Stereochemical Changes Upon Ligation

Upon binding a second axial ligand, the iron center together with the axial base move toward the plane of the porphyrin, initiating a change in spin state from high-spin to low-spin when the sixth ligand is O2 or CO or any other strong ligand with an even number of valence electrons. Given these general features, what are the structural differences between systems that bind O2 with high affinity and those that bind O2 with low affinity? The answers to this question are relevant to understanding at the molecular level the mechanism of cooperativity, where a low-affinity conformation, the T state, and a high-affinity conformation, the R state, are in dynamic equilibrium in one tetrameric molecule. In looking at crystallographic data one sees a particular conformation frozen in the crystal, usually the one of lowest free energy among many in equilibrium in the solution state. The R ~ T equilibrium for hemoglobin is moderately rapid, at 4 x 10 3 S - 1; hemocyanin also switches quaternary conformations with a similar rate constant.

Human hemoglobins are a heterogeneous group. Many mutants are known, and several have been structurally characterized. A structural alteration that affects the equilibrium between Rand T states has a marked effect on ligand affinity and cooperativity in hemoglobin. If a specific amino-acid substitution destabilizes the T state, then the transition to the R state will occur earlier in the ligation process, and the hemoglobin will have an increased oxygen affinity. Hemoglobin Kempsey is an example. In this mutant an aspartic acid on the f3 chain is replaced by asparagine. Conversely, if the R state is destabilized, then the hemoglobin will have a lowered oxygen affinity. Hemoglobin Kansas is an example. Here an asparagine on the f3 chain has been replaced by threonine.