Why the Flavin Adenine Dinucleotide (FAD) Cofactor Needs To Be Covalently Linked to Complex II of the Electron-Transport Chain for the Conversion of FADH into FAD.

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.