Lack of conventional oxygen-linked proton and anion binding sites does not impair allosteric regulation of oxygen binding in dwarf caiman hemoglobin

Authors

Roy E. Weber, Aarhus University, DenmarkFollow
Angela Fago, Aarhus University, DenmarkFollow
Hans Malte, Aarhus University
Jay F. Storz, University of Nebraska-LincolnFollow
Thomas A. Gorr, University of Zurich

Date of this Version

2013

Citation

American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 305 (2013), pp. R300-R312; doi:10.1152/ajpregu.00014.2013.

Comments

Copyright © 2013 the American Physiological Society
http://www.ajpregu.org

Abstract

In contrast to other vertebrate hemoglobins (Hbs) whose high intrinsic O₂ affinities are reduced by red cell allosteric effectors (mainly protons, CO₂, organic phosphates, and chloride ions), crocodilian Hbs exhibit low sensitivity to organic phosphates and high sensitivity to bicarbonate (HCO₃^—), which is believed to augment Hb-O₂ unloading during diving and postprandial alkaline tides when blood HCO₃^— levels and metabolic rates increase. Examination of α- and β-globin amino acid sequences of dwarf caiman (Paleosuchus palpebrosus) revealed a unique combination of substitutions at key effector binding sites compared with other vertebrate and crocodilian Hbs: β82Lys→Gln, β143His→Val, and β146His→Tyr. These substitutions delete positive charges and, along with other distinctive changes in residue charge and polarity, may be expected to disrupt allosteric regulation of Hb-O₂ affinity. Strikingly, however, P. palpebrosus Hb shows a strong Bohr effect, and marked deoxygenation-linked binding of organic phosphates (ATP and DPG) and CO₂ as carbamate (contrasting with HCO3^— binding in other crocodilians). Unlike other Hbs, it polymerizes to large complexes in the oxygenated state. The highly unusual properties of P. palpebrosus Hb align with a high content of His residues (potential sites for oxygenation-linked proton binding) and distinctive surface Cys residues that may form intermolecular disulfide bridges upon polymerization. On the basis of its singular properties, P. palpebrosus Hb provides a unique opportunity for studies on structure-function coupling and the evolution of compensatory mechanisms for maintaining tissue O₂ delivery in Hbs that lack conventional effector-binding residues.