The Metabolism of Sulfate

Of the elements required by living systems, carbon, nitrogen, and sulfur undergo extensive metabolic transformations. Sulfur, unlike carbon and nitrogen, can be utilized to some extent in its most highly oxidized naturally occurring form, sulfate; sulfate reduction is necessary for the formation of sulfur-containing amino acids and proteins (35). Figure 1 shows the relation of sulfate to the many reactions which sulfur undergoes in the biosphere. Sulfate is utilized by living systems to form sulfate esters of polysaccharides, phenols, steroids, and other organic compounds through a series of activation and transfer reactions which will be discussed sub­ sequently. It is also reduced by certain anaerobic organisms such as the bac­ terium Desulfovibrio in their energy metabolism where the process is called dis­ similatory sulfate reduction and the product is hydrogen sulfide. Many organisms reduce sulfate to the thiol level found in the amino acids cysteine and methio­ nine, and these serve as building blocks for proteins; this process is called assimiCopyright 1973. All rights reserved