CORRELATION BETWEEN PROTEIN CONFORMATION AND PROSTHETIC GROUP CONFIGURATION AS TESTED BY PH EFFECTS - A HOLE-BURNING STUDY ON MESOPORPHYRIN-IX-SUBSTITUTED HORSERADISH-PEROXIDASE

The optical absorption spectrum of mesoporphyrin-IX-substituted horser adish peroxidase shows a series of electronic origins with a typical s pacing on the order of 100 cm(-1). These origins correspond with diffe rent 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 dis tribution of the tautomer origins. In addition, the pattern of photoch emical tautomer transformation changes significantly. The straightforw ard interpretation is that a change in pH leads to a structural accomm odation of the apoprotein. This structural rearrangement influences th e energy hypersurface of the prosthetic group leading to the change in the origin spectrum observed. In turn, there seems to be a feedback b etween the actual structure of the prosthetic group and the substructu re of the apoprotein, as is clearly seen in the pressure-tuning behavi or of spectral holes in the various tautomer bands.