Membrane Lipid Degradation and Lipid Cycles in Microbes

Abstract

All living cells are delimited from the exterior world by a membrane, and membrane-forming lipids are the structural determinants for membrane assembly and maintenance. Although biosynthesis of membrane-forming lipids is well understood in many organisms, turnover, degradation, and remodeling of these lipids are less studied. An initial degradation of glycerol-containing membrane lipids may occur by (phospho)lipases or transferases which remove distinct groups from the membrane lipid converting it into a lysolipid or diacylglycerol. These degradation intermediates can either be totally degraded into low-molecular-weight metabolites or missing groups can be reintroduced onto the intermediates to convert them into fully functional membrane lipids again, thereby completing a lipid cycle. Classic examples in Escherichia coli are the lyso-phosphatidylethanolamine cycle, the diacylglycerol cycle, or cycles involving the isoprenoid undecaprenol. It is evident that many more lipid cycles exist in other proteobacteria and in gram-positive bacteria and that these cycles play major roles in decorating biomolecules located outside the cytoplasmic compartment.