Sulfur Cycle Enzymes

Many sulfur (S) transformation reactions are catalyzed by enzymes released into the soil environment by microorganisms, plant roots, and soil fauna. This chapter summarizes the biogeochemical cycling of organic and gaseous S compounds in soil catalyzed by some of these enzymes. The various pathways of hydrogen sulfide production from the amino acids cysteine and cystine can involve several enzymes. One mechanism studied in soils, and reported by Morra and Dick, is catalyzed by enzymes called cystathionine lyases. These enzymes belong to the family of lyases and specifically to the class of carbon-sulfur lyases. Ester sulfates represent an important fraction of total organic S in soil, and are considered to be the most labile form of organic S in soil. Sulfatases (sulfohydrolases) are enzymes of the esterase class that catalyze the hydrolysis of ester sulfates, where the linkage with sulfate is in the form of R–O–S and R represents a diverse group of organic moieties.