Sulfonates and organotrophic sulfite metabolism

One is used to considering sulfite oxidation as part of a lithotrophic process (e.g. SorAB or Sox system), much of which involves neutral or ionic inorganic sulfur species on the outer surface of the cytoplasmic membrane. In contrast, the processes referred to in this chapter involve organic compounds, which (1) include a highly stable sulfonate substituent (C-SO3-) (2) are involved in the organotrophic growth of the organism and (3) much of whose metabolism takes place in the cytoplasm. Many phenomena are associated with this life-style. The sulfonate may be a natural product, e.g. taurine or sulfoquinovose, whose synthesis can involve sulfite, or a xenobiotic laundry detergent, but it is effectively always a charged species, so an uptake system is essential. Two known systems are mentioned, ATP binding cassette transporters and tripartite ATP-independent periplasmic transporters. Annual dissimilation of megatonnes of organosulfonates essentially always involves intracellular sulfite generated by diverse enzymic cleavages in bacteria, archaea and possibly eukarya. The fate of this sulfite in anaerobes is often sulfide. Aerobes occasionally excrete sulfite directly; more frequently, the sulfite is oxidized to sulfate. Many aerobic bacteria excrete only sulfate, but many others excrete also some sulfite, which is rapidly oxidized to sulfate, even under anoxic conditions. The nature and location of these sulfite dehydrogenases are still unclear, but periplasmic SorAB is apparently used by some bacteria. In contrast to the cytochrome c coupled SorAB, there is at least one widespread, uncharacterized sulfite dehydrogenase, which is assayed with ferricyanide as the electron acceptor. Dissimilation of a sulfonate releases sulfite (sulfate) in about 500-fold excess of the sulfur requirement for growth, so exporters, two classes of which have been detected, are essential to prevent the cell from suffering osmotic stress.