© 1980 by The Society for Integrative and Comparative Biology
Structural Aspects of Hemerythrin and Myohemerythrin1
Department of Biochemistry, University of New Hampshire Durham New Hampshire 03824
The hemerythrins are oxygen transport proteins with molecular architecture distinct from the other classes of respiratory pigments the hemoglobins and the hemocyanins. Hemerythrins occur with molecular weights of 14 000, 40 000, 56 000, and 108 000 and can be monomeric trimeric tetrameric or octameric in their subunit stoichiometry. Octameric hemerythrins consist of two layers of subunits each having four monomers arranged head to side, the layers facing each other to give the overall shape of a square doughnut. The conformation of the hemerythrin subunit designated the hemerythrin fold, has four nearly parallel 
-helical segments surrounding the pair of non-heme iron atoms, the oxygen binding site. These iron atoms are linked to the protein through specific histidine and tyrosine side chains. Distinct hemerythrins are found in the muscle (myohemerythrin) vascular system and coelom of sipunculans. The coelomic and vascular proteins are extensively polymorphic while myohemerythrin has been found to be more homogeneous. Oxygen affinities of hemerythrins from different species and different tissues within animals fall in a narrow range of 1–15 mm of Hg with myohemerythrin having a relatively high oxygen affinity compared to that of the blood hemerythrins. Naturally occurring modulators of oxygen affinity have not been discovered for hemerythrin, though certain chemical manipulations can effect changes in P50. Cooperativity between oxygen binding sites has been observed in only one hemerythrin with the remainder having Hill n values near 1.