Amino acids cationic adduct structure

The structures of EX and El were deduced by resolution of El into apoenzyme and free flavin-substrate adduct. The structure of this adduct was determined as 5-cyanoethy 1-1,5-dihydro FAD and that of EX was deduced to be a cationic imine resulting from elimination of NO2" from the initial 5-nitroethy 1-1,5-dihydro FAD adduct formed in the process controlled by k2 by nucleophilic attack of nitroethane carbanion on the position of oxidized flavin. The chemistry of flavin reduction by nitroethane carbanion at the active site of D-amino acid oxidase is given by the following scheme (Equation 19) in which the kinetically important... 

Silverman and Zieske have rationalized how a protein nucleophile other than flavin is involved in MAO inactivation reactions, and why different inactivator compounds specifically react with flavin, protein amino acids, or both (100). Hydrogen atom donation from a cysteine residue to the flavin semiquinone radical would produce a thiyl radical, which could then capture the primary or secondary alkyl radical generated on cyclopropyl ring opening from the amine radical cation of the inactivator. The hydrogen atom abstraction reaction between the flavin and active site amino acid may be an equilibrium process such that either species could be present at any turnover. Hence, a combination of steric constraints and proximity to either the flavin semiquinone radical or the thiol radical will determine the site of adduct formation for a particular inactivator structure. A two-dimensional representation is shown in Scheme 23 (compounds 40-42), which illustrates the proposed equilibrium between the flavin semiquinone radical and amino acid as well as the proposed intermediates for the inactivation of MAO by A-(l-methylcyclopropyl)benzylamine 40 (104), rrradical center relative to the particular protein radical is consistent with proposed site of attachment of inactivator to protein 40 is near the flavin radical, such that exclusive flavin attachment occurs, 41 is positioned closer to the amino... 

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