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Abstract：
Malate dehydrogenase (MDH) and citrate synthase (CS) are two pacemaking enzymes involved in the tricarboxylic acid (TCA) cycle. Oxaloacetate (OAA) molecules are the intermediate substrates that are transferred from the MDH to CS to carry out sequential catalysis. It is known that, to achieve a high flux of intermediate transport and reduce the probability of substrate leaking, a MDH-CS metabolon forms to enhance the OAA substrate channeling. In this study, we aim to understand the OAA channeling within possible MDH-CS metabolons that have different structural orientations in their complexes. Three MDH-CS metabolons from native bovine, wild-type porcine, and recombinant sources, published in recent work, were selected to calculate OAA transfer efficiency by Brownian dynamics (BD) simulations and to study, through electrostatic potential calculations, a possible role of charges that drive the substrate channeling. Our results show that an electrostatic channel is formed in the metabolons of native bovine and recombinant porcine enzymes, which guides the oppositely charged OAA molecules passing through the channel and enhances the transfer efficiency. However, the channeling probability in a suggested wild-type porcine metabolon conformation is reduced due to an extended diffusion length between the MDH and CS active sites, implying that the corresponding arrangements of MDH and CS result in the decrease of electrostatic steering between substrates and protein surface and then reduce the substrate transfer efficiency from one active site to another.
摘要：
苹果酸脱氢酶(MDH)和柠檬酸合酶(CS)是参与三羧酸(TCA)循环的两种起搏酶。草酰乙酸（OAA）分子是从MDH转移到CS以进行顺序催化的中间底物。众所周知,为了实现中间传输的高通量并降低衬底泄漏的可能性，MDH-CS代谢物形成以增强OAA底物通道。在这项研究中,我们的目的是了解在复合物中具有不同结构取向的可能的MDH-CS代谢物中的OAA通道。来自天然牛的三种MDH-CS代谢物，野生型猪,和重组来源，发表在最近的工作，选择通过布朗动力学(BD)模拟来计算OAA转移效率，通过静电势计算，驱动衬底沟道的电荷的可能作用。我们的结果表明，在天然牛和重组猪酶的代谢产物中形成静电通道，引导带相反电荷的OAA分子通过通道并提高转移效率。然而,由于MDH和CS活性位点之间的扩散长度延长，建议的野生型猪代谢子构象中的通道形成概率降低，这意味着MDH和CS的相应排列导致底物和蛋白质表面之间的静电转向减少，然后降低了从一个活性位点到另一个活性位点的底物转移效率。
