Active Site Generation Of A Protonically Unstable Suicide Substrate From A Stable Precursor: Glucose Oxidase And Dibromonitromethane

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Bromonitromethane is an inefficient suicide substrate for glucose oxidase (in contrast to the case of CH₃CCl=NO₂- and D-amino acid oxidase) because, in the enzyme-substrate encounter step, the required ionization states of enzyme (EH₀⁺, pKₐ ~ 3.5) and substrate (CHBr=NO₂-, pKₐ ~ 8.3) cannot be highly populated simultaneously. Because reprotonation of CHBr=NO₂- is rapid at the pH value used for the assay of glucose oxidase, presentation of the enzyme with the preformed anion could not be exploited in this case. We circumvent this difficulty by allowing the enzyme to reductively dehalogenate CHBr₂-NO₂, thereby generating the desired protonically unstable suicide substrate in situ (Eᵣ + CHBr₂NO₂ --> E₀ + CHBr=NO₂- + HBr + H⁺). Irreversible inactivation of the enzyme, because of the formation of a dead-end N-5 formylflavin adduct, is more than 100-fold faster when CHBr=NO₂- is generated in situ than when it is externally applied. The remaining competitive fates of CHBr=NO₂- at the active site are protonation and release or oxidation to HCOBr (or HCONO₂). Strong support for these conclusions comes from (1) the brisk evolution of CH3CBr=NO₂- (which is too bulky to act further as an efficient suicide substrate) from the enzyme-catalyzed reductive debromination of CH₃CBr₂NO₂, (2) the 1:1 stoichiometry of enzyme inactivation, and (3) the identification of the modified flavin as 5-formyl-1,5-dihydro-FAD.

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