The flexibility of the structure and compressibility of the respective active site of cytochromes P450 3A4 (CYP3A4) and BM-3 (CYP102) were studied using absorption spectroscopy in the ultraviolet and visual regions. Conformational changes in the overall protein structures of both CYP3A4 and CYP102 due to the effects of temperature and pressure are reversible. However, the enzymes differ in the properties of their active sites. The CYP3A4 enzyme denatures to the inactive P420 form relatively easy, at 3000 bar over half is converted to P420. The compressibility of its active site is lower than that of CYP102 and is greater with the substrate bound, which is in line with the observed lack of a stabilizing effect of the substrate on its conformation under pressure. In contrast, CYP102, although having the most compressible active site among the P450s, possesses a structure that does not denature easily to the inactive (P420) form under pressure. In this respect, it resembles the P450 isolated from acidothermophilic archaebacteria [McLean, M.A., Maves, S.A., Weiss, K.E., Krepich, S. & Sligar, S.G. (1998) Biochem. Biophys. Res. Commun. 252, 166-172].