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The common sequence of amino acids about a reactive serine in four proteolytic enzymes derived from four different strains of B. subtilis has been reported.1' 2 The sequence is -threonyl-seryl-methionyl-alanine-. This sequence is significantly different from that reported for a variety of proteolytic enzymes derived from mammalian sources which also contain one uniquely reactive serine, namely, -glycyl-aspartyl-seryl-glycine.A 4 A previous report' indicated unusual chemical reactivity in synthetic peptide analogues of this mammalian "active site" sequence. We have now prepared a blocked tetrapeptide analogue of the (partial) active site sequence of subtilisin: N-carbobenzyloxy-L-threonyl-L-methionyl-I-seryl-L-alanine methyl ester (I). The stoichiometric reactions of a variety of "active-serine" proteolytic enzymes with acylimidazoles have been reported elsewhere.6-8 The reactions of subtilisin and of chymotrypsin with various acylimidazoles in every case parallel the reactions of a-chymotrypsin with cinnamoylimidazole (eq. 1) originally reported by Bender et al.6' 7

PROPERTIES OF A BLOCKED TETRAPEPTIDE ANALOGUE OF THE ACTIVE SITE OF SUBTILISIN