Kluger Ronald, Rathgeber Steven
Davenport Chemical Laboratories, Department of Chemistry, University of Toronto, Canada.
FEBS J. 2008 Dec;275(24):6089-100. doi: 10.1111/j.1742-4658.2008.06739.x. Epub 2008 Nov 7.
Thiamin diphosphate-dependent decarboxylases form addition intermediates between thiamin diphosphate (ThDP) and 2-ketoacids. Although it appears that the intermediate should react without the intervention of catalysts, evidence has clearly shown that Brønsted acid catalysis occurs through a pre-associated system. This can promote separation of carbon dioxide from the residual carbanion by protonation of the carbanion. Proteins operate through pre-association and may readily promote the separation of carbon dioxide by protonating or oxidizing the nascent carbanion. Alternatively, a nucleophilic side chain may trap carbon dioxide as an unstable hemi-carbonate. Mutagenesis experiments by others have shown that enhanced activity due to the protein in the presence of thiamin diphosphate does not depend on the presence of any one proton donor, consistent with pooled activity within the active site. This form of catalysis has not been widely recognized, but should be considered an integral aspect of enzyme-promoted decarboxylation.
硫胺素二磷酸依赖性脱羧酶在硫胺素二磷酸（ThDP）和2-酮酸之间形成加成中间体。尽管看起来中间体应该在没有催化剂干预的情况下发生反应，但证据清楚地表明，布朗斯特酸催化是通过预结合系统发生的。这可以通过碳负离子的质子化促进二氧化碳与残余碳负离子的分离。蛋白质通过预结合起作用，并且可以通过使新生的碳负离子质子化或氧化来容易地促进二氧化碳的分离。或者，亲核侧链可能将二氧化碳捕获为不稳定的半碳酸盐。其他人的诱变实验表明，在硫胺素二磷酸存在下由于蛋白质而增强的活性并不依赖于任何一种质子供体的存在，这与活性位点内的综合活性一致。这种催化形式尚未得到广泛认可，但应被视为酶促脱羧的一个不可或缺的方面。
