Bacterial sulfite-oxidizing enzymes.

Enzymes belonging to the Sulfite Oxidase (SO) enzyme family are found in virtually all forms of life, and are especially abundant in prokaryotes as shown by analysis of available genome data. Despite this fact, only a limited number of bacterial SO family enzymes has been characterized in detail to date, and these appear to be involved in very different metabolic processes such as energy generation from sulfur compounds, host colonization, sulfite detoxification and organosulfonate degradation. The few characterized bacterial SO family enzymes also show an intriguing range of structural conformations, including monomeric, dimeric and heterodimeric enzymes with varying numbers and types of redox centres. Some of the bacterial enzymes even catalyze novel reactions such as dimethylsulfoxide reduction that previously had been thought not to be catalyzed by SO family enzymes. Classification of the SO family enzymes based on the structure of their Mo domain clearly shows that three distinct groups of enzymes belong to this family, and that almost all SOEs characterized to date are representatives of the same group. The widespread occurrence and obvious structural and functional plasticity of the bacterial SO family enzymes make this an exciting field for further study, in particular the unraveling of the metabolic roles of the three enzyme groups, some of which appear to be associated almost exclusively with pathogenic microorganisms.