Reaction products and intermediates of tryptophan tryptophylquinone enzymes

Methylamine dehydrogenase and aromatic amine dehydrogenase each possess the tryptophan tryptophylquinone (TTQ) cofactor which catalyzes the oxidative deamination of primary amines and transfer of substrate-derived electrons to type I copper proteins. The reaction steps and intermediates in the overall oxidation–reduction reactions catalyzed by these enzymes are described. An important feature of the reaction mechanism of TTQ-dependent amine dehydrogenases is that the product of the reductive half-reaction is the reduced aminoquinol in which the substrate-derived amino group is covalently incorporated into the reduced TTQ cofactor. This has been proven by use of 15N NMR spectroscopy to monitor the fate of nitrogen from 15N-labeled substrate. This intermediate is significant because the covalent incorporation of N into the reduced TTQ has profound effects on the rates of electron transfer and factors which regulate the electron transfer reactions from the reduced enzymes. Study of the oxidative half-reaction of methylamine dehydrogenase showed that incorporation of the substrate-derived amino group into TTQ caused this reaction to become gated by a proton transfer from the aminoquinol, and revealed an important role for monovalent cations in the regulation of this reaction step. Analysis of monovalent cation-induced perturbations of the absorption spectrum of TTQ-dependent enzymes has provided insight into the mechanism of cation–protein interactions.