Phosphorylated and Dephosphorylated Structures of Pig Heart, GTP-specific Succinyl-CoA Synthetase

Succinyl-CoA synthetase (SCS) catalyzes the reversible phosphorylation/dephosphorylation reaction:¿¿¿rm succinyl ¿hbox ¿-¿CoA+NDP+P_i¿leftrightarrow succinate+CoA+NTP¿¿where N denotes adenosine or guanosine. In the course of the reaction, an essential histidine residue is transiently phosphorylated. We have crystallized and solved the structure of the GTP-specific isoform of SCS from pig heart (EC 6.2.1.4) in both the dephosphorylated and phosphorylated forms. The structures were refined to 2.1 A resolution. In the dephosphorylated structure, the enzyme is stabilized via coordination of a phosphate ion by the active-site histidine residue and the two "power" helices, one contributed by each subunit of the alphabeta-dimer. Small changes in the conformations of residues at the amino terminus of the power helix contributed by the alpha-subunit allow the enzyme to accommodate either the covalently bound phosphoryl group or the free phosphate ion. Structural comparisons are made between the active sites in these two forms of the enzyme, both of which can occur along the catalytic path. Comparisons are also made with the structure of Escherichia coli SCS. The domain that has been shown to bind ADP in E. coli SCS is more open in the pig heart, GTP-specific SCS structure.