1,1,1-TRICHLOROETHANE-BOUND CYTOCHROME P450CAM DYNAMICS - DOES ACTIVE-SITE WATER MAKE A DIFFERENCE

Cytochrome P450cam is capable of reductively dehalogenating several ch lorinated alkanes at slow, but measurable, rates. To gain insight into the dynamic properties of the enzyme having an influence over its rea ctivity, molecular dynamics (Mo) simulations are performed on the comp lex of wild-type P450cam and 1,1,1-trichloroethane (TCA), a long-lived pollutant. Previous simulations on this complex indicated that, unlik e the enzyme complexed with camphor and camphor analogs, the behavior of TCA is sensitive to electrostatic interactions with the heme-Fe cof actor. Using recently developed partial charges for the heme, we condu cted two 300 ps MD simulations on TCA-bound P450cam. Since water bound in the enzyme active site is thought to be an important mediator of c atalysis, in one simulation three additional water molecules are docke d into space left void when the native substrate, camphor, is replaced with TCA. For the majority of configurations sampled, TCA in close pr oximity to the heme-Fe, suggesting that, if a high degree of spin conv ersion were achieved, degradation of TCA might proceed at a measurable rate. In the simulation without active site water, TCA samples an alt ernate binding pocket that opens due to rearrangement of active site r esidues, leaving room for water to presumably coordinate Fe. This resu lt is consistent with the experimental observation that TCA causes onl y a minor degree of heme spin conversion. The presence of active site water inhibits these significant dynamic fluctuations in the active si te, indicating that including extra water molecules in this binding po cket does not provide information crucial to understanding dynamic pro perties of the enzymes that influence the behavior of TCA.