Biosynthesis of 5-aminolevulinic acid in Methanobacterium thermoautotrophicum

The pathway of 5-aminolevulinic acid (ALA) synthesis in growing cells of Methanobacterium thermoautotrophicum was studied. Advantage was taken of the fact that this anaerobic archaebacterium excretes ALA into the medium when growing in the presence of levulinic acid, which specifically inhibits the biosynthesis of tetrapyrroles at the level of ALA dehydratase. It was found that [1,4-14C]-succinate rather than [2-14C]glycine is incorporated into ALA. Evidence is provided that succinate incorporation into ALA probably proceeds via 2-oxoglutarate, which in M. thermoautotrophicum is synthesized from succinyl-CoA by reductive carboxylation. The data are consistent with the operation of the C-5 pathway for ALA synthesis and exclude the Shemin pathway in this methanogenic bacterium. The presence of l-alanine: 4,5-dioxovaleric acid aminotransferase activity and the apparent absence of ALA synthase in cell extracts support this conclusion.