title: A re-appraisal of the structural basis of stereochemical recognition in papain. Insensitivity of binding-site-catalytic-site signalling to P2-chirality in a time-dependent inhibition.
abstract: 1. 2-(N'-Acetyl-D-phenylalanylamino)ethyl 2'-pyridyl disulphide (compound I) [m.p. 123-124 degrees C; [alpha]20D -7.1 degrees (c 0.042 in methanol)] was synthesized, and the results of a study of the pH-dependence of the second-order rate constant (k) for its reaction with the catalytic-site thiol group of papain (EC 3.4.22.2), together with existing kinetic data for the analogous reaction of the L-enantiomer (compound II), were used to evaluate the consequences for transition-state geometry of the difference in chirality at the P2 position of the probe molecule. 2. The kinetic data suggest that the D-enantiomer binds approx. 40-fold less tightly to papain than the L-enantiomer but that the binding-site--catalytic-site signalling that results in a (His-159)-Im(+)-H-assisted transition state occurs equally effectively in the interaction of the former probe as in that of the latter. This results in pH-k profiles for the reactions of both enantiomers each characterized by four macroscopic pKa values (3.7-3.9, 4.1-4.3, 7.9-8.3 and 9.4-9.5) in which k is maximal at pH approx. 6 where the -Im(+)-H-assisted transition state is most fully developed. 3. Model building indicates that both enantiomers can bind to papain such that the phenyl ring of the N-acetylphenylalanyl group makes hydrophobic contacts in the binding pocket of the S2 subsite with preservation of the three hydrogen-bonding interactions involving the substrate analogue reagent and (Asp-158) C = O, (Gly-66) C = O, and (Gly-66)-N-H of papain. Earlier predictions that binding of N-acyl-D-phenylalanine derivatives to papain would be prevented on steric grounds [Berger & Schechter (1970) Philos. Trans. R. Soc. London B 257, 249-264; Lowe & Yuthavong (1971) Biochem. J. 124, 107-115; Lowe (1976) Tetrahedron 32, 291-302] were based on assumed models that are not consistent with the X-ray-diffraction data for papain inhibited by alkylation of Cys-25 with N-benzyloxycarbonyl-Phe-Ala-chloromethane [Drenth, Kalk & Swen (1976) Biochemistry 15, 3731-3738]. 4. The possibility that the kinetic expression of P2-S2 stereospecificity may depend on the nature of the chemistry occurring in the catalytic site of papain is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Biochemistry, Medical College of St. Bartholomew's Hospital, University of London, U.K.
authors.forename: W
authors.initials: W
authors.lastname: Templeton
authors.forename: D
authors.initials: D
authors.lastname: Kowlessur
authors.forename: E W
authors.initials: EW
authors.lastname: Thomas
authors.forename: C M
authors.initials: CM
authors.lastname: Topham
authors.forename: K
authors.initials: K
authors.lastname: Brocklehurst
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D055598:Chemical Phenomena; D002621:Chemistry; D003545:Cysteine / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D010206:Papain / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010649:Phenylalanine / Q000031:analogs & derivatives* / Q000378:metabolism; D011725:Pyridines / Q000378:metabolism*; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D011725:Pyridines; D010649:Phenylalanine; C053807:2-(N'-acetylphenylalanylamino)ethyl-2'-pyridyl disulfide; D010206:Papain; D003545:Cysteine