Hydroxymethylbilane
Hydroxymethylbilane, also known as preuroporphyrinogen, is an organic compound that occurs in living organisms during the synthesis of porphyrins, a group of critical substances that include haemoglobin, myoglobin, and chlorophyll. The name is often abbreviated as HMB.
Structure
The compound is a substituted bilane, a chain of four pyrrole rings interconnected by methylene bridges. The chain starts with a hydroxymethyl group and ends with a hydrogen, in place of the respective methylene bridges. The other two carbon atoms of each pyrrole cycle are connected to an acetic acid group and a propionic acid group, in that order.Metabolism
HMB is generated from four molecules of porphobilinogen by the enzyme porphobilinogen deaminase in the overall reaction:The enzyme uroporphyrinogen III synthase catalyses the cyclisation reaction of hydroxymethylbilane into uroporphyrinogen III via a spiro intermediate which allows one of the pyrrole rings to convert its initial acetate to propionate configuration into a propionate-acetate one.
Uroporphyrinogen III is a porphyrinogen, which is a class of compounds with the hexahydroporphine macrocycle. In the absence of the enzyme, the compound undergoes spontaneous cyclization and becomes uroporphyrinogen I.