The hemerythrin family

The biological occurrence of hemerythrins (Hr in Figure 4.8), the third class of dioxygen carriers, is relatively rare, being restricted to the sipunculid family (nonsegmented worms), a few members of the annelid (segmented worm) family

,a couple of brachiopods (shrimps), and a couple of priapulids. The oxygenbinding site contains, like hemocyanin, a pair of metal atoms, in this case, iron. Upon oxygenation the colorless protein becomes purple-red. Monomeric (myohemerythrin), trimeric, and octameric forms of hemerythrin are known; all appear to be based on a similar subunit of about 13.5 kDa. When hemerythrin is extracted from the organism, its oxygen binding is at best only weakly cooperative, with Hill coefficients in the range 1.1 to 2.1. 18 In coelomic cells (the tissue between the inner membrane lining the digestive tract and the outer membrane of the worm-analogous to flesh in vertebrates), oxygen apparently binds with higher cooperativity (n ~ 2.5).43 Perchlorate ions have been observed to induce cooperativity: since CI04 - has no biological role, it appears that in protein purifications the biological allosteric effector is lost. No Bohr effect occurs. Dioxygen binding data are accumulated in Table 4.2.

The structure of hemerythrin in a variety of derivatives (oxy, azido, met, and deoxy) is now well-characterized. With three bridging ligands, a distinctive cofacial bioctahedral stereochemistry is seen (Figure 4.10).