The cholesterol‐side‐chain‐cleaving cytochrome P450 spin‐state equilibrium: 1. Thermodynamic analysis

Abstract:

We have investigated the spin‐state equilibrium of adrenal mitochondrial P450scc (cholesterol‐side‐chain‐cleaving, CYP11A1) by absorption spectroscopy in the Soret band as a function of pH and temperature. The van't Hoff plot of the high‐spin/low‐spin equilibrium is not linear and is shifted towards high spin by lowering the pH. This non‐linearity resolves clearly into two phases when the temperature range is extended from 37°C to −20°C using ethylene glycol as anti‐freeze cosolvent. This enabled us to measure the enthalpy and entropy changes which are ΔHA= 0.7 kJ · mol−1 and ΔSA= 5 J · K−1· mol−1 at low temperatures and ΔHB=−42 kJ · mol−1 and ΔSB=−152 J · K−1· mol−1 at high temperatures. The transition temperature, Tbreak, between both phases decreases as a function of pH. The experimental data can be fitted by a minimal reactional model comprising a temperature dependent conformational transition and two ionisation steps (one for each conformation), the pK of which is 1.5 ± 0.5 higher in the low‐temperature conformation. The deduced conformational equilibrium is affected by physiological effectors: Tbreak depends on the nature of the substrate intermediate and on the presence of the physiological electron donor, adrenodoxin. Copyright © 1992, Wiley Blackwell. All rights reserved

Año de publicación:

1992

Keywords:

Fuente:

scopus