B12-Dependent Reactions in Enteric Bacteria

      The enzymes listed below are found in one or more enteric bacteria, and all of them except glycerol dehydratase are found in S. enterica. The reactions catalyzed by each of these enzymes are diagrammed in Figure 1-2. A more detailed description of their role in metabolism is reserved for later.

Propanediol dehydratase

      Diol dehydratase is one of the best-studied Ado-B12-dependent enzymes. Research on this enzyme, which is found in nearly all enteric bacteria tested except E. coli (Toraya et al. 1979, Lawrence and Roth 1996), has provided a wealth of information on the mechanisms of Ado-B12-dependent reactions (Toraya 2002). Ado-B12-dependent diol dehydratase is the initiating enzyme in the degradation of PD where it catalyzes the conversion of PD to propionaldehyde (Abeles and Lee Jr. 1961). All enterics possessing this enzyme are capable of aerobically respiring PD, which provides both an energy source via substrate level phosphorylation and a three-carbon compound that can enter central metabolism via the 2-methylcitric acid pathway (Horswill and EscalanteSemerena 1997, Tsang et al. 1998). Only some of the enterics ferment PD by first converting it to propionaldehyde and then subsequently dismutating it to propionate and propanol (Figure 1-2). In Salmonella this process produces a source of energy from substrate-level phosphorylation but no source of carbon since propanol is excreted in order to get rid of excess reducing equivalents (Price-Carter et al. 2001). Recently it has been found that the use of tetrathionate as a terminal electron acceptor allows PD to be used as both an energy and carbon source under anaerobic conditions (Price-Carter et al. 2001).