Inter-aspartic hydrogen bond at the α1β2 interface functions as a proton sink motif in fish hemoglobins

The cooperativity for oxygen binding exhibited by tetrameric Hbs (αβ)2 is explained by assuming that these proteins can adopt two distinct quaternary states, one with a low affinity (tense, T state) and the other with a high affinity (relaxed, R state) for the oxygen. Long time ago, Root observed that the hemoglobin, Hb, of a number of fish species show an extreme response to solution pH, reaching less than half saturation with oxygen when equilibrated in air at acidic pH. The so called Root effect is associated with 1) the existence of a swim bladder to alter the natural buoyancy of the fish to optimize energy use during swimming at various depths; and 2) the vascularization of retina which needs to be oxygenated to maintain function even at low ambient oxygen concentrations. The search for the molecular origins of the Root effect have been reached in a combination of sequence comparisons, chemical modification studies and more recently structural analysis on Antarctic fish [1-3], trout, spot and tuna Hbs in the ferrous states. A recent structural analysis of the cathodic component Hb from the Antarctic fish Trematomus newnesi [4] showed that the hydrogen bond formed between Asp95α and Asp101β at the α1β2 interface, which is stabilized by Asp99β, is per se sufficient to generate Root effect, and it is the minimal structural requirement needed for the design of Root effect Hbs. The same Asp-Asp motif was observed in the crystal structure of the deoxy hemoglobin from the fish Trematomus bernacchii, HbTb at pH=6.0 [2]. Herein we present some crystal structures of HbTb in ferrous deoxy and carbomonoxy and several ferric states at different pH values. These structures show how finely the inter-aspartic motif is modulated by the pH. In particular, in the deoxy form (T state) the Asp95α -Asp101β hydrogen bond persists from pH=6 up to 8.4, with a H-bond length that is 2.4 Ǻ at pH=6 and 2.6/3.2 Ǻ at pH=8.4. The pKa associated to one of the Asp residues involved in the H-bond does not cover the distribution of experimental pKa values reported in literature. [5] The high pKa, attributed to Asp95α, is justified also by a low solvent accessibility area and by the assistance of the deprotonated Asp99β. Upon oxygenation, in the R state, this H-bond breaks down and consequently two protons per tetramer are released. Also in the ferric states HbTb displays a pH sensitive modulation. Infact, at pH=7.6, the ferric HbTb exhibits a T/R intermediate quaternary structure [6] in which the Asp-Asp hydrogen bond is absent or mediated by a water molecule, depending on the ligand in β. At pH=6.0 the ferric form assumes a T-like quaternary structure accompanied by a partial ligation α(aquomet)- β(penta-coordinated Fe3+) suggesting that a sort of Root effect is active also in the ferric state. [1] Camardella et al. J. Mol. Biol. 1992, 224, 449; [2] Ito et al. J. Mol. Biol. 1995, 250, 648 [3] Mazzarella et al. J. Mol. Biol. 1999, 287, 897; [4] Mazzarella et al. Submitted; [5] Forsyth et al. Proteins 2002, 48, 388.; [6] Riccio et al. PNAS 2002, 99, 9801.