H. Schulze et al., STRUCTURAL-CHANGES IN CYTOCHROME P-450(CAM) EFFECTED BY THE BINDING OF THE ENANTIOMERS (1R)-CAMPHOR AND (1S)-CAMPHOR, Biochemistry, 35(45), 1996, pp. 14127-14138
A comparative study of the enantiomeric substrate [(1R)-camphor- and (
IS)-camphor)-bound cytochrome P-450(cam) concerns the spin-state equil
ibrium, substrate dissociation, the thermal unfolding of the protein s
tructure, and the subconformer equilibria observed in the infrared spe
ctra of the carbon monoxide (GO) complex of cytochrome P-450(cam). The
behavior of the different conformational equilibria in dependence on
temperature, pressure, pH-value, cosolvent, and cation binding led us
to suggest that (1S)-camphor is more loosely and less optimally bound
in the heme pocket, which facilitates the access of solvent molecules
into the heme-iron environment, The spin reaction volume difference me
asured using the high pressure technique is smaller by 16 +/- 9 cm(3)/
mol for (1S)-camphor-bound P-450(cam) compared to the (1R)-camphor-bou
nd P-450(cam), which might indicate a higher water content in the prot
ein and in the heme environment in the (IS)-camphor complex, The half-
transition temperature of the thermal unfolding of 53.8 degrees C for
the (1S)-camphor-bound oxidized cytochrome P-450(cam) is one degree lo
wer than the value for the (IR)-camphor-bound protein (54.8 degrees C)
, In the reduced, GO-bound form of cytochrome P-450(cam) at 290 K the
(1S)-camphor complex reveals another CO stretch vibration population d
istribution with slightly higher frequencies [1940.2 cm(-1) (major ban
d) and 1946.3 cm(-1) (minor band)] compared to the (1R)-camphor comple
x [1939.7 cm(-1) (major band) and 1930 cm(-1) (minor band)], A looseni
ng of the contact between the iron-bound CO ligand and amino acids of
the I-helix, probably induced by compensating effects of the increased
water content, is suggested. Assuming the carbon monoxide complex as
a model for the dioxygen complex, the more loosened binding of (1S)-ca
mphor, therefore the increased water accessibility, and the weaker con
tact of the iron ligand to the I-helix might explain the higher amount
of uncoupling of the cytochrome P-450 reaction cycle compared to that
when (1R)-camphor is used as substrate.