P450 active site architecture and reversibility: Inactivation of cytochromes P450 2B4 and 2B4 T302A by tert-butyl acetylenes

The inactivations of P450 2B4 and the T302A mutant of 2B4 by tert-butyl acetylene (tBA) and the inactivation of 2B4 T302A by tert-butyl 1-methyl-2-propynyl ether (tBMP) have been investigated. tBA and tBMP inactivated both enzymes in a mechanism-based manner with the losses in enzymatic activity corresponding closely to losses in P450 heme. HPLC and ESI-LC-MS analysis detected two different tBA- or tBMP-modified heme products with masses of 661 and 705 Da, respectively. Interestingly, the inactivations of the P450s 2B4 by tBA and tBMP were partially reversible by dialysis, and the tBA- or tBMP-modified heme products could only be observed with ESI-LC-MS/MS when the inactivated samples were acidified prior to analysis, indicating a requirement for protons in the formation of stable heme adducts in both the wild-type and mutant 2B4 enzymes. Results of studies using artificial oxidants to support enzyme inactivation suggest that the oxenoid-iron activated oxygen species is preferentially utilized during the inactivation of the P450s 2B4 by tBA. These results argue against the use of a peroxo-iron species by P450 2B4 T302A. Molecular dynamics studies of wild-type P450 2B4 reveal that contiguous hydrogen bond networks, including structural waters, link a conserved glutamate (E301) to the distal oxygen of the peroxo-heme species via threonine 302. Interestingly, models of 2B4 T302A reveal that a compensatory, ordered hydrogen bond network forms despite the removal of T302. These results indicate that while T302 may play a role in proton delivery in the formation of the oxenoid-iron complex and in the stabilization of acetylene heme adducts in 2B4, it is not essential for proton delivery given the presence of E301 in the binding site

Blobaum, AL., Harris, D., & Hollenberg, PF. (2005). P450 active site architecture and reversibility: Inactivation of cytochromes P450 2B4 and 2B4 T302A by tert-butyl acetylenes. Biochemistry, 44(10), 3831-3844. https://doi.org/10.1021/bi0478953