Enzymology and molecular biology of prokaryotic sulfite oxidation

Despite its toxicity, sulfite plays a key role in oxidative sulfur metaboli sm and there are even some microorganisms which can use it as sole electron source. Sulfite is the main intermediate in the oxidation of sulfur compou nds to sulfate, the major product of most dissimilatory sulfur-oxidizing pr okaryotes. Two pathways of sulfite oxidation are known: (1) direct oxidatio n to sulfate catalyzed by a sulfite: acceptor oxidoreductase, which is thou ght to be a molybdenum-containing enzyme; (2) indirect oxidation under the involvement of the enzymes adenylylsulfate (APS) reductase and ATP sulfuryl ase and/or adenylylsulfate phosphate adenylyltransferase with APS as an int ermediate. The latter pathway allows substrate phosphorylation and occurs i n the bacterial cytoplasm. Direct oxidation appears to have a wider distrib ution; however, a redundancy of pathways has been described for diverse pho to- or chemotrophic, sulfite-oxidizing prokaryotes. In many pro- and also e ukaryotes sulfite is formed as a degradative product from molecules contain ing sulfur as a heteroatom. In these organisms detoxification of sulfite is generally achieved by direct oxidation to sulfate. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.