The reactivity of lactyl-oxythiamin implies the role of the amino-pyrimidine in thiamin catalyzed decarboxylation
作者:Yasaman Heidari、Graeme W. Howe、Ronald Kluger
DOI:10.1016/j.bioorg.2016.10.008
日期:2016.12
enzymes. The amino group has also been implicated in assisting the departure of the aldehydic product formed after loss of CO2 from ketoacid substrates. However, the potential role for the pyrimidine amino group in the key decarboxylation step has not been assessed. Oxythiamin contains a hydroxyl group in place of the pyrimidine amino group in thiamin, providing a basis for comparison of reactivity. Lactyl-oxythiamin
先前已经确定,硫胺素二磷酸的嘧啶(ThDP)的嘧啶的去质子化的氨基取代基充当内部碱基,以在ThDP依赖性酶中接受噻唑鎓的C 2H。氨基也与协助从酮酸底物上损失掉CO2之后形成的醛产物的离开有关。然而,尚未评估嘧啶氨基在关键脱羧步骤中的潜在作用。硫胺素在硫胺素中取代了嘧啶氨基而含有羟基,为比较反应性提供了基础。通过丙酮酸乙酯和羟基保护的羟硫胺的缩合反应,然后对该乙酯进行脱保护和酸水解,制得了丙酮酸和羟硫胺酸的共轭物-乳酸-氧硫胺素(LOTh)。在中性和酸性溶液中观察到的LOTh脱羧速率常数约为相应的含有氨基乳酰硫胺素的化合物的速率常数的四倍。反应性的差异与氨基通过允许竞争性加成途径产生碳酸氢根并影响相应的酶促反应而参与促进脱羧步骤的过程一致。