Aspects of the Origin and Evolution of Non-Vertebrate Hemoglobins

Abstract

Hemoglobins, found in members of almost all invertebrate phyla, display an extraordinary diversity of form and function. Although some are intracellular with chains and assemblies similar in size to those of vertebrates, others are giant extracellular proteins with masses as large as 8,000 kilodaltons. Two very different strategies have evolved for the stabilization of these large molecules. The first is the formation of both intra- and interchain disulfide bonds that effectively immobilize segments of the protein, and the second is the evolution by gene duplication of multi-domain chains with from two to eighteen myoglobin-like, heme-containing domains in a single polypeptide that may be as large as ˜260 kilodaltons. The genes for vertebrate globins have a characteristic two-intron, threeexon structure. The gene encoding chain c of the hemoglobin of the earthworm Lumbricus terrestris has precisely the same organization and splice junction positions. This shows that these positions have been conserved for at least 600 million years, the estimated time of divergence of annelids and the ancestor to chordates. The gene encoding a hemoglobin (leghemoglobin) of higher plants shows exactly the same splice junctions as in the globin genes of vertebrates except that the middle exon is split by an additional intron which is believed to have been lost early in animal evolution. The occurrence of hemoglobins in diverse higher plants suggests that they might be present in all plants albeit at very low concentrations and perhaps serving an enzymatic function. Hemoglobin, broadly defined as a heme-containing protein capable of reversible combination with oxygen, also occurs in bacteria and fungi. The discovery of a bacterial hemoglobin that is 26% identical with lupin leghemoglobin indicates a procaryotic origin. The possibility that hemoglobin may have evolved from a cytochrome is suggested by the presence of hemoglobins in the yeast, Candida , and the bacterium, Alcaligenes , that contain both heme and flavin. They must therefore have evolved by the fusion of the genes for two different proteins. However, the possible homology of the heme domains with plant or animal hemoglobins remains to be determined. The lactic dehydrogenase of yeast, cytochrome b2, is also a soluble flavoheme protein with a heme domain that is homologous with mammalian cytochrome b5. Globin may have evolved in part from a member of the cytochrome b5 family, but if so, the event must have occurred so early that only a borderline perhaps random correspondence of amino acid sequences remains