The crystal structure of cytochrome P450cam complexed with (S)-nicotine has been determined at 2.3 ? resolution. (S)-nicotine has been extensively studied as a substrate of both bacterial P450cam and human P450 enzymes. Previous kinetic studies have examined (S)-nicotine binding to P450cam (Jones et al., 1993, J. Am. Chem. Soc. 115, 381-387.). These demonstrate that (S)-nicotine undergoes regiospecific monooxygenation at the pyrrolidine 5' carbon, implying that the molecule orients in the active site with the pyrrolidine ring close to the catalytic heme iron. In contrast, the crystal structure shows the nicotine molecule in a reversed orientation with the pyridine nitrogen coordinated with the heme iron. The is consistent with UV spectral studies, which indicate formation of a low-spin hexacoordinate iron upon nicotine binding to P450cam in solution (Ibid.). However, the pyrrolidine 5' carbon is approximately 8 ? from the heme iron in the crystal structure, indicating that this represents a nonproductive, though energetically favorable, binding mode. These findings will be presented at the 17th International Congress of Biology and Molecular Biology, August 1997, San FRancisco, and are being written up for publication (see below). Additional studies are now examining possible reorientation of the ligand for productive binding. These include examination of (S)-nicotine binding to P450cam in the presence of CO under reducing conditions.
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