Hu Qingyan, Kluger Ronald
Davenport Chemical Laboratory, Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6.
J Am Chem Soc. 2002 Dec 18;124(50):14858-9. doi: 10.1021/ja027976r.
Benzoylformate decarboxylase forms a covalent intermediate from thiamin diphosphate (TDP) and benzoylformate, alpha-mandelylTDP. This loses carbon dioxide to form a carbanion (enamine). Protonation of the carbanion and elimination of benzaldehyde regenerate enzyme-bound TDP. We synthesized alpha-mandelylthiamin and found that the rate of the loss of carbon dioxide is one-millionth that of the enzymic reaction. Thus, the enzyme provides an environment that facilitates the unimolecular decarboxylation process. However, the resulting nonenzymic carbanion reacts very rapidly to give products that lead to the irreversible destruction of the cofactor. This contrasts with the normal process on the enzyme. Brønsted acids on the enzyme may divert the reaction to the benzaldehyde precursor, or the enzyme may block access to the pathway that leads to destruction of the cofactor.
苯甲酰甲酸脱羧酶由硫胺素二磷酸（TDP）和苯甲酰甲酸形成共价中间体α-扁桃酰TDP。该中间体失去二氧化碳形成碳负离子（烯胺）。碳负离子质子化并消除苯甲醛后再生与酶结合的TDP。我们合成了α-扁桃酰硫胺，发现二氧化碳损失速率是酶促反应的百万分之一。因此，该酶提供了一个促进单分子脱羧过程的环境。然而，生成的非酶促碳负离子反应非常迅速，生成导致辅因子不可逆破坏的产物。这与酶上的正常过程形成对比。酶上的布朗斯特酸可能使反应转向苯甲醛前体，或者酶可能阻止通往导致辅因子破坏的途径。
