A 175-PSEC MOLECULAR-DYNAMICS SIMULATION OF CAMPHOR-BOUND CYTOCHROME-P-450CAM

The structure and internal motions of cytochrome P-450cam, a monooxygenase heme enzyme with 414 amino acid residues, with camphor bound at the active site have been evaluated on the basis of a 175-psec molecular dynamics simulation carried out at 300 K. All hydrogen atoms were explicitly modeled, and 204 crystallographic waters were included in the simulation. Based on an analysis of the time course of the trajectory versus potential energy, root mean square deviation, radius of gyration, and hydrogen bonding, the simulation was judged to be stable and representative of the average experimental structure. The averaged structural properties of the enzyme were evaluated from the final 135 psec of the simulation. The average atomic displacement from the X-ray structure was 1.39 angstrom for all heavy atoms and 1.17 angstrom for just C-alpha-atoms. The average root-mean-square (rms) fluctuations of all heavy atoms and backbone atoms were 0.42 and 0.37 angstrom, respectively. The computed rms fluctuations were in reasonable agreement with the experimentally determined temperature factors. All 13 segments of alpha-helix and 5 segments of beta-sheet were well preserved with the exception of the N-terminal half of helix F which alternated between an alpha-helix and a 3(10)-helix. In addition there were in general only small variations in the relative orientation of adjacent alpha-helices. The rms fluctuations of the backbone dihedral angles in the secondary structure elements were almost uniformly smaller, with the fluctuation in alpha-helices and beta-sheets, 31 and 10% less, respectively, than those in nonsecondary structure regions. The reported crystal structure contains kinks in both helices C and I. In the simulation, both of these regions showed high mobility and large deviations from their starting positions. Since the kink in the I helix is at the oxygen binding site, these motions may have mechanistic implications.