Interactions of GTP with the ATP-grasp Domain of GTP-specific Succinyl-CoA Synthetase

Two isoforms of succinyl-CoA synthetase exist in mammals, one specific for ATP and the other for GTP. The GTP-specific form of pig succinyl-CoA synthetase has been crystallized in the presence of GTP and the structure determined to 2.1 Å resolution. GTP is bound in the ATP-grasp domain, where interactions of the guanine base with a glutamine residue (Gln-20β) and with backbone atoms provide the specificity. The γ-phosphate interacts with the side chain of an arginine residue (Arg-54β) and with backbone amide nitrogen atoms, leading to tight interactions between the γ-phosphate and the protein. This contrasts with the structures of ATP bound to other members of the family of ATP-grasp proteins where the γ-phosphate is exposed, free to react with the other substrate. To test if GDP would interact with GTP-specific succinyl-CoA synthetase in the same way that ADP interacts with other members of the family of ATP-grasp proteins, the structure of GDP bound to GTP-specific succinyl-CoA synthetase was also determined. A comparison of the conformations of GTP and GDP shows that the bases adopt the same position but that changes in conformation of the ribose moieties and the α- and β-phosphates allow the γ-phosphate to interact with the arginine residue and amide nitrogen atoms in GTP, while the β-phosphate interacts with these residues in GDP. The complex of GTP with succinyl-CoA synthetase shows that the enzyme is able to protect GTP from hydrolysis when the active-site histidine residue is not in position to be phosphorylated.