Structure and Evolution of Prokaryotic Cell Envelopes

B acterial cytoplasmic membranes consist primarily of amphipathic phospholipids with lesser amounts of glycolipids, providing a degree of protection from deleterious, toxic substances that is lacking in organisms having a single membrane. Almost all bacterial membranes are assembled in bilayers with embedded integral and associated peripheral membrane proteins. In the case of gram-negative bacterial outer membranes, phospholipids are present in the inner leaflet of the bilayer, while lipopolysaccharides predominate in the outer leaflet. Carbohydrates in lipopolysaccharides and in glycolipids and glycoproteins usually extend outwards. Meanwhile, many gram-positive bacteria, particularly firmicutes and mollicutes with genomes of low or moderate G C content, lack an outer membrane. However, acid-fast, high G C gram-positive bacteria such as mycobacteria and corynebacteria have mycolic acid-containing outer membranes that are structurally very different from the outer membranes of gramnegative bacteria. Some archaea contain membranes of very different protein and lipid constituents, typically consisting of hydrophobic tails linked by ether rather than ester bonds to the glycerol-containing lipid backbone. A few archaea have complex envelopes which, like those in some bacteria, consist of inner and outer membranes that differ in their lipid and protein constituents. These prokaryotic outer membranes are so different as to suggest independent evolutionary origins.