Ion Catalysis Carboxypeptidase

Lipscomb has commented that glutamic acid-245 might act either as a general base or a nucleophile. The available mechanistic information has been reviewed by Kaiser and Kaiser (1972), who postulated that the carboxylate anion of glutamic acid-245 acts as a nucleophile forming an anhydride intermediate [equation (29)]. The divergent D2O solvent isotope effects, = 

There are three fundamental questions concerning the mechanism of action of carboxypeptidase (1) How do metal ions affect ester and amide hydrolysis (2) Under what conditions will a neighbouring carboxyl group participate in ester and amide hydrolysis emd what is the mechanism of such participation (3) How will a metal ion affect 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl 

Cu -catalysed hydrolysis and 5 4 for alkaline hydrolysis of 0-labelled ester. Formation of the tetrahedral intermediate in the scheme of equation (31) is indicated by the observed 0-exchange. However, interactions of the carbonyl oxygen and the metal ion 

The following relative second-order rate constants have been obtained for hydroxide ion-catalysed hydrolysis glycine ethyl ester, 1 protonated glycine ethyl ester, 41 and the cupric ion complex of glycine ethyl ester, F3 x 10 (Conley and Martin, 1965). The large effect of the cupric ion cannot be due entirely to electrostatic effects, but rather to catalysis by direct co-ordination with the ester function. 

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