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Biosynthesis

Vitamin B is derived from a tetrapyrrolic structural framework created by the enzymes deaminase and cosynthetase which transform aminolevulinic acid via porphobilinogen and hydroxymethylbilane to uroporphyrinogen III. The latter is the first macrocyclic intermediate common to heme, chlorophyll, siroheme and B itself. Later steps, especially the incorporation of the additional methyl groups of its structure, were investigated using C methyl-labelled S-adenosyl methionine. It was not until a genetically engineered strain of Pseudomonas denitrificans was used, in which eight of the genes involved in the biosynthesis of the vitamin had been overexpressed, that the complete sequence of methylation and other steps could be determined, thus fully establishing all the intermediates in the pathway.