title: Inhibition of cathepsin B and papain by peptidyl alpha-keto esters, alpha-keto amides, alpha-diketones, and alpha-keto acids.
abstract: A series of peptidyl alpha-keto esters, alpha-keto amides, alpha-keto acids, and alpha-diketones were synthesized which reversibly inhibit papain and cathepsin B. Methyl 3-(N-benzyloxycarbonyl-L-phenylalanyl)amino-2-oxopropionate (a dicarbonyl compound) inhibits papain with a Ki value of 1 microM, whereas the Ki of 3-(N-acetyl-L-phenylalanyl)aminopropanone (a monocarbonyl compound) is 1.5 mM (M. R. Bendall et al., 1979. Eur. J. Biochem. 79, 201-209). Both carbonyl groups are required for effective inhibition. Extension of these inhibitors by addition of P substituents (e.g., hexyl) does not affect the Ki for papain, but reduces Ki for cathepsin B 33-fold. For these two enzymes slow binding inhibition was observed with slow on rates (kappa on, 5.2 X 10(2) M-1 s-1 for papain, and 2.7 X 10(3) M- s-1 for cathepsin B). Addition of a P3 substituent (glycine) has no effect on Ki. We propose that the mechanism of inhibition involves the formation of a hemithioketal by addition of the active-site thiol to the carbonyl group of the inhibitor closer to the N-terminus. The hemithioketal intermediate is most likely stabilized by the electron withdrawing effect of the second carbonyl group.
authors.affiliation: Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254.
authors.forename: L Y
authors.initials: LY
authors.lastname: Hu
authors.forename: R H
authors.initials: RH
authors.lastname: Abeles
mesh_terms: D000577:Amides / Q000494:pharmacology*; D002401:Cathepsin B / Q000037:antagonists & inhibitors*; D055598:Chemical Phenomena; D002621:Chemistry; D007651:Keto Acids / Q000494:pharmacology*; D007659:Ketones / Q000494:pharmacology*; D010206:Papain / Q000037:antagonists & inhibitors*; D010455:Peptides / Q000494:pharmacology*; D013329:Structure-Activity Relationship
chemical_list: D000577:Amides; D007651:Keto Acids; D007659:Ketones; D010455:Peptides; D002401:Cathepsin B; D010206:Papain