The enigma of cobalamin (vitamin B-12) biosynthesis in Porphyromonas gingivalis - Identification and characterization of a functional corrin pathway

The ability of Porphyromonas gingivalis to biosynthesize tetrapyrroles de n ovo has been investigated. Extracts of the bacterium do not possess activit y for 5-aminolevulinic-acid dehydratase or porphobilinogen deaminase, two k ey enzymes involved in the synthesis of uroporphyrinogen III. Similarly, it was not possible to detect any genetic evidence for these early enzymes wi th the use of degenerate polymerase chain reaction. However, the bacterium does appear to harbor some of the enzymes for cobalamin biosynthesis since cobyric acid, a pathway intermediate, was converted into cobinamide. Furthe rmore, degenerate polymerase chain reaction with primers to cbiP, which enc odes cobyric-acid synthase, produced a fragment with a high degree of ident ity to Salmonella typhimurium cbiP. Indeed, the recently released genome se quence data confirmed the presence of cbiP together with 14 other genes of the cobalamin pathway. A number of these genes were cloned and functionally characterized. Although P. gingivalis harbors all the genes necessary to c onvert precorrin-2 into cobalamin, it is missing the genes for the synthesi s of precorrin-2. Either the organism has a novel pathway for the synthesis of precorrin-2, or more likely, it has lost this early part of the pathway . The remainder of the pathway may be being maintained to act as a salvage route for corrin synthesis.