The biosynthesis of the tetrapyrrole framework has been investigated in the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough by characterization of the enzymes required for the transformation of aminolaevulinic acid into sirohydrochlorin. PBG (porphobilinogen) synthase (HemB) was found to be a zinc-dependent enzyme that exists in its native state as a homohexamer. PBG deaminase (HemC) was shown to contain the dipyrromethane cofactor. Uroporphyrinogen III synthase is found fused with a uroporphyrinogen III methyltransferase (HemD-CobA). Both activities could be demonstrated in this amalgamated protein and the individual enzyme activities were separated by dissecting the relevant gene to allow the production of two distinct proteins. A gene annotated in the genome as a bifunctional precorrin-2 dehydrogenase/sirohydrochlorin ferrochelatase was in fact shown to act only as a dehydrogenase and is simply capable of synthesizing sirohydrochlorin rather than sirohaem. Genome analysis also reveals a lack of any uroporphyrinogen III decarboxylase, an enzyme necessary for the classical route to haem synthesis. However, the genome does encode some predicted haem d1 biosynthetic enzymes even though the bacterium does not contain the cd1 nitrite reductase. We suggest that sirohydrochlorin acts as a substrate for haem synthesis using a novel pathway that involves homologues of the d1 biogenesis system. This explains why the uroporphyrinogen III synthase is found fused with the methyltransferase, bypassing the need for uroporphyrinogen III decarboxylase activity.

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