Molecular properties of membrane lipids and activity of a membrane adenosine triphosphatase from Acholeplasma laidlawii B.

The effects of membrane lipid fluidity and phase state on enzyme activity were evaluated by careful analysis of Arrhenius plots of the Na+, Mg2+-dependent adenosine triphosphatase (referred to hereafter as ATPase) from Acholeplasma laidlawii B membranes and differential thermal analysis of the membrane lipid phase transitions. Arrhenius plots of the ATPase activity in membranes containing only liquid-crystalline lipids are gently curved and have the same form regardless of the membrane fatty liver composition. The absolute ATPase activity at temperatures well above the lipid phase transition temperature varies in a complex fashion with fatty acid composition. As the membrane enters its lipid phase transition, ATPase activity begins to fall off more rapidly than it does above the transition. Quantitative analysis of the results suggests that ATPase is reversibly inactivated when its vicinal lipids undergo a transition to a state of reduced plasticity at low temperatures. This transition is driven by the conversion of the bulk membrane lipid phase to the gel state, but it is less cooperative and occurs at lower temperatures than the bulk lipid transition. The results suggest that the familial "biphasic linear" Arrhenius plots reported for many membrane enzymes and transport systems may, in fact, have more complex shapes, analysis of which can furnish useful information about the behavior of the enzyme molecule or transport system in its membrane environment.