title: 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 degrees C to -20 degrees C using ethylene glycol as anti-freeze cosolvent. This enabled us to measure the enthalpy and entropy changes which are delta HA = 0.7 kJ.mol-1 and delta SA = 5J.K-1.mol-1 at low temperatures and delta HB = -42 kJ.mol-1 and delta SB = -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.
authors.affiliation: Institut National de la Santé et de la Recherche Médicale, Unité 128, Montepellier, France.
authors.forename: R
authors.initials: R
authors.lastname: Lange
authors.forename: C
authors.initials: C
authors.lastname: Larroque
authors.forename: P
authors.initials: P
authors.lastname: Anzenbacher
mesh_terms: D000302:Adrenal Cortex / Q000201:enzymology; D000818:Animals; D002417:Cattle; D002786:Cholesterol Side-Chain Cleavage Enzyme / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008433:Mathematics; D008928:Mitochondria / Q000201:enzymology; D011487:Protein Conformation; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D008024:Ligands; D002786:Cholesterol Side-Chain Cleavage Enzyme