Lipid Phase Transitions and Regulation of Membrane Fluidity in Prokaryotes

Abstract

Publisher Summary Normal cell function requires fluid membrane bilayers. In addition to selecting mixtures that form stable bilayers for their membrane lipids, prokaryotes maintain their membranes in a fluid state under varying environmental conditions. This chapter discusses the reason for the cells requirement of fluid membranes; and presents some of the strategies they use to maintain their membranes in a fluid state. The roles different lipid classes play in membrane function is considered from the perspective of their physical behavior. The properties of membrane lipids in water are also described. Because of the hydrophobic effect, lipid molecules aggregate in water to minimize the contact of nonpolar regions with water while allowing the contact of head groups with water. The membrane bilayers of most organisms are entirely or mostly fluid at physiological temperatures, but at lower temperatures undergo a major reversible change of state. Membrane bilayers are composed of many different lipids and melt over a wide range of temperatures. Membranes from a variety of prokaryotes crystallized at lower than physiological temperature. The lipid molecules act to keep bilayers in a stable state despite changes in environmental conditions, but are not themselves bilayer formers. The functions of the membrane bilayer as acceptors and sinks are interrelated; and the two functions provide a sensitive, unified means to control the fatty acid composition and hence the physical state of biological membranes, whether fatty acids are supplied exogenously in the growth medium or are endogenously biosynthesized by the organism.