The pathway of PD degradation in S. enterica

      The degradation of PD has been investigated (Abeles and Lee Jr. 1961, Obradors et al. 1988). The putative pathway is shown in Figure 1-5. The process is initiated by the conversion of PD to propionaldehyde, mediated by the B12-dependent enzyme, diol dehydratase. Currently it is thought that propionaldehyde is subsequently catabolized to propionic acid by CoA-dependent aldehyde dehydrogenase, phosphotransacylase, and propionate kinase; and propanol by alcohol dehydrogenase (Toraya et al. 1979, Obradors et al. 1988). This pathway provides a source of ATP, an electron sink, and 3-carbon compounds which can be channeled to central metabolism via the 2-methyl-citrate pathway (Horswill and Escalante-Semerena 1997, Tsang et al. 1998). In the 2-methylcitrate pathway, propionyl-CoA is joined to oxaloacetate, which is then subsequently converted to succinate and pyruvate. Pyruvate is then converted to acetyl- CoA and enters the TCA cycle (Witt et al. 1994, Horswill and Escalante-Semerena 1997, Textor et al. 1997, Walter et al. 1997, Tsang et al. 1998). Under anaerobic conditions PD does not serve as a sole carbon and energy source for S. enterica. In theory, energy could be provided by the conversion of propionyl-phosphate to propionate and ATP and reducing power could be regenerated by the concomitant reduction of propionaldehyde to propanol which is subsequently excreted. However, in Salmonella fermentation of PD does not provide a carbon source, and anaerobic growth on this small molecule only occurs when dilute yeast extract is included in the growth medium (Price-Carter et al. 2001). Until recently, this phenomenon seemed paradoxical as S. enterica only synthesizes B12 de novo under anaerobic conditions. The discovery that tetrathionate could be used as a terminal electron acceptor that would support