Oxidized flavin, deprotonation

The redox and proton transfer reactions undergone by the flavin prosthetic group are summarized in Scheme 5.2. The vertical reactions are oxidations by Q regenerating P. From the standard potential values (V vs. SCE) of the four flavin redox couples that are involved in Scheme 5.2 and those of the mediators (Table 5.1), all four oxidation steps may be regarded as irreversible. The horizontal reactions are deprotonations by the bases present in the buffer. From the pA values of the various flavin acid-base couples indicated in Scheme 5.2 (over or below the horizontal arrows), reactions H2 and H4 may be regarded as irreversible and reactions HI and... 

Over the years there have been a number of mechanistic proposals for substrate oxidation by TMADH. An early proposal considered a carbanion mechanism in which an active site base deprotonates a substrate methyl group to form a substrate carbanion [69] reduction of the flavin was then achieved by the formation of a carbanion-flavin N5 adduct, with subsequent formation of the product imine and dihydroflavin. A number of active site residues were identified as potential bases in such a reaction mechanism. Directed mutagenesis and stopped-flow kinetic studies, however, have been used to systematically eliminate the participation of these residues in a carbanion-type mechanism [76-79], thus indicating that a proton abstraction mechanism initiated by an active site residue does not occur in TMADH. Early proposals also invoked the trimethylammonium cation as the reactive species in the enzyme-substrate complex, owing to the high (9.81) of free... 

The Use of an Inverse Isotope Effect to Delineate an Enzyme Mechanism D-Amino acid oxidase catalyzes the oxidation of amino acids to imino acids via the transfer of a hydride to the coenzyme flavin adenine dinucleotide (FAD). The mechanism first involves deprotonation of the amino acid to create a carbanion (see below). This carbanion can then undergo either a nucleophilic addition to the flavin of FAD (Path A) or an electron transfer to the flavin, creating radicals that combine to give the same product as the nucleophilic addition (Path B). Expulsion of the flavin as a leaving group, concomitant with some proton transfers, gives the oxidized imino acid product. 

---