Redox properties of tryptophan tryptophylquinone enzymes. Correlation with structure and reactivity

The pH dependence of the redox potentials for the oxidized/reduced couples of methylamine dehydrogenase (MADH) and aromatic amine dehydrogenase (AADH) were determined. For each enzyme, a change of -30 mV/pH unit was observed, indicating that the two-electron transfer is linked to the transfer of a single proton. This result differs from what was obtained from redox studies of a tryptophan tryptophylquinone (TTQ) model compound for which the two-electron couple is linked to the transfer of two protons. This result also distinguishes the redox properties of the enzyme-bound TTQ from those of the membrane-bound quinone components of respiratory and photosynthetic electron transfer chains that transfer two protons per two electrons. This difference is attributed to the accessibility of TTQ to solvent in the enzymes. One of the quinol hydroxyls is shielded from solvent and thus is not protonated. The unusual property of TTQ enzymes of stabilizing the anionic form of the reduced quinol is important for the reaction mechanism of MADH because it allows stabilization of physiologically important reaction intermediates. Examination of the extent to which disproportionation of the MADH and AADH semiquinones occurred as a function of pH revealed that the equilibrium concentration of semiquinone increased with pH. This indicates that the proton transfer is linked to the semiquinone/quinol couple. Therefore, the quinol is singly protonated, and the semiquinone is unprotonated and anionic. It was also shown that the oxidation-reduction midpoint potential for AADH is 20 mV less positive than that of MADH over the range of pH values that was studied and that the TTQ semiquinone of AADH was less stable than that of MADH. This may be explained by differences in the active site environments of the two enzymes, which modulate their respective redox properties.