nondenaturing polyacrylamide gel systems, and in macromolecular inhibitor assays. The interaction between alpha 2M and thrombin was also assessed by comparison of sodium dodecyl sulfate-gel electrophoretic patterns of peptides produced by Staphylococcus aureus V-8 proteinase digests of denatured alpha 2M-125I-thrombin and alpha 2M-125I-trypsin complexes. In experiments measuring fluorescence changes and sulfhydryl group exposure caused by methylamine, we found that thrombin produced its maximum effects at a mole ratio of approximately 1.3:1 (thrombin:alpha 2M). Measurements of the ability of alpha 2M to bind trypsin after prior reaction with thrombin indicated that thrombin binds rapidly at one site on alpha 2M, but occupies the second site with some difficulty. Intrinsic fluorescence studies of trypsin binding to alpha 2M at pH 5.0, 6.5, and 8.0 not only revealed striking differences in trypsin's behavior over this pH range, but also some similarities between the behavior of thrombin and trypsin not heretofore recognized. Structural studies, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to measure alpha 2M-125I-thrombin covalent complex formation, indicated that covalency reached a maximum at a mole ratio of approximately 1.5:1. At this ratio, only 1 mol of thrombin is bound covalently per mol of alpha 2M. These gel studies and those of proteolytic digests of denatured alpha 2M-125I-trypsin and alpha 2M-125I-thrombin complexes suggest that proteinases form covalent bonds with uncleaved alpha 2M subunits. The sum of our results is consistent with a mechanism of proteinase binding to alpha 2M in which the affinity of the proteinase for alpha 2M during an initial reversible interaction determines its binding ratio to the inhibitor.

The effects of fibrinogen and its cleavage products on the kinetics of plasminogen activation by urokinase and subsequent plasmin activity

The effects of fibrinogen and its plasmic cleavage fragments on the activation of Glu-, Lys-, and Val442- plasminogen by urokinase were investigated. A possible explanation for the large variations in the published steady state parameters for Glu-plasminogen activation is the undetected formation of Lys-plasminogen and its subsequent more rapid activation to plasmin. When Lys-plasminogen formation was avoided, the Km for Glu-plasminogen activation by urokinase was 2.5 microM with or without lysine present and the catalytic rate constant (kcat) was 3.4 min-1 in the absence of lysine, but increased to 49.0 min-1 in its presence. For Lys-plasminogen activation, both the Km of 2.7 microM and the kcat of 57.8 min-1 were only slightly increased by lysine. With Val442-plasminogen, the absence of the first 4 kringle structures of Lys-plasminogen resulted in a 6-fold higher Km and a 3-fold higher kcat, both of which were relatively unchanged by lysine. The specificity of urokinase for Val442-plasminogen, as measured by the quotient kcat/Km was thus half that for Lys-plasminogen. Fibrinogen, Fragment D, and Fragment E enhanced the rate of activation of Glu-plasminogen to Glu-plasmin as measured by the irreversible binding of plasmin to fluorescently labeled bovine pancreatic trypsin inhibitor. Both fibrinogen and Fragment D increased the value of kcat/Km about 4-fold whereas Fragment E caused a 2-fold enhancement. In contrast to Glu-plasminogen activation, the urokinase activation of Lys-plasminogen was not affected by fibrinogen or its fragments, yet a marked inhibition of Lys-plasmin autolysis occurred in their presence, with the half-life of plasmin being increased 13-fold by fibrinogen, 5-fold by Fragment D, and 3-fold by Fragment E. The K4 kringle region may be particularly involved in the plasmin-plasmin interaction that results in autolysis, since it significantly reduced degradation when incubated with Lys-plasmin. Val442-plasmin displayed essentially no autolysis, which further implicates the first 4 kringles in the autolytic reactions. In addition to these effects, the rate of Glu-plasminogen conversion to Lys-plasminogen by plasmin was increased 4-fold by fibrinogen or Fragment E, but only 2-fold by Fragment D. This augmentation was not merely due to inhibition of Lys-plasmin autolysis since Fragment D has a greater effect in that regard. The sum of these interactions indicates that Glu-plasminogen binds to the Fragment D region of fibrinogen/fibrin through its low affinity binding site(s) and, as when lysine binds at these sites, the activation to Glu-plasmin is then accelerated.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of protease binding by alpha 2-macroglobulin on intrinsic fluorescence

We have evaluated intrinsic protein fluorescence as a method for investigating the reactions of alpha 2-macroglobulin (alpha 2M) with proteases and amines. Changes in fluorescence intensity of alpha 2M in the presence of proteases and amines were shown to correlate with structural and functional changes in the alpha 2M molecule. By intrinsic fluorescence we found that 2 mol of trypsin bound to 1 mol of alpha 2M whereas thrombin and plasmin each bound in a stoichiometry closer to 1:1. Studies showed that changes in fluorescence caused by ammonium ion paralleled the loss of the ability of alpha 2M to protect trypsin from soybean trypsin inhibitor. The exposure of sulfhydryl groups on alpha 2M by a small organic amine (methylamine) also correlated with fluorescence change that could be quantitatively eliminated by prior reaction of alpha 2M with trypsin. Cleavage of alpha 2M by four serine proteases (plasmin, thrombin, trypsin, and elastase) as determined by sodium dodecyl sulfate gel electrophoretic analyses and the binding of plasmin and thrombin as measured by macromolecular inhibitor assays corresponded to the increase in fluorescence intensity. In addition, the rate of thrombin inhibition for clotting fibrinogen was the same as the rate of fluorescence change observed when thrombin was incubated with alpha 2M. Our results indicate that intrinsic protein fluorescence is an easy and rapid technique for assessing both qualitative and quantitative aspects of protease-alpha 2M interactions.