A Central Role for Water in the Control of the Spin State of Cytochrome P‐450scc

A previous thermodynamic study [Lange, R., Larroque, C. & Anzenbacher, P. (1992) Eur. J. Biochem. 207, 69–73] demonstrated two conformations (A and B) of cytochrome P-450scc (SCC), the enzyme which initiates steroid biosynthesis by cleaving the side chain of cholesterol. The conformation found at the lowest temperatures (form A) displays a six-ligand high-spin heme iron [Hildebrandt, P., Heibel, G., Anzenbacher, P., Lange, R., Kruger, V. & Stier, A. (1994) Biochemistry 33, 12920–12929]. Analytical centrifugation shows that the oligomeric composition of SCC is the same for the A and the B conformers. However, as revealed by fourth-derivative ultraviolet spectroscopy, the two conformers differ in the mean environment of the tryptophan residues, which was more polar in the A form. The structural role of water in these two conformations was investigated using the pressure-jump technique under various pH, temperature and osmotic-stress conditions. Applying hydrostatic pressure to SCC induced very slow (τ >30 min) biexponential relaxation kinetics corresponding to the high-spin to low-spin transition. Analysis of the activation volumes suggested a dissociative mechanism for the A conformer (+45 ml/ mol), and an associative mechanism for the B conformer (−39 ml/mol). Applying osmotic stress to the A form changed its kinetic characteristics to those of the B form. These results are consistent with a model comprising a solvent intake (ten water molecules) between the B and the A conformers and protonation of their respective high-spin states. The sixth ligand of the high-spin form in the A conformer involves a water molecule and an unknown constraining structure.