The Phosphoglycerate Kinase Reaction and its Metal Ion Specificity

The present study shows that MgATP 2‐ or MnATP 2‐ as substrates of phosphoglycerate kinase can be replaced by CaATP 2‐ , ZnATP 2‐ , CoATP 2‐ or NiATP 2‐ . MnATP 2‐ and NiATP 2‐ are about 90% and 15%, respectively, and the other ATP 4‐ ‐metal ion complexes roughly 70% as good substrates as MgATP 2‐ . No measurable activity was found with Be(II) or Fe(III). The effectiveness of the different substrate species appears to be determined by factors such as the size and polarizing capability of the metal ion as well as of the structure of the relevant metal ion complex and the rate of ligand dissociation processes. Detailed kinetics with Zn(II), Mn(II) and Co(II) showed that: (a) Zn 2+ is a strong uncompetitive inhibitor of ZnATP 2‐ , K i approx. 0.02 mM. (b) Mn 2+ is a competitive inhibitor of MnATP 2‐ at concentrations < 0.1 mM, K i approx. 2.3 mM. This inhibition is dependent on the 3‐ P ‐glycerate concentration. At about 1 mM and higher concentrations Mn 2+ acts as an uncompetitive inhibitor of MnATP 2‐ . (c) Co 2+ is a competitive inhibitor of CoATP 2‐ at about 1 mM and higher concentrations, K i approx. 3 mM. With CoATP 2‐ as substrate the activity is slightly increased in the presence of free Co 2+ and/or free ATP 4‐ at concentrations < 0.5 mM. When the CoATP 2‐ concentration is varied, the activity seems not to become constant at concentrations ten times the apparent Michaelis constant for CoATP 2‐ . Equilibrium dialysis studies on the binding of Co 2+ to the enzyme showed that this ion binds more strongly to the enzyme than, for example, Mn 2+ . This probably explains the differences in activation and inhibition observed with these two ions. The kinetic patterns obtained with the different metal ions indicate that Ni(II) behaves as Co(II), Cd(II) as Zn(II) and Ca(II) as Mg(II), which is somewhat similar in behaviour to Mn(II).