Association of alcohols with heme proteins: optical analysis and thermodynamic models

At concentrations lower than those causing denaturation, methanol, ethanol, and 1-propanol produce changes in optical absorption of alkaline ferricytochrome c. These changes arise from weak equilibrium associations characterized by dissociation constants at 25 degrees C of about 4 and 2 M, respectively, for the methanol- and 1-propanol-ferricytochrome c complexes. The difference spectra and temperature dependence of enthalpy and entropy changes accompanying formation of methanol and 1-propanol complexes, as well as changes induced in the EPR spectra, are different and suggest distinct binding modes. Considered in conjunction with related parameters from ferrihemoglobin and ferrimyoglobin, the spectral and thermodynamic data are consistent with models in which methanol is bound directly to the ferric ion of cytochrome c, methanol and ethanol are bound directly to the ferric ions of hemoglobin and myoglobin, and 1-propanol is bound to a hydrophobic region of cytochrome c. Both the absolute and alcohol-induced optical difference spectra of these proteins have been simulated, the former through summation of Gaussian bands and the latter as the difference between two such summations, one with parameters slightly altered from the other. This analysis reveals and characterizes previously unresolved structure, which is discussed in terms of electronic transitions of the heme group and changes caused by differing interactions of the heme with surroundings. Similarity between the difference spectra produced by IHP perturbation of ferrihemoglobin and that from the difference between absolute spectra of ferrimyoglobin and ferrihemoglobin suggests that, with ferrihemoglobin as reference, the conformations about the hemes of ferrimyoglobin and of ferrihemoglobin-IHP are in some way similar.