Heme‐pocket‐hydration change during the inactivation of cytochrome P‐450camphor by hydrostatic pressure

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Abstract
Hydrostatic pressure has been used to convert cytochrome P-450camphor to cytochrome P-420. The latter is an inactivated but soluble and undenaturated form of cytochrome P-450camphor. Using camphor analogues as probes of the active site we show that the inactivation volume change is directly correlated to the initial degree of hydration of the heme pocket. The values range between -73 ml/mol and -197 ml/mol [Di Primo, C., Hui Bon Hoa, G., Douzou, P. & Sligar, S. G. (1990) Eur. J. Biochem. 193, 383-386] for a totally hydrated (substrate-free, low-spin, six coordinated heme iron) and a non-hydrated (camphor-bound, high-spin, five coordinated heme iron) heme pocket. These results suggest that the larger value, -197 ml/mol, for the inactivation volume change is due to a hydration change of the heme pocket resulting from the displacement of the substrate during the compression and the subsequent entrance of water molecules. Similarly, the stability of the protein against compression is correlated with water accessibility to the active site. Increase in substrate mobility by loss of specific interactions with both regions of well defined secondary structure of cytochrome P-450camphor results in an increase of water accessibility and decrease of stability. Thus for camphor and adamantanone which strongly interact with the protein and exclude water from the active site [Poulos, T. L., Finzel, B. C. & Howard, A. J. (1987) J. Mol. Biol. 195, 687-700; Raag, R. & Poulos, T. L. (1989) Biochemistry 28, 917-922] the increase in stability compared to the free protein is roughly 30 kJ/mol at 20 degrees C. With smaller substrates such as norcamphor, which loosely fits into the active site and does not completely exclude water [Raag, R. & Poulos, T. L. (1989) Biochemistry 28, 917-922], the increase in stability is only 7 kJ/mol. Finally these results suggest that cytochrome P-420 induced by hydrostatic pressure is a unique form where the active site is hydrated and camphor is displaced from its binding site.