Open AccessThe Pressure Dependence of the Spin Equilibrium in Camphor‐Bound Ferric Cytochrome P‐450
Author(s) -
HOA Gaston,
MARDEN Michael C.
Publication year - 1982
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1982.tb06593.x
Subject(s) - chemistry , bar (unit) , ferric , camphor , cytochrome , analytical chemistry (journal) , solvent , volume (thermodynamics) , spin (aerodynamics) , molar volume , crystallography , inorganic chemistry , thermodynamics , chromatography , organic chemistry , physics , meteorology , enzyme
The spin equilibrium of camphor‐bound ferric cytochrome P‐450 has been measured between 1–1000 bar (10 5 ‐10 8 Pa). Increasing pressure shifts the absorption spectrum from the high‐spin form at 302 nm to the low‐spin form at 417 nm. The molar volume change for the spin states ΔV= ‐RTôln K e ôP and the equilibrium coefficient K e = [high spin]/[low spin] depend on the solvent conditions. At pH 5.6 the equilibrium coefficient at 1 bar, K 1 = 0.5 and ΔV = 31 cm 3 /mol. A sample with 10 mM KCl at pH 7 has K 1 = 7.0 and ΔV = 52 cm 3 /mol. Solvent changes producing a larger K 1 also result in a larger ΔV which ranged over 16–74 cm 3 /mol. The correlation can be approximated as ΔV = 36 + 18 log K 1 . which implies that there is a pressure, 3000 bar, for camphor‐bound ferric cytochrome P‐450 at 4°C, at which the changes in ΔV are compensated by the other thermodynamic parameters leaving K e independent of the solvent conditions. Although the protein is not stable above 1000 bar for most sample conditions, the extrapolated log K e versus pressure curves for all sample conditions intersect near 3000 bar. Camphor‐bound cytochrome P‐450 appears to be a rather flexible protein, having a low denaturing pressure, a large volume change, and a high sensitivity to the protein environment.