Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.4.21.1 (chymotrypsin)
 10,938 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Serpins form enzymatically inactive covalent complexes (designated E*I*) with their target proteinases, corresponding most likely to the acyl enzyme that resembles the normal intermediate in substrate turnover. Formation of E*I* involves large changes in the conformation of the reactive center loop (residues P17 to P9') and of the serpin molecule in general. The "hinge" region of the reactive center loop, including residues P10-P14, shows facile movement in and out of beta-sheet A, and this movement appears to be crucial in determining whether E*I* is formed (the inhibitor pathway) or whether I is rapidly hydrolyzed to I* (the substrate pathway). Here, we report stopped-flow and rapid quench studies investigating the pH dependence of the conversion of the alpha1-antichymotrypsin.alpha-chymotrypsin encounter complex, E.I, to E*I*. These studies utilize fluorescent derivatives of cysteine variants of alpha1-antichymotrypsin at the P11 and P13 residues. Our results demonstrate three identifiable intermediates, EIa, EIb, and EIc, between E.I and E*I* and permit informed speculation regarding the nature of these intermediates. Partitioning between inhibitor and substrate pathways occurs late in the process of E*I* formation, most likely from a species occurring between EIc and E*I*.
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PMID:Antichymotrypsin interaction with chymotrypsin. Intermediates on the way to inhibited complex formation. 1036 15

Serpins are a superfamily of serine proteinase inhibitors which function to regulate a number of key biological processes including fibrinolysis, inflammation, and cell migration. Poxviruses are the only viruses known to encode functional serpins. While some poxvirus serpins regulate inflammation (myxoma virus SERP1 and cowpox virus [CPV] crmA/SPI-2) or apoptosis (myxoma virus SERP2 and CPV crmA/SPI-2), the function of other poxvirus serpins remains unknown. The rabbitpox virus (RPV) SPI-1 protein is 47% identical to crmA and shares all of the serpin structural motifs. However, no serpin-like activity has been demonstrated for SPI-1 to date. Earlier we showed that RPV with the SPI-1 gene deleted, unlike wild-type virus, fails to grow on A549 or PK15 cells (A. Ali, P. C. Turner, M. A. Brooks, and R. W. Moyer, Virology 202:306-314, 1994). Here we demonstrate that in the absence of a functional SPI-1 protein, infected nonpermissive cells which exhibit the morphological features of apoptosis fail to activate terminal caspases or cleave the death substrates PARP or lamin A. We show that SPI-1 forms a stable complex in vitro with cathepsin G, a member of the chymotrypsin family of serine proteinases, consistent with serpin activity. SPI-1 reactive-site loop (RSL) mutations of the critical P1 and P14 residues abolish this activity. Viruses containing the SPI-1 RSL P1 or P14 mutations also fail to grow on A549 or PK15 cells. These results suggest that the full virus host range depends on the serpin activity of SPI-1 and that in restrictive cells SPI-1 inhibits a proteinase with chymotrypsin-like activity and may function to inhibit a caspase-independent pathway of apoptosis.
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PMID:SPI-1-dependent host range of rabbitpox virus and complex formation with cathepsin G is associated with serpin motifs. 1051 6

alpha(1)-Antichymotrypsin is a member of the serine proteinase inhibitor, or serpin, family that typically forms very long-lived, enzymatically inactive 1:1 complexes (denoted E*I*) with its target proteinases. Serpins share a conserved tertiary structure, in which an exposed region of amino acid residues (called the reactive center loop or RCL) acts as bait for a target proteinase. Within E*I*, the two proteins are linked covalently as a result of nucleophilic attack by Ser(195) of the serine proteinase on the P1 residue within the RCL of the serpin. This species is formally similar to the acyl enzyme species normally seen as an intermediate in serpin proteinase catalysis. However, its subsequent hydrolysis is extremely slow as a result of structural changes within the enzyme leading to distortion of the active site. There is at present an ongoing debate concerning the structure of the E*I* complex; in particular, as to whether the enzyme, bound to P1, maintains its original position at the top of the serpin molecule or instead translocates across the entire length of the serpin, with concomitant insertion of RCL residues P1-P14 within beta-sheet A and a large separation of the enzyme and RCL residue P1'. We report time-resolved fluorescence energy transfer and rapid mixing/quench studies that support the former model. Our results indicate that the distance between residue P1' in alpha(1)-antichymotrypsin and the amino terminus of chymotrypsin actually decreases on conversion of the encounter complex E.I to E*I*. These results led us to formulate a comprehensive mechanism that accounted both for our results and for those of others supporting the two different E*I* structures. In this mechanism, partial insertion of the RCL, with no large perturbation of the P1' enzyme distance, is followed by covalent acyl enzyme formation. Full insertion can subsequently take place, in a reversible fashion, with the position of equilibrium between the partially and fully inserted complexes depending on the particular serpin-proteinase pair under consideration.
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PMID:Formation of the covalent chymotrypsin.antichymotrypsin complex involves no large-scale movement of the enzyme. 1102 95