It has been proposed that the decarboxylation of mandelylthiamin, the adduct of benzoylformate and thiamin, is uniquely catalyzed by protonated pyridines through a preassociation mechanism in which proton transfer competes with the internal return of carbon dioxide. Application of this mechanism suggests that the observed primary (12)C/(13)C kinetic isotope effect in the absence of catalyst is reduced in magnitude because diffusion of carbon dioxide is partially rate-determining. Where proton transfer blocks the internal return of carbon dioxide, the separation of carbon dioxide is facilitated, and the observed isotope effect increases toward the intrinsic value for carbon-carbon bond breaking. Headspace analysis of carbon dioxide formed over the course of the reaction shows that protonated pyridine increases the magnitude of the observed (12)C/(13)C KIE, consistent with the proposed mechanism.
Decarboxylases and intermediatesThiamin diphosphate (ThDP) is a cofactor that promotes the decarboxylation of 2-ketoacids through formation of covalent derivatives between its C2 thiazolium and the carbonyl of the substrate. Combination of a protein and ThDP in a holoenzyme provides substrate specificity and the general enzymic advantage of reduced translational entropy that favors addition processes [1,2]. The covalent intermediate undergoes cleavage of a bond to a carboxylate group derived from the 2-ketoacid, resulting in production of carbon dioxide. This also produces a residual acyl anion equivalent [3-6] with a delocalized structure that can also be represented as a neutral enamine. The sequence is illustrated for the decarboxylation of pyruvic acid by pyruvate decarboxylases in Scheme 1.Protonation at the basic carbon and elimination of ThDP leads to formation of an aldehyde (giving a net substitution of a proton for carbon dioxide). Oxidation of the same intermediate would yield an acid, while reaction with a carbonyl carbon gives a condensation product. The general route is based on concepts originally developed by Breslow [7][8][9] based on studies of model compounds related to ThDP. Details of reaction patterns within that pathway reveal previously unrecognized aspects of enzymic catalysis [3,10].Synthetic analogs of the covalent intermediates have been prepared and studied in order to arrive at a quantitative understanding of the separate functions of the cofactor and protein [11,12]. Spectroscopic analysis of the conjugates of thiamin and ketoacids has enabled specific and quantitative identification of the coenzyme derivatives bound to proteins in enzymic reactions [13][14][15].
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