Membrane lipid physical state and modulation of the Na+,Mg2+-ATPase activity in Acholeplasma laidlawii B.

Abstract

Careful analysis of the Arrhenius plot of the Na+,Mg2+-ATPase (ATP pyrophosphohydrolase, EC 3.6.1.8) activity in A. laidlawii B membranes of varying fatty acid composition was combined with differential thermal analysis of the membrane lipid phase transitions to evaluate the effects of membrane lipid properties on the enzyme activity. The enzyme is active only in association with liquid-crystalline lipids, exhibiting a significant heat capacity of activation, .**GRAPHIC**. [heat capacity of activation], for the ATP hydrolytic reaction in this case. Quantitative analyses of Arrhenius plots for the enzyme activity in membranes whose lipids exhibit a gel-to-liquid-crystalline phase transition in the physiological temperature range suggest that the ATPase is inactivated when its boundary lipids undergo a phase transition that is driven by the bulk lipid phase transition but is less cooperative than the latter. The familiar biphasic linear Arrhenius plots obtained for many membrane enzymes may in fact have a more complex shape, analysis of which can furnish useful information regarding the behavior of the enzyme molecule.