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The X-ray crystal structures of cytochrome P-450CAM complexed with both
enantiomers of a chiral, multifunctional inhibitor have been refined to
R-factors of 21.0% [(+)-enantiomer] and 19.6% [(-)-enantiomer] at approximately
2.1-A resolution. Binding of either enantiomer, both considerably larger than
the natural substrate camphor, results in similar, dramatic structural changes
in the enzyme. In contrast to all previous P-450CAM crystallographic structures,
the Tyr96 side chain is not pointing "down" toward the heme but is rather
directed "up" into the proposed substrate access channel. This conformational
change is accompanied by the displacement of the Phe193 side chain out into the
solvent at the enzyme surface. These changes are consistent with the assignment
of this region of the enzyme as the access channel [Poulos et al. (1986)
Biochemistry 25, 5314-5322] and suggest that several aromatic residues lining
the channel may be involved in substrate recognition and channeling to the
active site. The cation usually observed coordinated to the Tyr96 carbonyl
oxygen is missing in the presence of the (+)-enantiomer but is present with the
(-)-enantiomer. The Phe87 side chain, located near the inhibitor binding site,
adopts different orientations depending upon which enantiomer is bound. Finally,
electron density reveals that although the inhibitor enantiomers were
dichlorinated as provided, when bound to P-450CAM the chlorine atoms are present
at only 0-20% occupancy, probably reflecting selective binding of impurities in
the samples. Coordinates of these inhibited P-450CAM complexes have been
deposited in the Brookhaven Protein Data Bank [Bernstein et al. (1977) J. Mol.
Biol. 112, 535-542].
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