Hajipour G, Schowen K B, Schowen R L
Department of Chemistry and Higuchi Biosciences Center, University of Kansas, Lawrence 66047, USA.
Bioorg Med Chem. 1999 May;7(5):887-94. doi: 10.1016/s0968-0896(98)00269-7.
The reaction catalyzed by the thiamin-diphosphate-dependent yeast pyruvate decarboxylase, which is hysteretically regulated by pyruvate, undergoes paracatalytic oxidative diversion by 2,6-dichlorophenolindophenol, which traps a carbanionic intermediate and diverts the product from acetaldehyde to acetate (Christen, P. Meth. Enzymol. 1977, 46, 48). This reaction is now shown to exhibit an oxidant on-rate constant somewhat faster than that for pyruvate in the normal catalytic cycle and a product off-rate constant about 60-fold smaller than that for acetaldehyde. Both on-rates and off-rates exhibit an inverse solvent isotope effect of 1.5-2, observed in normal catalysis as a signal of sulfhydryl addition to the keto group of pyruvate at the allosteric regulatory site. The findings are consistent with a model for regulation in which the sulfhydryl-addition process mediates access to a fully catalytically competent active site, the oxidative-diversion reaction being forced to make use of the normal entry exit machinery.
由硫胺二磷酸依赖性酵母丙酮酸脱羧酶催化的反应，该酶受丙酮酸的滞后调节，会被2,6 - 二氯酚靛酚进行副催化氧化转移，2,6 - 二氯酚靛酚会捕获一个碳负离子中间体并使产物从乙醛转变为乙酸盐（克里斯汀，P. 《酶学方法》1977年，第46卷，第48页）。现已表明该反应的氧化剂结合速率常数比正常催化循环中丙酮酸的结合速率常数稍快，产物解离速率常数比乙醛的解离速率常数小约60倍。结合速率和解离速率均表现出1.5 - 2的反向溶剂同位素效应，在正常催化中，这是硫氢基在变构调节位点加成到丙酮酸酮基的信号。这些发现与一种调节模型一致，在该模型中，硫氢基加成过程介导了进入完全具有催化活性的活性位点的途径，氧化转移反应被迫利用正常的进出机制。
