Methane monooxygenase and compound Q P450 chemistry without heme

Méthane monooxygenasc (MMO) catalyzes the oxidation of CH, to CH(OH in mcthanotrophs, thereby limiting the atmospheric egress of 10'' tons of (his greenhouse gas annually. The soluble form of the enzTne contains roductase (MMOR). hydrolase (MMOH) and regulatory "B" (MMOB) components MMOH contains a nonheme. hdroxo-bridged, diiron (Felu)Fe(III)) cluster at the active site, which replaces the function of the heme cofactor found in P45Q Single or multiple turnover of MMOH gives high vields of a wide anel> of oxidized saturated and unsaturated. linear, branched, cyclic, aromatic. heterocyclic. and halogenated hydrocarbons as observed for the P450 famiK However, the unique abiliu of MMOH to oxidize these substrates in a single active sue and to cleave the ( -H bond of CH, sets it apart. Rapid mixing of MMOH ( + MMOB) in (he Fc(ll)Fe(ll) state with 0- results in formation and deea> of at least 4 transient intermediates (O,P,Q,T) Compound O is probably a Michaelis complex with O. lnle P has spectroscopic characteristics suggesting an Fe(IlI)Fc(III) perox adduct Compound 0 reacts directh ith hydrocarbons to yield the product complex. T Mösbauer and EXAFS studies of Q show that the active site cluster contains mo Fe(lV) atoms bridged by 2 oxo groups to form a "diamond core". Q is equivalent to the Fe(IV) porphynn K cation radical proposed as the reactive species of P450. but it can be trapped in high yield for study Using Q, it is now possible to directlv observe the reaction step(s) in which oxgen insertion occurs It is found that in this step, oxidation of CHt occurs with a kinetic deuterium isotope effect ot 50-10(1. one of the largest ever observed This strongly supports a hdrogen atom abstraction ' rebound mechanism similar to that proposed for P450 and implies the presence of a radical intermediate (R) between 0 -in T.