Dehydrogenation Mechanism in Flavoprotein Catalysis

I "C -X-H+Acc(ox)--> C =X+Acc(red)H 2 I / H This type of reaction should be differentiated from the oxygenase reaction, in which the substrate electrons are transferred directly to oxygen. The limiting chemical step of the dehydrogenation, i.e., the rupture of the kinetically stable substrate C-H bond, is catalyzed by enzymes which have either pyridine nucleotide, pyridoxal-phosphate, flavin-, or pteridine as coenzymes. The mechanism of the biological dehydrogenation step is undoubtedly a cardinal question and has been addressed since the discovery of redox enzymes. Michaelis' discovery of flavin radicals prompted the proposal that biological redox processes mandatorily proceed through single-electron transfer steps (I). This type of mechanism was revived in the early sixties by Beinert's discovery of stable flavoprotein radicals (2). Cornforth, however, was the first to make a concrete mechanistic suggestion proposing in 1959 that the oxidation of Acyl-CoA substrates by Acyl-CoA dehydrogenase is initiated by abstraction of the proton, and that it goes to completion by subsequent transfer of radical entities (3). In 1964, Hemmerich independently proposed a carbanion-initiated mechanism for the oxidation of "activated" substrates (4). It was not until the late 60's, however, that new inputs and ideas were put forward which constituted the stimulus for the progress that led to our present knowledge. This experimental work will be discussed below.