Oxygen Binding to Myoglobins and Their Cobalt Analogues

Thermodynamic measurements for O₂ binding to native myoglobins and their cobalt analogues are discussed for proteins from three aquatic mammals: sperm whale, grey whale, and sea lion; reference is also made to studies on horse myoglobin. Emphasis is placed on the differences between O₂ binding by the Fe and Co forms of the same protein. By comparing these differences within the set of myoglobins, and between proteins and the Fe and Co “picket-fence” porphyrin models of Collman et al., we are able to draw inferences that would be impossible with, say, the direct comparison of myoglobin and the Fe-por-phyrin model, or Co-myoglobin and the Co-porphyrin model. In particular, such comparisons confirm the fact that the cobalt-substituted myoglobins are true analogues of the natural proteins. They also suggest that there can be significant differences between oxygen binding in the proteins and the porphyrin models: similar affinities are achieved by different enthalpy-entropy compensation effects.