title: The effect of cytochrome P-450cam on the NMR relaxation rate of water protons.
abstract: Cytochrome P-450cam in the native, substrate-free state (Fe3+, S = 1/2) substantially reduces the NMR relaxation times, T1 and T2, of water protons. Temperature and frequency dependences of T1 and T2 were measured; they are consistent with a model of one or two protons exchanging between a binding site on a heme ligand and bulk water. The relevant parameters of this model have been deduced from the data. The spin relaxation time of the heme iron, tau S similar to 0.5 ns at 25 degrees C, is unusually long for a low spin ferric heme protein but is compatible with the line widths measured for paramagnetically shifted heme resonances. The proton residence time on the ligand, tau M similar to 1 microsecond at 25 degrees C, follows an Arrhenius law with activation energy EM similar to 15 kcal/mol. A scalar hyperfine interaction A/h = 2.2 MHz (3.1 MHz for one-proton exchange) of the found proton(s) with the heme iron is deduced from the difference between T1 and T2 observed in the fast exchange limit. The iron-proton distance is found to be 2.9 A (2.6 A for one-proton exchange). Variation of pH between pH 6.4 and 8.6 does not affect T1. The bearing of these results on the question of the axial heme ligand is discussed.
mesh_terms: D001665:Binding Sites; D003577:Cytochrome P-450 Enzyme System; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme; D007501:Iron; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D011485:Protein Binding; D011487:Protein Conformation; D011522:Protons; D013696:Temperature; D014867:Water
chemical_list: D008024:Ligands; D011522:Protons; D014867:Water; D006418:Heme; D003577:Cytochrome P-450 Enzyme System; D007501:Iron