Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The mechanism for the inhibition of thrombin by the serpins antithrombin and protease nexin 1 has been investigated using several kinetic techniques at pH 7.9 and 37 degrees C with an ionic strength of 0.3 M. Rapid kinetic studies demonstrated that a two-step mechanism for the formation of the stable thrombin-serpin complex applied to both serpins. The inhibition constant for the initial thrombin-antithrombin complex was 265 microM, and the rate constant for the conversion of this complex to the final one was 3.9 s-1; the corresponding values for PN1 were 3.4 microM and 6.0 s-1. By using slow-binding kinetics, it was possible to obtain estimates of the second-order rate constants for the formation of the stable thrombin-serpin complexes (1.2 x 10(4) and 1.5 x 10(6) M-1 s-1 for antithrombin and protease nexin 1, respectively) and the dissociation constants for these complexes (< 1 nM for both serpins). The influence of viscosity on the reactions indicated that the rate of interaction of both serpins with thrombin was diffusion-controlled. Moreover, the results indicated that the initial complex reacted more rapidly to form the stable complex than it dissociated to free enzyme and inhibitor; i.e., the behavior of the serpins was analogous to that of "sticky" substrates. By using the results from slow-binding, viscosity, and rapid kinetic studies, it was possible to set values for all of the rate constants for the interactions of antithrombin and protease nexin 1 with thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Inhibitory mechanism of serpins. Interaction of thrombin with antithrombin and protease nexin 1. 771 Oct 36

An overlapping synthetic peptide library was constructed representing most of the mature protease nexin I (PN1) sequence from the amino terminus to the reactive center. This library, along with peptides from the heparin binding domain and from the region carboxyl-terminal to the P1 residue of the cleavage site, was screened for the inhibition of 125I-thrombin (Th)-PN1 complex binding and degradation. A peptide corresponding to residues Pro47-Ile58 in the PN1 sequence was identified as a potent inhibitor of 125I-Th-PN1 complex degradation, although it did not affect binding significantly. Pro47-Ile58 was shown to competitively inhibit the low density lipoprotein receptor-related protein (LRP)/alpha2-macroglobulin receptor-mediated endocytosis of 125I-Th-PN1 complexes in mouse embryo fibroblasts. Pro47-Ile58 is an apparent transition sequence in PN1, separating sheet-6B and helix-B. The sequence of Pro47-Ile58, PHDNIVISPHGI, suggests that it forms a loop structure defined by the seven underlined amino acids bordered by proline residues at each end. These studies are the first to identify a putative binding site in a serine protease inhibitor that is required for LRP-mediated internalization.
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PMID:Identification of a binding site in protease nexin I (PN1) required for the receptor mediated internalization of PN1-thrombin complexes. 913 67