No abstract

The structure of Ca-prothrombin fragment 1 (residues 1-156 prothrombin) has been solved and refined at 2.2-A resolution by X-ray crystallographic methods. The first two-thirds of the Gla domain (residues 1-48) and two carbohydrate chains (approximately 5 kDa) are disordered in crystals of apo-fragment 1. When crystals are grown in the presence of Ca2+ ions, the Gla domain exhibits a well-defined structure binding seven Ca2+ ions, but the carbohydrate is still disordered. Even so, the crystallographic R factor reduced to 0.171. The folding of the Gla domain is dominated by 9-10 turns of three different alpha-helices. These turns produce two internal carboxylate surfaces composed of Gla side chains. A polymeric array of five Ca2+ ions separated by about 4.0 A intercalates between the carboxylate surfaces. The coordination of the Ca2+ ions with Gla carboxylate oxygen atoms and water molecules leads to distorted polyhedral arrangements with mu-oxo bridges in a highly complex array that most likely orchestrates the folding of the domain. The overall mode of interaction of the Ca2+ ions is new and different from any Ca2+ ion-protein interactions heretofore observed or described. The fluorescence quenching event observed upon Ca2+ ion binding is due to a disulfide-pi-electron interaction that causes a 100 degrees reorientation of Trp42 of the Gla domain. The Ca2+ ion interaction also affords the N-terminus protection from acetylation because the latter is buried in the folded structure and makes hydrogen-bonding salt bridges with Gla17, Gla21, and Gla27. The Gla domain and its trailing disulfide unit associate intimately and together give rise to a domain-like structure. Electrostatic potential calculations indicate that the Gla domain is very electronegative. Since most of the carboxylate oxygen atoms of Gla residues are involved in Ca2+ ion binding, leaving only a few for bridging Ca2+ ion-phospholipid interactions, the role of bridging Ca2+ ions might be generally unspecific, with Ca2+ ions simply intervening between the negative Gla domain and negative head groups of the membrane surface. The folding of the kringle structure in apo- and Ca-fragment 1 is essentially the same. However, the Ser36-Ala47 helix of the Gla domain pivots around Cys48, shifting by approximately 30 degrees, and the helix encroaches on the kringle producing some concomitant changes. These might be related to the protection of carbohydrate carrying Asn101 from acetylation in the Ca-fragment 1 structure.

Many of the vital actions of thrombin on platelets and other cells appear to be mediated by the recently cloned seven-transmembrane-domain thrombin receptor. Thrombin activates this receptor by a novel proteolytic mechanism. The amino-terminal exodomain of the receptor contains the sequence LDPRSFLLRNPNDKYEPF. Structure-activity studies with mutant receptors and receptor peptides suggest that this sequence binds to thrombin at two sites: LDPR with the active center of thrombin and KYEPF with the fibrinogen recognition exosite of thrombin. Thrombin then cleaves the Arg41-Ser42 bond to unmask a new amino terminus, which functions as a tethered peptide ligand binding to as yet undefined sites within the body of the receptor to effect receptor activation. We have determined eight crystal structures of thrombin complexed with receptor-based peptides. Each of the two components of the bidentate docking model was captured in individual cocrystals. In one crystal type, the LDPR sequence docked in the active center of thrombin in a manner analogous to d-PheProArg chloromethyl ketone. In other crystals, the KYEPF sequence bound in the fibrinogen anion binding exosite of thrombin in a manner analogous to the DFEEI sequence of the carboxylate-terminal peptide of hirudin. Strikingly, however, generation of a single crystal that includes both components of the anticipated bidentate binding mode was not achieved, apparently because the peptides have a dominant solution S-like conformation that does not bind in a productive way at the active center. This peptide structure apparently favored a novel alternative mode of receptor peptide-thrombin interaction in which the receptor peptides formed an intermolecular bridge between neighboring thrombin molecules, resulting in an infinite peptide thrombin chain in crystals. In this structure, the KYEPF sequence docked in the expected manner at the exosite of one thrombin molecule, but the LDPR sequence docked in an unusual nonproductive mode with the active center of a neighboring molecule. Mutations that removed important determinants of the S-like receptor peptide structure underlying the bridging mode in the receptor itself did not significantly alter thrombin signaling. Additionally, a comparison of receptor density to the responsiveness of a cell did not support a role for receptor oligomerization in signaling. The physiological role for this unexpected intermolecular binding mode, if any, remains to be identified.(ABSTRACT TRUNCATED AT 400 WORDS)

The X-ray crystal structure of prethrombin2 (pre2), the immediate inactive precursor of a-thrombin, has been determined at 2.0 A resolution complexed with hirugen. The structure has been refined to a final R-value of 0.169 With the determination of isomorphous structures of hirugen-thrombin and D-Phe-Pro-Arg chloromethyl ketone (PPACK)-thrombin, the changes that occur in the active site that affect the kinetics of chromogenic substrate hydrolysis on binding to the fibrinogen recognition exosite have been determined. The backbone of the Ala 190-Gly 197 segment in the active site has an average RMS difference of 0.55 A between the 2 structures (about 3 . 7~ compared to the bulk structure). This segment has 2 type I1 0-bends, the first bend showing the largest shift due to hirugen binding. Another important feature was the 2 different conformations of the side chain of Glu 192.The side chain extends to solvent in hirugen-thrombin, which is compatible with the binding of substrates having a n acidic residue in the P 3 position (protein-C, thrombin platelet receptor). In PPACK-thrombin, the side chain of Asp 189 and the segment Arg 221A-Gly 223 move to provide space for the inhibitor, whereas in hirugenthrombin, the Ala 190-Gly 197 movement expands the active site region. Although 8 water molecules are expelled from the active site with PPACK binding, the inhibitor complex is resolvated with 5 other water molecules. Keywords: activation; exosite binding; hirugen-thrombin; PPACK-thrombin; prethrombin2Prothrombin activation to a-thrombin by the prothrombinase complex (Factor Xa-Factor Va-phospholipids-Ca*+) initially proceeds through cleavage at Arg 320 (Arg 15, chymotrypsinogen numbering; Fig. 1) to give rise to the intermediate product meizothrombin (Krishnaswamy et al., 1987;Mann, 1987 Abbreviations: pre2, prethrombin2; Pre2 (with a capital), hirugenpre2 complex; PPACK, D-Phe-Pro-Arg chloromethyl ketone; Throm, or-thrombin; hirugen (Hir), hirudin 53-64; BPTI, bovine pancreatic trypsin inhibitor; DFP, di isopropylfluorophosphate; PEG, polyethyleneglycol; DAPA, dansyl-arginine N-(3-ethyl-1,5 pentanediy1)amide. This is followed by cleavage at Arg 271 to produce a-thrombin, which autolytically cleaves at Arg 284 to give a-thrombin (des Thr 272-Arg 284) (Downing et al., 1975). The stable form of athrombin is thus composed of the original residues from prothrombin Thr 285 to Arg 320, which corresponds to the A chain, and Ile 321 to Glu 579 in the B chain. In contrast, the activation of human prothrombin to a-thrombin by the catalyst Factor Xa-phospholipid-Ca'+ proceeds by cleavage at Arg 271 that leads to the noncovalently associated products, prothrombin fragment 1.2 (Ala 1-Arg 271) and pre2 (Thr 272-Glu 579). Subsequently, the catalytically inactive pre2 is cleaved at Arg 320 to a-thrombin and, ultimately, to the des Thr 272-Arg 284 cleavage product of a-thrombin. Thus, the simplest precursor form 2254

The structure of a complex between thrombin and a GGTTGGTGTGGTTGG DNA 15-mer has been analyzed crystallographically. The solution NMR structure of the 15-mer has two stacked G-quartets similar to that found in the previous X-ray structure determination of the 15-mer-thrombin complex [Padmanabhan, Padmanabhan, Ferrara, Sadler & Tulinsky (1993). J. Biol. Chem. 268, 17651-17654]; the strand polarity, however, is reversed from that of the crystallographic structure. The structure of the complex here has been redetermined with better diffraction data confirming the previous crystallographic structure but also indicating that the NMR solution structure fits equally well. Both 15-mer complex structures refined to an R value of about 0.16 presenting a disconcerting ambiguity. Since the two 15-mer structures associate with thrombin in different ways (through the TGT loop in the X-ray and TT loop in the NMR model), other independent lines of physical or chemical evidence are required to resolve the ambiguity.

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The crystallographic structure of the plasminogen kringle 4-epsilon-aminocaproic acid (ACA) complex (K4-ACA) has been solved by molecular replacement rotation-translation methods utilizing the refined apo-K4 structure as a search model (Mulichak et al., 1991), and it has been refined to an R value of 0.148 at 2.25-A resolution. The K4-ACA structure consists of two interkringle residues, the kringle along with the ACA ligand, and 106 water molecules. The lysine-binding site has been confirmed to be a relatively open and shallow depression, lined by aromatic rings of Trp62, Phe64, and Trp72, which provide a highly nonpolar environment between doubly charged anionic and cationic centers formed by Asp55/Asp57 and Lys35/Arg71. A zwitterionic ACA ligand molecule is held by hydrogen-bonded ion pair interactions and van der Waals contacts between the charged centers. The lysine-binding site of apo-K4 and K4-ACA have been compared: the rms differences in main-chain and side-chain positions are 0.25 and 0.69 A, respectively, both practically within error of the determinations. The largest deviations in the binding site are due to different crystal packing interactions. Thus, the lysine-binding site appears to be preformed, and lysine binding does not require conformational changes of the host. The results of NMR studies of lysine binding with K4 are correlated with the structure of K4-ACA and agree well.

Both human and bovine prothrombin fragment 2 (the second kringle) have been cocrystallized separately with human PPACK (D-Phe-Pro-Arg)-thrombin, and the structures of these noncovalent complexes have been determined and refined (R = 0.155 and 0.157, respectively) at 3.3-A resolution using X-ray crystallographic methods. The kringles interact with thrombin at a site that has previously been proposed to be the heparin binding region. The latter is a highly electropositive surface near the C-terminal helix of thrombin abundant in arginine and lysine residues. These form salt bridges with acidic side chains of kringle 2. Somewhat unexpectedly, the negative groups of the kringle correspond to an enlarged anionic center of the lysine binding site of lysine binding kringles such as plasminogens K1 and K4 and TPA K2. The anionic motif is DGDEE in prothrombin kringle 2. The corresponding cationic center of the lysine binding site region has an unfavorable Arg70Asp substitution, but Lys35 is conserved. However, the folding of fragment 2 is different from that of prothrombin kringle 1 and other kringles: the second outer loop possesses a distorted two-turn helix, and the hairpin beta-turn of the second inner loop pivots at Val64 and Asp70 by 60 degrees. Lys35 is located on a turn of the helix, which causes it to project into solvent space in the fragment 2-thrombin complex, thereby devastating any vestige of the cationic center of the lysine binding site. Since fragment 2 has not been reported to bind lysine, it most likely has a different inherent folding conformation for the second outer loop, as has also been observed to be the case with TPA K2 and the urokinase kringle. The movement of the Val64-Asp70 beta-turn is most likely a conformational change accompanying complexation, which reveals a new heretofore unsuspected flexibility in kringles. The fragment 2-thrombin complex is only the second cassette module-catalytic domain structure to be determined for a multidomain blood protein and only the third domain-domain interaction to be described among such proteins, the others being factor Xa without a Gla domain and Ca2+ prothrombin fragment 1 with a Gla domain and a kringle.