RESPIRATORY PROTEINS OF INVERTEBRATES - STRUCTURE, FUNCTION AND EVOLUTION

Invertebrate organisms utilize a variety of proteins for the purposes of oxygen storage and transport. Hemoglobins are found in virtually ev ery phylum, but take on an almost infinite variety of structures. Alth ough the ''globin fold'' is a common motif, this can be found as indiv idual subunits, as linked subunits, and as extremely large aggregates of globins, sometimes accompanied by non-globin linker units. Hemoglob ins are the primary oxygen carriers among the annelida but they are ut ilized in many other phyla as well. A number of invertebrate phyla, pr incipally sipunculids, brachiopods, priapulids, and a few annelids, bi nd oxygen via a non-heme iron protein called hemerythrin. Hemerythrins occur as single chain molecules (myohemerythrins) and in a variety of associated forms used in oxygen transport. In these proteins the oxyg en is bound by an iron pair. Two phyla, the arthropoda and mollusca, e mploy as their principal oxygen transport agents copper proteins calle d hemocyanins. Arthropodan and molluscan hemocyanins exhibit little se quence or structural similarity. However, the oxygen binding sites are similar in the two classes; in each case oxygen is bound between a pa ir of copper atoms. All hemocyanins are very large molecules, consisti ng of numerous subunits and carrying many oxygen binding sites. Arthro pod hemocyanins are built from hexameric aggregates of polypeptide cha ins of about 70 KDa mass with one binding site on each. Molluscan hemo cyanins, on the other hand, are constructed from much larger subunits, each carrying seven or eight binding sites. Consideration of the dive rsity and distribution of these proteins indicates that most of them ( except the hemoglobins) evolved as oxygen transporters after the initi al dispersion of the major invertebrate phyla, almost certainly in pre -Cambrian times. Arthropod and molluscan hemocyanins, hemerythrins, an d the invertebrate hemoglobins represent individual solutions to a com mon physiological problem.