Heme A (HEA)

Heme A is a coordination complex consisting of a macrocyclic ligand called a porphyrin, chelating an iron atom. Heme A is a biomolecule and is produced naturally by many organisms. Heme A differs from heme B in that a methyl side chain at ring position 8 is oxidized to a formyl group and a hydroxyethylfarnesyl group, an isoprenoid chain, has been attached to the vinyl side chain at ring position 2 of the iron tetrapyrrole heme. Heme A is similar to heme o, in that both have this farnesyl addition at position 2 but heme O does not have the formyl group at position 8, still containing the methyl group. The final structural question of the exact geometric configuration about the first carbon at ring position 3 of ring I, the carbon bound to the hydroxyl group, has been recently published as the chiral S configuration. Like heme B, heme A is often attached to the apoprotein through a coordinate bond between the heme iron and a conserved amino acid side-chain.