An exchange-inert Rh(H2O)4ATP complex showed a one-turnover substrate activity with yeast phosphoglycerate kinase. Transfer of the phosphoryl group between ATP and 3-phosphoglycerate occurs with both substrates in the coordination sphere of the metal ion. Because of the slow ligand exchange rates of Rh3+, the reaction product 1,3-diphosphoglycerate (1,3-dPGA) remained coordinated to the metal ion. During the course of the reaction, the enzyme was inactivated, suggesting that the metal ion is coordinated to a protein side chain. Thus the product Rh(H2O)nADP.1,3-dPGA remained bound to the enzyme even after removal of excess substrate. These results suggested that the metal ion may not only act as an electron sink to activate the electrophile, but it may also help to optimally align both substrates for phosphoryl transfer by coordination to both substrates. It is therefore likely that entry of 3-phospho-D-glycerate into the coordination sphere of metal of a metal-ATP complex may start the proposed hinge-bending motion of yeast phosphoglycerate kinase to form a "closed" active site between the two substrate binding domains of the enzyme.