The evolution of oxygen as a biosynthetic reagent.

THE BIOSYNTHESIS OF CERTAIN CELL CONSTITUENTS

monounsaturated fatty acids, tyrosine, and nicotinic acid, is oxygen-dependent in many higher organisms. The same compounds can be synthesized by different, oxygen-independent pathways in lower organisms. The general outlines of these pathways are described and the importance of the compounds synthesized is discussed. An examination of the distribution of these pathways among living organisms reveals that oxygen-dependent pathways replaced the "anaerobic" pathways at different branch points on the evolutionary tree. Other groups of compounds are discussed, which are not distributed as widely among living organisms, but are found in all higher organisms. These compounds have specialized functions and their biosynthesis requires molecular oxygen. The oxygen-dependent portions of the biosynthetic pathways leading to porphyrins, quinone coenzymes, carotenoids, sterols, and polyunsaturated fatty acids are summarized. The distribution and functions of these compounds are also considered and an attempt is made to place them in the framework of evolution. While sterols and polyunsaturated fatty acids are found exclusively in the higher Protista and multicellular organisms, carotenoids, porphyrins, and quinones are also found in bacteria. The possibility of oxygen-independent mechanisms for their biosynthesis is discussed.