ENZYME-CATALYZED DEHALOGENATION OF PENTACHLOROETHANE - WHY F87W-CYTOCHROME P450CAM IS FASTER THAN WILD-TYPE

Under anaerobic conditions, cytochromes P450 can reductively dehalogen ate heavily halogenated hydrocarbons, such as one- and two-carbon orga nic solvents, This catalytic capacity has drawn attention to the poten tial use of engineered forms of P450s in the remediation of contaminat ed deep subsurface ecosystems. Loida (1994, PhD Thesis, University of Illinois at Urbana-Champaign, IL) and S.G.Sligar (personal communicati on) have observed recently that an active-site variant of cytochrome P 450cam (F87W) dechlorinates pentachloroethane approximately three time s faster than the wild-type enzyme, Molecular dynamics simulations hav e revealed that the mutant enzyme binding pocket remains smaller, and that pentachloroethane assumes configurations closer to the heme-Fe in the F87W mutant twice as often as in the wild-type enzyme, This resul t is consistent with a collisional model of dehalogenation, which agre es with experimental observations[li and Wackett (1993) Biochemistry, 32, 9355-9361] that solutions containing wild-type P450cam dehalogenat e pentachloroethane 100 times faster than those containing free heme, The simulations suggest that it is unlikely that Trp87 significantly s tabilizes the developing negative charge on the substrate during carbo n-halogen bond reduction, The design of improved microbial enzymes tha t incorporate both steric and electronic effects continues for use in remediating halogenated contaminants in situ.