The Effect of the Viscosity of a Trehalose Solution on ATP Hydrolysis by Chloroplast F1-ATPase

Resumo

The effect of trehalose solution viscosity on the kinetics of ATP hydrolysis by CF₁-ATPase (pea chloroplast F₁-ATPase) was studied. Trehalose added to the reaction mixture had a double effect: the Mg-dependent enzyme activity was stimulated at trehalose levels below 20 wt % and suppressed at higher trehalose levels. The Ca-dependent activity of CF₁ decreases monotonically with the increasing trehalose level. It was shown that stimulation of the Mg-dependent enzyme activity was a result of diminished Mg-ADP-dependent enzyme inactivation. In the case of negligible inactivation, the elevated trehalose content caused a decrease in the maximum rate of ATP hydrolysis and the apparent Michaelis constant increased. The changes in the values of the Michaelis constant and enzyme activity indicate that the delivery of reaction substrates and the conformational changes in CF₁-ATPase accompanying the hydrolysis are impeded by the viscosity of the medium. In accordance with Cramer’s rule, the energy loss due to interaction with the medium leads to the conclusion that the efficiency of energy conversion by CF₁-ATPase never reaches 100%.