Quaternary structure has little influence on spin states in mixed-spin human methemoglobins

A key feature of the Perutz stereochemical model for cooperativity in hemoglobin is a strong coupling between quaternary structure and the spin state of the heme iron [Perutz, M. F. (1979) Annu. Rev. Biochem. 48, 327-386]. While this coupling appears to be present for carp azide methemoglobin, it should also be present for all liganded forms of human methemoglobin that exhibit a thermal high-spin in equilibrium low-spin equilibrium. To test this hypothesis, we have measured the changes in spin equilibria upon conversion of six mixed-spin forms of human methemoglobin from the R (high-affinity) to the T (low-affinity) quaternary structure by addition of inositol hexaphosphate. These experiments were done with a sensitive superconducting magnetic susceptibility instrument on solutions at 20 degrees C in 20 mM maleate buffer, pH 6. The data show zero or small increases in high-spin content upon switching from R to T, changes that are equivalent to a relative stabilization of the high-spin form by only 0-300 cal mol-1 heme-1. These changes in energy are far less than the 1200 cal mol-1 heme-1 predicted from the Perutz stereochemical model [Cho, K. C., & Hopfield, J. J. (1979) Biochemistry 18, 5826-5833]. That is, these data do not support a view that the low affinity of the T state is due to restraints acting through the iron-proximal histidine linkage. The mechanistic implications of these results and the differences between species and ferric ligands are discussed.