Hydrolysis of adenosine 5'-triphosphate by Escherichia coli GroEL: effects of GroES and potassium ion

Abstract

The potassium-ion activation constant (Kact) for the ATPase activity of Escherichia coli chaperonin groEL is inversely dependent upon the ATP concentration over at least 3 orders of magnitude. The ATPase activity shows positively cooperative kinetics with respect to ATP and K+. Both the K0.5 for ATP and cooperativity (as measured by the Hill coefficient) decrease as the K+ concentration increases. Equilibrium binding studies under conditions where hydrolysis does not occur indicate that MgATP binds weakly to groEL in the absence of K+. In the absence of groES, the K(+)-dependent hydrolysis of ATP by groEL continues to completion. In the presence of groES, the time course for the hydrolysis of ATP by groEL becomes more complex. Three distinct kinetic phases can be discerned. Initially, both heptameric toroids turn over once at the same rate that they do in the absence of groES. This leads to the formation of an asymmetric binary complex, groEL14-MgADP7-groES7, in which 7 mol of ADP is trapped in a form that does not readily exchange with free ADP. In the second phase, the remaining seven sites (containing readily exchangeable ADP) turn over, or have the potential to turn over, at the same rate as they do in the absence of groES, so that the overall rate of hydrolysis is maximally 50%. These remaining sites of the asymmetric binary complex do not hydrolyze all of the available ATP. Instead, the second phase of hydrolysis gives way to a third, completely inhibited state, the onset of which is dependent upon the relative affinities of the remaining sites for MgATP and MgADP.(ABSTRACT TRUNCATED AT 250 WORDS)