FIG. 3.

Suggested pathway for the metabolism of carnitine to γ-butyrobetaine in anaerobic E. coli cultures. Carnitine imported into the cells is converted to its carnitinyl-CoA thioester by CaiB in a transfer reaction with γ-butyrobetainyl-CoA that releases γ-butyrobetaine. CaiD converts carnitinyl-CoA to crotonobetainyl-CoA in a dehydration reaction, and the latter is reduced to γ-butyrobetainyl-CoA by CaiA. Electrons for this reaction are postulated to be contributed by the FixABCX proteins, which are reduced during the oxidation of other substrates available to the cell.