19F nuclear magnetic resonance as a probe of the spatial relationship between the heme iron of cytochrome P-450 and its substrate.

The distance between the heme iron of ferrous cytochrome P-450-CAM and a fluorine label attached to the 9-methyl carbon of its substrate, (1R)-(+)-camphor, has been determined using 19F NMR. This investigation uses the Solomon-Bloembergen equation to measure the distance from a paramagnetic heme iron to a fluorine probe incorporated into a substrate that is not in fast exchange. The structural identity of the substrate analogue, 9-fluorocamphor, has been established using one- and two-dimensional NMR methods and mass spectrometry. The relaxation rate of 9-fluorocamphor bound to high-spin paramagnetic ferrous P-450-CAM has been studied at 188, 282, and 376 MHz, and the correlation time has been directly determined from the frequency dependence of the relaxation rate. When the substrate analogue was bound to the low-spin diamagnetic ferrous-CO derivative of the enzyme, the relaxation rate was found to be 100 times slower and was therefore neglected in the distance calculation. The relaxation data for the paramagnetic system and the correlation time have been used to calculate a distance of 3.8 A between the heme iron and the C-9 fluoride. A fit of the distance and the chemical shift data to the pseudocontact shift equation predicts an angle of approximately 52 degrees between the heme normal and the Fe-F vector. The solution state Fe-F distance is somewhat shorter and the angle between the heme normal and the Fe-F vector slightly larger for the substrate-bound ferrous enzyme reported herein than the analogous values for the substrate-bound ferric enzyme determined in the solid state by x-ray crystallography. These differences may reflect a structural change at the substrate-binding site upon reduction of the iron.