Charge-Transfer Interactions of Flavoenzymes

采用x射线晶体学、13C-NMR和电子吸收等方法研究了黄酮酶酰基辅酶a脱氢酶（ACD）与底物类似物3-硫代酰基辅酶a之间的电荷转移（CT）相互作用。与底物不同的是，类似物在酶活性位点提取α-质子后抑制氢化物转移过程。用密度泛函理论（DFT）计算了由光黄素和3-硫代酸去质子化乙硫酯构建的最简单模型。优化后的模型复合物的结构与x射线晶体学测定的排列非常相似。模型对13C化学位移和电子激发能的模拟结果与观测值吻合较好。该配合物的最高占据分子轨道（HOMO）表明，成键轨道是在类似物的S(3)原子和黄素环的N5原子之间产生的。Mulliken种群分析评估从阴离子类似物到氧化黄素的电荷转移量为-0.28个电荷。S(3)和N5原子之间的成键轨道类似于底物氧化还原反应中的氢化物转移过程；因此3-硫代酰基辅酶a被认为是过渡态类似物。对ACD与黄酮类酶的CT相互作用的研究为DFT计算的有效性提供了依据。进一步的研究适用于ACD反应的整个过程和其他黄酶。

The charge-transfer (CT) interaction between flavoenzyme acyl-CoA dehydrogenase (ACD) and substrate analog, 3-thiaacyl-CoA, was investigated by the combination of the X-ray crystallography, the spectroscopy (13C-NMR and electronic absorption) and the theoretical treatments. Different from the substrate, the analog inhibits the hydride transfer process after the α-proton is abstracted in the active site of the enzyme. The density functional theory (DFT) calculations were applied to the simplest model constructed with lumiflavin and deprotonated ethylthioester of 3-thiabtanoic acid. The optimized structure of the model complex is quite similar to the alignments determined by the X-ray crystallography. And the simulations of the 13C chemical shifts and the electronic excitation energy of the model are coincident with the observed ones. The highest occupied molecular orbital (HOMO) of the complex revealed that the bonding orbital is generated between the S(3) atom of the analog and the N5 atom of the flavin ring. Mulliken population analysis evaluates the quantity of the charge transfer from anionic analog to oxidized flavin as to be -0.28 charges. The bonding orbital between the S(3) and N5 atoms is analogous to the hydride transfer process in the case of the redox reaction of a substrate; therefore 3-thiaacyl-CoA is regarded as a transition-state analog. The research on the CT interaction of ACD provided the validity of the DFT calculations to the flavoenzymes. Further researches are applicable to the entire process of the ACD reaction and the other flavoenzymes.