Hemerythrin - Quaternary Structure and Cooperativity

Quaternary Structure and Cooperativity

Hemerythrin typically exists as a homooctamer or heterooctamer composed of α- and β-type subunits of 13-14 kDa each, although some species have dimeric, trimeric and tetrameric hemerythrins. Each subunit has a four-α-helix fold binding a binuclear iron centre. Because of its size hemerythrin is usually found in cells or "corpuscles" in the blood rather than free floating.

Unlike hemoglobin, most hemerythrins lack cooperative binding to oxygen, making it roughly 1/4 as efficient as hemoglobin. In some brachiopods though, hemerythrin shows cooperative binding of O₂. Cooperative binding is achieved by interactions between subunits: the oxygenation of one subunit increases the affinity of a second unit for oxygen.

Hemerythrin affinity for carbon monoxide (CO) is actually lower than its affinity for O₂, unlike hemoglobin which has a very high affinity for CO. Hemerythrin's low affinity for CO poisoning reflects the role of hydrogen-bonding in the binding binding O2, a pathway mode that is incompatible with CO complexes which usually do not engage in hydrogen bonding.