Distinct Contributions of Residue 192 to the Specificity of Coagulation and Fibrinolytic Serine Proteases

Abstract

Archetypal members of the chymotrypsin family of serine proteases, such as trypsin, chymotrypsin, and elastase, exhibit relatively broad substrate specificity. However, the successful development of efficient proteolytic cascades, such as the blood coagulation and fibrinolytic systems, required the evolution of proteases that displayed restricted specificity. Tissue-type plasminogen activator (t-PA), for example, possesses exquisitely stringent substrate specificity, and the molecular basis of this important biochemical property of t-PA remains obscure. Previous investigations of related serine proteases, which participate in the blood coagulation cascade, have focused attention on the residue that occupies position 192 (chymotrypsin numbering system), which plays a pivotal role in determining both the inhibitor and substrate specificity of these enzymes. Consequently, we created and characterized the kinetic properties of new variants of t-PA that contained point mutations at position 192. These studies demonstrated that, unlike in coagulation serine proteases, Gln-192 does not contribute significantly to the substrate or inhibitor specificity of t-PA in physiologically relevant reactions. Replacement of Gln-192 with a glutamic acid residue did, however, decrease the catalytic efficiency of mature, two-chain t-PA toward plasminogen in the absence of a fibrin co-factor.Appreciation that thrombotic disorders are the major cause of morbidity and mortality in many countries sparked intense interest in the human fibrinolytic system, which normally provides a counterbalance to the blood coagulation cascade (1-5). The rate-limiting step in the fibrinolytic cascade, conversion of the circulating zymogen plasminogen into the active protease plasmin, is catalyzed by t-PA, 1 a member of the chymotrypsin family of serine proteases (4, 6, 7). Discovery of this important physiological role encouraged extensive scrutiny of t-PA, and the efforts of many investigators have placed this enzyme among the most thoroughly characterized human enzymes (6 -8). Administration of wild type t-PA has now become standard therapy for acute myocardial infarction (5, 9 -13), and several variants of the enzyme have either recently entered phase III clinical trials for testing as improved thrombolytic agents or already received approval by the Food and Drug Administration (14, 15).One of the most important and intriguing biochemical properties of t-PA is the highly stringent substrate and inhibitor specificity (6, 7), which is in stark contrast to that of archetypal chymotrypsin family enzymes such as chymotrypsin, trypsin, and elastase (16). The striking substrate specificity of t-PA is mediated in part by interactions of both the enzyme and its substrate with the co-factor fibrin (6, 7). However, even in the absence of a co-factor, t-PA maintains strict specificity for plasminogen, the primary physiological substrate, and we have shown that this specificity is an inherent property of the isolated protease domain of t-PA (17). Recent structural...

The Role of Glu192 in the Allosteric Control of the S2′ and S3′ Subsites of Thrombin

“…Exosite 1 Modulation of the S 2 Ј and S 3 Ј Subsites-In a previous study (9), we challenged the hypothesis that the E192Q mutation mimicked the allosteric alteration induced by Hir [52][53][54][55][56][57][58][59][60][61][62][63][64][65] in the S 3 subsite of thrombin (4). The data were in favor of exosite binding, causing changes in the conformation of the S 2 and/or S 1 subsites of thrombin rather than mitigating unfavorable interactions between Glu 192 and acidic residues in P 3 position of the substrate.…”

“…Materials-Fluorescence-quenched substrates and Hir [52][53][54][55][56][57][58][59][60][61][62][63][64][65] (peptide NDGDFEEIPEEYLQ) were prepared via Fmoc/t-butyl strategy, as described previously (3). Lyophilized substrates were resuspended in N,Ndimethylformamide, and their concentrations were determined spectrophotometrically by assuming an absorption coefficient of 10…”

“…Our results reveal that the mutation dramatically modifies the P 2 Ј and P 3 Ј preferences, which become much less restricted in E192Q as compared with thrombin. In an attempt to correlate this alteration with those triggered by exosite binding, we have systematically determined the rates of cleavage in the presence of an analog of hirugen based on the sequence of hirudin variant 1 (Hir [52][53][54][55][56][57][58][59][60][61][62][63][64][65] ), of heparin, and of both ligands simultaneously. Our results established opposing modulation for thrombin and E192Q.…”

“…4 Hirugen is a C-terminal analog of hirudin variant 2 (NGDFEE-IPEEYL). Hir [52][53][54][55][56][57][58][59][60][61][62][63][64][65] (used in this study) is the corresponding C-terminal analog of hirudin variant 1 (NDGDFEEIPEEYLQ). For both analogs, when Tyr 63 is sulfated, the K d for thrombin decreases about 10-fold.…”

“…Binding of a peptide derived from hirudin (Hir [52][53][54][55][56][57][58][59][60][61][62][63][64][65] ) and/or of heparin to these opposing exosites alters catalysis. We have investigated the contribution of subsites S 2 and S 3 to this allosteric transition by comparing the hydrolysis of two sets of fluorescence-quenched substrates having all natural amino acids at positions P 2 and P 3 .…”