Second distinct conformation of the phosphohistidine loop in succinyl-CoA synthetase

Succinyl-CoA synthetase (SCS) catalyzes a reversible reaction that is the only substrate-level phosphorylation in the citric acid cycle. One of the essential steps for the transfer of the phosphoryl group involves the movement of the phosphohistidine loop between active site I, where CoA, succinate and phosphate bind, and active site II, where the nucleotide binds. Here, the first crystal structure of SCS revealing the conformation of the phosphohistidine loop in site II of the porcine GTP-specific enzyme is presented. The phosphoryl transfer bridges a distance of 29 Å between the binding sites for phosphohistidine in site I and site II, so these crystal structures support the proposed mechanism of catalysis by SCS. In addition, a second succinate-binding site was discovered at the interface between the α- and β-subunits of SCS, and another magnesium ion was found that interacts with the side chains of Glu141β and Glu204β via water-mediated interactions. These glutamate residues interact with the active-site histidine residue when it is bound in site II.