Regulatory role of GDP in the phosphoenzyme formation of guanine nucleotide: specific forms of succinyl coenzyme A synthetase.

We have previously shown that micromolar concentrations of GDP stimulate the GTP-mediated phosphorylation of p36, the alpha subunit of succinyl-CoA synthetase (SCS), in lysates prepared from Dictyostelium discoideum. In this study, we report that this phenomenon represents an enhanced catalytic capacity of SCS to form the phosphoenzyme intermediate. Low concentrations of GDP stimulate phosphoenzyme formation by either GTP, or succinyl-CoA and P(i). Under these conditions GDP enhances the apparent rate of phosphoenzyme formation but does not significantly alter the fraction of phosphorylated enzyme. This effect is retained during purification of the protein and is also observed with purified pig heart SCS, indicating that GDP directly alters the enzyme to enhance its rate of phosphorylation. Under these conditions, GDP does not function at the catalytic site, implying an allosteric regulation of SCS.