Ferric cold-adapted fish haemoglobins exhibit heterogeneous coordination at the α (aquo-met) and β (bis-histidyl) subunits. Herein we report two EPR-distinct bis-histidyl conformers (I and II) of the β 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 β subunits. The X-ray crystal structure of the partially cyanided conformer I reveals both an order-disorder transition and details of a communication between α and β subunits.
