Heme A

Heme A (or haem A) is a heme, 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, often appears a dichroic green/red when in solution, is a structural relative of heme B, a component of hemoglobin, the red pigment in blood. 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 correct structure of heme A, based upon NMR and IR experiments of the reduced, Fe(II) form of the heme, was published in 1975. The structure was confirmed by synthesis of the dimethyl ester of the iron-free form. Heme A was first isolated by the German biochemist Otto Warburg in 1951 and shown by him to be the active component of the integral membrane metalloprotein cytochrome c oxidase. 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 shown to be 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. In the important respiratory protein cytochrome c oxidase (CCO) this ligand 5 for the heme A at the oxygen reaction center is a histidyl group. Histidine is a common ligand for many hemeproteins including hemoglobin and myoglobin. Heme A in the cytochrome a portion of cytochrome c oxidase, bound by two histidine residues (shown in pink) An example of a metalloprotein that contains heme A is cytochrome c oxidase. This very complicated protein contains heme A at two different sites, each with a different function.