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       COLLOQUIUM OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
           Institute for Computational Sciences and Informatics
				  CSI 929
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             OXIDATION IN COMPOUND ES OF CYTOCHROME C PEROXIDASE  

                                Steven W. Bunte
           U.S. Army Research Laboratory, Aberdeen Proving Ground, MD

				   and

                       Gerard M. Jensen and David B. Goodin
                   The Scripps Research Institute. La Jolla, CA

     The peroxidases comprise a class of enzymes that catalyze the hydrogen 
peroxide-mediated oxidation of an enormous range of biological substrates.  
The heme-containing yeast enzyme cytochrome c peroxidase (CCP) reacts with 
hydrogen peroxide to form a two electron oxidized intermediate:  compound ES. 
This intermediate in turn accepts electrons sequentially from two ferrous 
cytochromes c.  One oxidizing equivalent is stored as an oxyferryl heme 
(Fe(IV)=O), while the other is stored as a radical species on the indole ring 
of tryptophan-191.  This is distinct from many other peroxidases, where the 
second oxidizing equivalent is stored on the porphyrin macrocycle as a 
pi-cation radical.  It has been difficult to determine whether the tryptophan 
radical is a neutral radical or a cation radical, resulting from oxidation 
with or without loss of a proton, respectively.  ENDOR experiments and 
Becke3LYP density functional ab initio calculations of  the spin densities of 
the oxidized forms of 3-methyl indole are consistent with the conclusion that 
the oxidized indole in compound ES is a cation radical.  The energetics of 
the preferential oxidation of Trp-191 to a cation radical have also been 
modeled using the Protein Dipoles Langevin Dipoles (PDLD) method of Warshel 
and co-workers in combination with the (ostensibly gas phase) ab initio 
calculations.  These latter include Mulliken and ESP partial charge sets and 
the gas phase energies for the 3-methyl indole and the neutral and cation 
radicals of 3-methyl indole.  These methods in combination lead to 
predictions for the oxidation potential of Trytophan-191 and for the pKa of 
the indole radical (for the neutral/cation radical equilibrium).

                           Monday , March 24, 1997
                                  4:30 pm
                          Room 206, Science & Tech. I
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