Photodissociation of ligands from heme and heme proteins. Effect of temperature and organic phosphate

The effect of temperature on ligand photodissociation from protoheme and the heme proteins hemoglobin (Hb) and myoglobin (Mb) has been examined. The quantum yield of photodissociation (phi) is greater at 40 degrees than at 0 degrees; in general, larger increases are seen in the less photosensitive complexes, while phi does not change in the most photosensitive complexes. The ratio of phi at 40 degrees to phi at 0 degrees is 1.8 for HbCO, 2.3 for n-butyl isocyanide Hb, 2.7 for HbO2, and 1.3 for HbNO, with initial phi values of 0.38, 0.26, 0.028, and 0.003, respectively. This pattern of quantum yield increases is seen in protoheme as well as Hb and Mb ligand photolysis. The allosteric effector inositol hexaphosphate increases the quantum yield of lignad photolysis from hemoglobin. As with temperature, inositol hexaphosphate addition has a larger effect on complexes of low quantum yield; phi increases 1.2-fold for HbCO and 2.2-fold for HbO2 at 0 degrees. The results are discussed in terms of a model containing a photoexcited intermediate (Phillipson, P.E., Ackerson, B.J., and Wyman, J. (1973) Proc. Natl. Acad. Sci. U.S.A. 70, 1550-1553).