Regulation of methane monooxygenase catalysis based on size exclusion and quantum tunneling

The hydroxylase component (MMOH) of the soluble form of methane monooxygenase (sMMO) isolated from Methylosinus trichosporium OB3b catalyzes both the O₂ activation and the CH₄ oxidation reactions at the oxygen-bridged dinuclear iron cluster present in its buried active site. During the reaction cycle, the diiron cluster forms a bis-μ-oxo-(Fe(IV)) ₂ intermediate termed compound Q (Q) that reacts directly with methane. Many adventitious substrates also react with Q, most at a relatively slow rate. We have proposed that Q reacts preferentially with CH₄ because the sMMO regulatory component MMOB induces a size selective pore into the MMOH active site as the two components form a complex. Support for this proposal has come through the observation of a nonlinear Arrhenius plot for the CH₄ oxidation, presumably due to a shift in rate-limiting step from substrate binding at low temperature to C-H bond cleavage at high temperature. Reactions of all substrates other than CH₄ fail to exhibit a break in the Arrhenius plot because binding is always rate limiting in the temperature range explored. Here we show that it is possible to induce a break in the Arrhenius plot for the ethane reaction with Q by using an MMOB mutant termed DBL2 (S109A/T111A) in which residues at the MMOH-MMOB interface are reduced in size. We hypothesize that this increases the ethane binding rate and shifts the Arrhenius breakpoint into the observable temperature range. As a result of this shift, the kinetic and activation parameters of the C-H bond breaking reaction for both methane and ethane can be observed using the DBL2 mutant. A ²H-KIE is observed for both substrate oxidation reactions when using DBL2, whereas only CH₄ oxidation exhibits an effect when using wild type MMOB, consistent with the C-H bond cleaving reaction becoming at least partially rate limiting for ethane. Analysis of the temperature dependence of the ₂H-KIE for ethane and methane for reactions using both mutant and wild type forms of MMOB suggests that quantum tunneling plays a significant role in methane oxidation but not ethane oxidation.