by Howard Goldfine
Some biologists go blissfully through life without paying much attention to lipids. They do this at their own risk, because there are innumerable things to be learned from their study, including, as we will see here, many relevant to the understanding of evolution. Lipids come in unexpected and exciting varieties, a point that has been acknowledged in this blog (for examples, see here and here)
The lipids that make up the membranes of prokaryotes are polar, that is, they have a moiety such as phosphate, linked to one of the carbons of their glycerol backbone (non-polar triglycerides are generally not known to be made by prokaryotes). The lipids of aerobic and facultative bacteria are mainly of the phospholipid or glycolipid type, in which the first two carbons of the glycerol backbone are linked to long-chain fatty acid esters (Figure). The situation, however, is different in many anaerobes, including both Gram-positive and Gram-negative species, in which the membranes contain both diacyl lipids and compounds known as plasmalogens, in which the chain linked to the first carbon of the glycerol is attached through an O-alk-1’-enyl ether bond (Figure) rather than an ester bond. Plasmalogens were accidentally discovered by Robert Feulgen in 1924. He observed that the cytosol of animal cells turned red when stained with a colorless fuchsin-sulfurous acid reagent, aka known as the Schiff reagent. Such staining reveals compounds that contain aldehyde groups. The red color did not appear if Feulgen’s preparations were first treated with alcohol. He called these compounds plasmalogens meaning “aldehyde-forming substances found in the plasma.” It wasn’t until the 1950s that the correct structure of these ether lipids was worked out. Feulgen and others found plasmalogens in many animal tissues; they are present in especially high concentrations in the brain, CNS, and muscles. Indeed, >60% of the phosphatidylethanolamine (PtdEtn) of our brain is in the plasmalogen form.