The aminolysis of a series of malonate thiolmonoesters ( H02CCH2COSR) has been studied kinetically at 25 "C. No indication of substrate ionisation was observed for the S-4-chlorophenyl and S-phenyl esters up to 0.&~-sodium hydroxide. Morpholinolysis showed saturation kinetics, i.e. the observed pseudofirst-order rate constant became insensitive to morpholine concentration at higher morpholine concentrations. This behaviour was analysed in terms of the rate equation kobs = kmax.[B]/(K + [B]), where kmax. is the limiting rate constant at higher buffer (B) concentrations and K the concentration of buffer required for kobs = 0.5 /?ma,. Saturation aminolysis was also observed for S-malonylcoenzyme A. Arrhenius parameters were determined for the above kinetic parameters (kmax., K, k m a x . / K ) for the S-benzyl ester. Trapping experiments with aniline showed that rate-determining and product-determining steps differed. The results were interpreted in terms of an intermediate keten formed from the ester dianion (-0,CCHCO-SR). However, to explain leaving-group dependences of kmax., K, etc., a degree of leaving-group protonation in the transition-state had to be invoked, although mechanisms involving the zwitterion-anion [-O,CCHCOk(H)R] could be excluded.
The biochemistry of S-acylcoenzymeA derivatives has been rationalised partly on the basis of nucleophilic attack by acceptor species on the acyl carbonyl group.' An alternative (ElcB) process has been demonstrated recently for S-acetoacetylcoenzyme A' and related thiolesters [equation (l)], CH,COCH,COSCoA CH,COCHCOSCoA * fast CH,COCH=C=Oproducts (1)although S-acetylcoenzyme A follows a bimolecular associative p a t h ~a y . ~ Many biological situations involve thiolesters, including the mechanisms of action of sulphydryl proteases, aldehyde dehydrogenases, glyoxalase, thiolase, etc. Acyl coenzyme A intermediates (especially acetyl, malonyl, and acetoacetyl) are pivotal intermediates in many pathways for amino-acids, carbohydrates, and fatty acids.' S-Malonylcoenzyme A functions as a donor of two-carbon units in the synthesis of P-ketoacyl acyl carrier protein, an intermediate in de novo fatty acid biosynthesis and in micro-soma1 elongation of fatty acids.' It is also a precursor of 6methylsalicylate in aromatic biosynthesis and is probably also involved in fatty acid regulation via its inhibitions of fatty acid synthetase and citrate activation of acety1CoA carboxylase ' and participates in bacterial degradation of propionic