Structural modifications induced by the switch from an endogenous bis-histidyl to an exogenous cyanomet hexa-coordination in a tetrameric haemoglobin

Ferric cold-adapted fish haemoglobins exhibit heterogeneous coordination at the a (aquo-met) and b (bishistidyl) subunits. Herein we report two EPR-distinct bis-histidyl conformers (I and II) of the b subunits of ferric haemoglobin from Trematomus bernacchii which react differently with CN. An EPR titration reveals that upon cyanidation the most distorted conformer I reacts faster than II, producing both a cyanided and penta-coordinate ferric forms in the b subunits. The X-ray crystal structure of the partially cyanided conformer I reveals both an order-disorder transition and details of a communication between a and b subunits.