BIOSYNTHESIS OF PORPHYRINS AND RELATED MACROCYCLES .35. DISCOVERY OF A NOVEL DIPYRROLIC COFACTOR ESSENTIAL FOR THE CATALYTIC ACTION OF HYDROXYMETHYLBILANE SYNTHASE (PORPHOBILINOGEN DEAMINASE)

The enzyme hydroxymethylbilane synthase constructs the open-chain hydroxymethylbilane by assembly of four porphobilinogen units head-to-tail, the first of these being covalently bound to the enzyme through a group X. The surprising discovery is made that X is a novel dipyrromethane cofactor constructed from two porphobilinogen units and bound to the protein via the sulphur of cysteine. This cofactor does not turn over in the catalytic process but acts as an anchor for the assembly of a hexapyrrole from which the tetrapyrrolic hydroxymethylbilane is cleaved leaving the dipyrromethane cofactor in place for a further building cycle.