Evidence for a pH-dependent irreversible formation of a stable conformation of phenacyl-alpha-chymotrypsin.
A reinvestigation of the modification reactions of alpha-chymotrypsin with phenacyl bromide was carried out. Results conclusively demonstrate that the chemically and physically different modified enzymes prepared at pH 4 and at pH 7 both contain the phenacyl group at methionine-192 in the sulphonium salt form. Evidence to suppoort this conclusion derives from 13C nuclear-magnetic-resonance spectroscopic observations on [methylene-13C]phenacyl-enriched enzymes. More conclusively, the methionine-192-containing C-chain, derived by performic acid oxidative cleavage of radioactively-labelled enzyme prepared at pH 7, was shown to contain the phenacyl moiety and to undergo dealkylation by 2-mercaptoethanol with loss of this moiety. In addition, thermolytic cleavage of the high-pH enzyme results in fragmentation of the polypeptide chain in a fashion analogous to model reactions of phenacylmethionyl dipeptides and other methionine-192 sulphonium salts. A rationalization of the unusual nature of the high-pH phenacyl-modified enzyme based on the irreversible formation of stable conformation in which the phenacyl moiety is rigidly located in interior regions of the enzyme is presented and discussed.
D000098:Acetophenones; D000596:Amino Acids / Q000032:analysis; D055598:Chemical Phenomena; D002621:Chemistry; D002918:Chymotrypsin / Q000032:analysis; D005561:Formates; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation
D000098:Acetophenones; D000596:Amino Acids; D005561:Formates; C013190:phenacyl bromide; D002918:Chymotrypsin