LIGAND-BINDING TO HEME-PROTEINS .5. LIGHT-INDUCED RELAXATION IN PROXIMAL MUTANTS L891 AND H97F OF CARBONMONOXYMYOGLOBIN

We have studied the proximal mutants L89I and H97F of MbCO with FTIR a nd temperature-derivative spectroscopy at temperatures between 10 and 160 K. The mutations give rise only to minor alterations of the stretc h spectra of the bound and photodissociated CO ligand. The most pronou nced difference is a larger population in the A, substate at approxima te to 1930 cm(-1) in the mutants. The barrier distributions, as determ ined by temperature-derivative spectroscopy, are very similar to nativ e MbCO after short illumination. Extended illumination leads to substa ntial increases of the rebinding barriers in native MbCO and the proxi mal mutants. A larger fraction of light-relaxed states is found in the proximal mutants, implying that the conformational energy landscape h as been modified to more easily allow light-induced transitions. These and other spectroscopic data imply that the large changes in the bind ing properties are brought about by a light-induced conformational rel axation involving the structure at the heme iron. Similarities with sp ectral hole-burning studies and physical models are discussed.