The optical absorption spectrum of mesoporphyrin-IX-substituted horseradish peroxidase shows a series of electronic origins with a typical spacing on the order of 100 cm(-1). These origins correspond with different tautomer states of mesoporphyrin-IX. A change in the pH from 8 to 5 induces severe changes in structure as well as in the intensity distribution of the tautomer origins. In addition, the pattern of photochemical tautomer transformation changes significantly. The straightforward interpretation is that a change in pH leads to a structural accommodation of the apoprotein. This structural rearrangement influences the energy hypersurface of the prosthetic group leading to the change in the origin spectrum observed. In turn, there seems to be a feedback between the actual structure of the prosthetic group and the substructure of the apoprotein, as is clearly seen in the pressure-tuning behavior of spectral holes in the various tautomer bands.