Heme a

Heme A differs from heme B in that a methyl side chain at ring position 8 is oxidized into a formyl group, and one of the vinyl side chains, at ring position 2, has been replaced by an isoprenoid chain. Like heme B, heme A is often attached to the apoprotein through a coordination bond between the heme iron and a conserved amino acid side-chain.

An example of a metalloprotein that contains heme A is cytochrome c oxidase.

Both the formyl group and the isoprenoid side chain are thought to play important roles in conservation of the energy of oxygen reduction by cytochrome c oxidase.