Characterization of the active site of human multicatalytic proteinase

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Abstract
The activity of multicatalytic proteinase against synthetic substrates and the kinetics of its inhibition by a range of class-specific inhibitors have been investigated. The enzyme was found to have a broader pH activity profile than previously noted, being active against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin optimally at pH 4.5 and against benzyloxycarbonyl-Gly-Gly-Arg-7-amino-4-methylcoumarin optimally at pH 10.5. Neither activity was inhibited by the class-specific inhibitors 1,10-phenanthroline, EDTA, pepstatin, di-isopropyl fluorophosphate, peptidyl chloromethanes, peptidyl diazomethanes or L-3-carboxy-2,3-trans-epoxypropionyl-leucylamido-(4-guanidin o)butane (E-64), indicating that the enzyme is not a typical metallo-, aspartic, serine or cysteine proteinase. Inhibition by HgCl2, iodoacetamide and N-ethylmaleimide suggests that free thiols are necessary for the enzyme to maintain activity, but that these thiols are not particularly reactive as is the case for cysteine proteinases of the papain superfamily. The peptidyl aldehydes chymostatin and leupeptin were found to be reversible inhibitors of multicatalytic proteinase. Chymostatin inhibited activity against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin at pH 4.5 (Ki 160 +/- 22 microM) whereas leupeptin (200 microM) was not inhibitory. Inhibition of activity against benzyloxycarbonyl-Gly-Gly-Arg-7-amino-4-methylcoumarin by these compounds was more complex, in that they behaved as slow tight-binding inhibitors. kon values were determined to be 12 +/- 2 M-1.s-1 and 1290 +/- 125 M-1.s-1 for chymostatin and leupeptin, respectively. The upper limit for Ki values for these two inhibitors was estimated as 5 +/- 1.5 microM and 25 +/- 5 nM, respectively. The different inhibition characteristics for each substrate were also apparent at an intermediate pH of 8.5, showing that the two activities are distinct. Dichloroisocoumarin, a mechanism-based inhibitor of serine proteinases, did inhibit activity against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin with a rate constant of 250 M-1.s-1, suggesting that multicatalytic proteinase is an atypical serine proteinase.