Haem d1 and other haem cofactors from bacteria.

Several bacterial haem prosthetic groups whose structures deviate significantly from the ubiquitous protohaem (Fe-protoporphyrin) have been discovered recently. These newly discovered pigments contain dramatic modifications in their aromatic core and/or side chains. Examples include the dioxoisobacteriochlorin-type haem d1 and the chlorin-type haem d as well as the haem a-like haem o. Total syntheses of these macrocycles have been accomplished. Synthetic haem d1 and its analogues were used in reconstitution studies with nitrite reductase which revealed the importance of the oxo groups and the acrylate side chain for enzymic activity. The structural features of these porphyrinoids immediately suggest some possible, but as yet unproven, biosynthetic pathways.