Comparative biological chemistry of cobalt hemoglobin.

Cobaltohemoglobin A binds oxygen cooperatively with a Hill coefficient of up to 2.3 and its ~1~2 is found to be 91 mm. Its oxygen afllnity is controlled by diphosphoglycerate which the protein binds very strongly. Cobaltohemoglobin A possesses only 40% of the alkaline Bohr effect of the native enzyme; carbon dioxide has no effect upon oxygen binding. The oxidation-reduction potential of cobaltohemoglobin A is +O.lOO volt which is about 50 mv less positive than that of native hemoglobin. The electron paramagnetic resonance of the nitroxide spin label attached to the sulfhydryl groups of cys(92)/? and the rates of reaction of these groups with paramercuribenzoate indicate that the “heme” crevice is more open near that residue. A detailed comparison of these properties with those of the native protein suggests subtle and intricate alterations of the quaternary structures of the oxy and deoxy species which result from substitution of the metal atom.