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This protein is not an enzyme, but an oxygen carrier. The heme group lies near the protein surface in a hydrophobic pocket. Thus an area of low dielectric constant is provided which favours oxygenation but not oxidation. When oxygen is taken up, the spin state of iron is changed and a small movement of iron occurs relative to the porphyrin ring. This shift is transmitted so that the constraints holding hemoglobin in the deoxy state are relaxed. As a result it is easier for the next subunit to take up oxygen (cooperativity). Neutron diffraction studies have shown that the Fe--O--O angle is 156.
Perutz, M. F., Kendrew, J. C. and Watson, H. C., J. Mol. Biol . 13 (1965) 669; Perutz, M. F., Nature 228 (1970) 726, 734: Padlan, E. A. and Love, W. E., J. Biol. Chem . 249 (1974) 4067; Ten Eyck, L. F. and Arnone, A., J. Mol. Biol . 100 (1976) 3; Baldwin, J. and Chothia, C., J. Mol. Biol . 129 (1979) 175; Shannan, B., Nature 296 (1982) 683.
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