A combined EPR and X-ray crystallography approach to the reactivity of hemichromes

Antarctic fish hemoglobins (AFHbs) exhibit a peculiar oxidation process. Indeed, upon oxidation, these tetrameric proteins show a remarkable propensity to evolve toward the formation of a bis-hystidine complex, hemichrome, [1,2]. The crystal structures of the oxidized AFHbs, isolated from Trematomus newnesi and Trematomus bernacchii, have shown that a) a chains go to aquo-met, whereas the b chains to hemichrome, and b) the quaternary structure is intermediate between the physiological R and T states [3]. The EPR analysis of the ferric AFHbs at physiological pH reveals two distinct hemichromes (I, II) in solution. The crystal structure (1.5 Å) of the oxidized form (hemichrome I) of the AFHbs isolated from T. bernacchii, HbTb, attributes the difference between the two EPR-distinct hemichromes to the deviation from the ideal hexa-coordination. The reactivity of the two HbTb hemichromes with CN- (studied via EPR, and confirmed via X-ray crystallography) is very different. In fact, only hemichrome I reacts in stechiometric conditions, converting into the cyano-met and a penta-coordinated ferric form. Interestingly, ferric AFHbs reveal a low peroxidasic activity with respect to mammalian and temperate fish hemoglobins. [1] Riccio A., et al. Proc. Natl. Acad. Sci. US, 2002, 99, 9801; [2] Vitagliano L., et al. Eur. .J. Biochem, 2004, 271, 1651. [3] Mazzarella L., et al. Proteins in press.