CHARACTERIZATION OF A RESTING STATE MODEL OF PEROXIDASES BY AB-INITIOMETHODS - OPTIMIZED GEOMETRIES, ELECTRONIC-STRUCTURES, AND RELATIVE ENERGIES OF THE SEXTET, QUARTET, AND DOUBLET SPIN STATES

Ferric aquo heme complexes comprise the active site of three major fam ilies of heme proteins, the peroxidases, the cytochrome P450s, and met myoglobin. There is ample evidence from a variety of spectroscopic stu dies of wild type (wt) and mutant enzymes that in these resting state complexes, the sextet, quartet, and doublet states are very close in e nergy and the predominant spin state observed is a sensitive function of the number, nature, and geometry of the axial ligands. One correlat ion that is very difficult to determine experimentally is the relation ship between spin state and geometry in the same heme complex. This co upling of geometry and spin state in the same complex can be functiona lly important since spin state changes are often a key part of functio n, for example, in the enzymatic cycle of the cytochrome P450s. To fur ther explore the relationship between geometry and spin state, we repo rt here for the first time the use of ab initio methods to calculate o ptimized geometries and electronic structure of a model for the restin g state of peroxidases in its sextet, quartet, and doublet states. The sextet state is found to be the lowest energy state in agreement with experimental results reported for a model diaquo heme compound. Altho ugh a longer Fe-water distance was obtained in the model compound, the unique feature of these calculations is their ability to monitor chan ges in geometry in the various spin states in the same complex. While the optimized quartet geometry is similar to the sextet geometry, the doublet state has a considerably shorter Fe-water distance. These resu lts suggest that the environment of the protein can modulate spin stat e changes by imposing geometric changes in this mobile Fe-ligand inter action by interaction from both the proximal and distal sides. Experim ental determination of spin slate populations by a number of spectrosc opic methods in wt and mutants of the Fe(III) resting form of cytochro me-C peroxidases (CCP) with known Fe-water distances from crystal stru ctures provide strong support for this hypothesis.