Structures of Invertebrate Hemoglobins

Abstract

Hemoglobin is widely distributed among the invertebrates. Intracellular hemoglobins consist of relatively small molecules with mol wts of 15–17,000 or dimeric, tetrameric or octameric aggregates of 15–17,000 mol wt subunits. Sequence homology is present but not extensive in those pigments which have been studied and the characteristic myoglobin fold of vertebrate hemoglobin occurs in at least two invertebrate hemoglobins. The wide array of aggregation states among invertebrate hemoglobins provides some simple models for understanding homotropic functional properties exhibited by many of these pigments. Polymeric extracellular hemoglobins are present in annelids molluscs crustacean arthropods and nematodes. Annelid extracellular hemoglobins and chlorocruorins consist of 3 × 106 mol wt two-tiered hexagonal arrays of submultiples which in turn are based on polypeptide chain subunits of mol wt 14–16 000. Molluscan extracellular hemoglobins are constructed from a different subunit arrangement. In the planorbid snail and clam extracellular hemoglobins the subunits appear to be 175 000 and 300 000 mol wt linear series of 15–17 000 dalton oxygen binding domains respectively. Planorbid snail native hemoglobin presents circular structures 200 Å in diameter in the electron microscope with 10-fold symmetry in at least one view, and clam extracellular hemoglobins are huge 345 by 1200 Å rodlike structures. Crustacean extracellular hemoglobins are also polymeric pigments and at least in a few species appear to have subunits which are tandemly linked oxygen binding domains. The polymeric hemoglobins of nematodes have molecular weights of about 330 000. The subunit molecular weight and heme content suggest a value of 40,000 daltons which set the nematode pigments apart from all other hemoglobins so far studied. An overview of invertebrate hemoglobin structures and some of the questions they pose are presented in this paper.