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.  The iron of the heme A of cytochrome a is hexacoordinated, that is bound with 6 other atoms.  The iron of the heme A of cytochrome a3 is sometimes bound by 5 other atoms leaving the sixth site available to bind dioxygen (molecular oxygen).  In addition, this enzyme binds 3 copper, magnesium, zinc, and several potassium and sodium ions.  The two heme A groups in CCO are thought to readily exchange electrons between each other, the copper ions and the closely associated protein cytochrome c.
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.  CCO is thought to be responsible for conserving the energy of dioxygen reduction by pumping protons into the inter-membrane mitochondrial space.  Both the formyl and hydroxyethylfarnesyl groups of heme A are thought to play important roles in this critical process, as published by the influential group of S. Yoshikawa.