Stereochemistry, formation and reactivity of hemichromes in Antarctic fish hemoglobins

Antarctic fish hemoglobins (AF-Hbs) exhibit an unusual auto-oxidation process. Our previous crystallographic and spectroscopic investigations have demonstrated that, upon oxidation, these tetrameric proteins show a remarkable propensity to evolve towards the formation of low-spin bis-histidine complexes, hemichromes, even under physiological conditions [1,2]. The crystal structures of the fully oxidized forms of Trematomus newnesi and Trematomus bernacchii AF-Hbs have shown that α and β chains follow different oxidation pathways. Indeed, α chains form aquo-met, whereas β chains form hemichromes. Interestingly, the quaternary structures of these ferric forms are intermediate [1,2] between the physiological R and T hemoglobin states [3]. Frozen solution EPR spectra of the ferric AF-Hbs at physiological pH reveal two distinct hemichrome signals (I, II), corresponding to two bis-histidine structures with different tilt angles, one of which (I) is consistent with the high resolution crystal structure (1.5 Å) of the oxidized form of T. bernacchii AF-Hb, HbTb. EPR studies show that one of the hemichromes (I) preferentially reacts with CN¯. With excess CN¯, a cyano-met and a rhombic high spin signal were resolved. These signals may correlate with the high resolution crystal structure of the ferric form of HbTb after cyanidation (1.4 Å), which exhibits two different β heme structures, attributed to a cyano-met and a penta-coordinated form. A possible functional role of hemichromes in Antarctic fish is discussed in terms of protection from oxidative stress [4].[1] Riccio A., Vitagliano L., Zagari A., di Prisco G., Mazzarella L. Proc. Natl. Acad. Sci. US, 2002, 99, 9801; [2] Vitagliano L., Bonomi G., Riccio A., di Prisco G., Smulevich G., Mazzarella L. Eur. .J. Biochem, 2004, 271, 1651. [3] Mazzarella L., Vergara A., Vitagliano L., Merlino A., Bonomi G., Scala S., Verde C., di Prisco G. Proteins, Str. Funct. Bioinf. in press. [4] Feng L., Zhou S., Gu L., Gell D.A., Mackay J. P., Weiss M.J., Gow A.J., Shi Y., Nature 2005, 435, 697.