School of Chemistry and Chemical Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast, BT9 5AG, Northern Ireland, UK.
Almac Sciences, Department of Biocatalysis and Isotope Chemistry, Almac House, 20 Seagoe Industrial Estate, Craigavon, BT63 5QD, Northern Ireland, UK.
Chemistry. 2018 Apr 6;24(20):5246-5252. doi: 10.1002/chem.201704622. Epub 2017 Dec 14.
A covalently bound flavin cofactor is predominant in the succinate-ubiquinone oxidoreductase (SQR; Complex II), an essential component of aerobic electron transport, and in the menaquinol-fumarate oxidoreductase (QFR), the anaerobic counterpart, although it is only present in approximately 10 % of the known flavoenzymes. This work investigates the role of this 8α-N3-histidyl linkage between the flavin adenine dinucleotide (FAD) cofactor and the respiratory Complex II. After parameterization with DFT calculations, classical molecular-dynamics simulations and quantum-mechanics calculations for Complex II:FAD and Complex II:FADH , with and without the covalent bond, were performed. It was observed that the covalent bond is essential for the active-center arrangement of the FADH /FAD cofactor. Removal of this bond causes a displacement of the isoalloxazine group, which influences interactions with the protein, flavin solvation, and possible proton-transfer pathways. Specifically, for the noncovalently bound FADH cofactor, the N1 atom moves away from the His-A365 and His-A254 residues and the N5 atom moves away from the glutamine-62A residue. Both of the histidine and glutamine residues interact with a chain of water molecules that cross the enzyme, which is most likely involved in proton transfer. Breaking this chain of water molecules could thereby compromise proton transfer across the two active sites of Complex II.
一个共价结合的黄素辅因子在琥珀酸-泛醌氧化还原酶（SQR；复合物 II）中占主导地位，这是需氧电子传递的必需组成部分，在menaquinol-延胡索酸氧化还原酶（QFR）中也是如此，尽管它仅存在于大约 10%的已知黄素酶中。这项工作研究了这个 8α-N3-组氨酸键在黄素腺嘌呤二核苷酸（FAD）辅因子和呼吸复合物 II 之间的作用。在使用 DFT 计算进行参数化后，对具有和不具有共价键的复合物 II:FAD 和复合物 II:FADH 进行了经典分子动力学模拟和量子力学计算。结果表明，共价键对于 FADH/FAD 辅因子的活性中心排列是必不可少的。去除这个键会导致异咯嗪基团的位移，这会影响与蛋白质的相互作用、黄素的溶剂化和可能的质子转移途径。具体来说，对于非共价结合的 FADH 辅因子，N1 原子会远离 His-A365 和 His-A254 残基，N5 原子会远离 Gln-62A 残基。两个组氨酸和谷氨酰胺残基都与穿过酶的水分子链相互作用，这很可能参与质子转移。打破这个水分子链可能会破坏复合物 II 的两个活性位点之间的质子转移。
