‘Novel’ factors that regulate oxygen binding in vertebrate hemoglobins

Abstract

Globins or their genes—that presumably evolved from a common ancestral molecules—appear to occur in all organisms and tissues, and exhibit a diversity of quaternary structures and a large array of functions besides transporting and storing oxygen (Burmester et al., 2000, 2002; Trent and Hargrove, 2002; Weber and Vinogradov, 2001). In contrast to the intensively-studied interactions of hemoglobin (Hb) with protons (the Bohr effect), inorganic anions like chloride, and red cell organic phosphates (2,3 diphosphoglycerate (DPG) in mammals, inositol pentaphosphate (IPP) in birds and ATP and guanosine triphosphate (GTP) in ectothermic vertebrates) little is known about Hb’s reactions with other cytosolic and protoplasmic factors and their adaptive significance. This communication focuses on the reactions with water molecules (hydration) and the red cell membrane protein, band 3. Changes in water activity exert relatively small effects on the O2 binding affinities of the high-molecular-weight, extracellular, invertebrate O2 binding proteins, indicating small O2-linked changes in their water accessible surfaces (Hundahl et al., 2003b). In contrast, the solvation effects observed in fish Hbs (Hundahl et al., 2003a) are similar to those observed in human Hb (Colombo et al., 1992). In humans the N-terminal, cytoplasmic domains of the erythrocyte membrane protein band 3 (cdB3) bind at the organic phosphate site between the two beta chains of deoxyHb, decreasing Hb–O2 affinity (Walder et al., 1984). Given that band 3 mediates transmembrane anion (HCO3 / Cl) exchange, and that cdB3 binds Hb in competition with glycolytic enzymes (such as aldolase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase) that govern the metabolic pathways in the red cells, it follows that Hb may function as a tranducer (Giardina et al., 1995) that regulates red cell metabolism and ion exchange in an oxygenation dependent manner. Curiously, however, peptides corresponding to trout cdB3 undergo oxygenation-linked binding to human Hb but exert no effect on the O2 affinity of trout isoHbs (Jensen et al., 1998). To investigate cd-B3/Hb interactions in vitro and discern their possible in vivo regulatory significance in humans and other vertebrates, we synthesized 10-mer peptides corresponding to N-termini of cd-B3 from trout, chickens and