Binding of monovalent cations to methylamine dehydrogenase in the semiquinone state and its effect on electron transfer

Catalytic control of redox reactivities of coenzyme analogs by metal ions

Redox coenzymes and analogs have their own redox reactivities for both thermal and photochemical redox reactions. The redox activities of coenzymes can be tuned by using metal ions that can bind the redox coenzymes and analogs. Quantitative measure to determine the Lewis acidity of a variety of metal ions is given in relation to the catalytic reactivities. The mechanistic viability of metal ion catalysis in redox reactions of coenzyme analogs is described by showing a number of examples of both thermal and photochemical reactions that are made possible to proceed by controlling the redox reactivities of coenzymes with metal ions.

Crystallographic and spectroscopic studies of native, aminoquinol, and monovalent cation-bound forms of methylamine dehydrogenase from Methylobacterium extorquens AM1

Various monovalent cations influence the enzymatic activity and the spectroscopic properties of methylamine dehydrogenase (MADH). Here, we report the structure determination of this tryptophan tryptophylquinone-containing enzyme from Methylobacterium extorquens AM1 by high resolution x-ray crystallography (1.75 A). This first MADH crystal structure at low ionic strength is compared with the high resolution structure of the related MADH from Paracoccus denitrificans recently reported. We also describe the first structures (at 1.95 to 2.15 A resolution) of an MADH in the substrate-reduced form and in the presence of trimethylamine and of cesium, two competitive inhibitors. Polarized absorption microspectrophotometry was performed on single crystals under various redox, pH, and salt conditions. The results show that the enzyme is catalytically active in the crystal and that the cations cause the same spectral perturbations as are observed in solution. These studies lead us to propose a model for the entrance and binding of the substrate in the active site.

Refined crystal structure of methylamine dehydrogenase from Paracoccus denitrificans at 1.75 A resolution

The three-dimensional structure of the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans has been refined at 1.75 A resolution utilizing the DNA-based protein sequence. The final model incorporates 8034 atoms per molecule, including 552 molecules of solvent, and gives an R-factor of 0.163. The molecule is an H2L2 hetero-tetramer containing a non-crystallographic 2-fold axis of symmetry. The 373-residue H subunit is folded into seven repeats of a four-stranded antiparallel beta-sheet motif, arranged in a propeller-like pattern about a pseudo-7-fold rotational axis of symmetry. Each L subunit contains 131 residues folded in a tight structure composed of five beta-strands in two sheets and crosslinked by six disulfide bonds. In addition there is an intrasubunit covalent linkage between two tryptophan side-chains that form the unique redox center, tryptophan tryptophylquinone (TTQ). The active site contains the O-6 carbonyl of TTQ, the side-chains of Asp32L Asp76L, Tyr119L and Thr122L, and two solvent molecules. A potential "gate" (Phe55H) separates the closed active-site cavity from a channel containing a group of highly ordered water molecules to bulk solvent. Phe55H and Tyr119L, and a number of neighboring oxygen atoms, may also provide a binding site for monovalent cations that are known to affect the reactivity and spectral properties of TTQ as well as the oxidative half reaction. The overall reaction has been dissected into a number of discrete steps that may require participation by several individual amino acid residues in the active site acting as general acids and bases.

Kinetic and chemical mechanisms for the effects of univalent cations on the spectral properties of aromatic amine dehydrogenase

Univalent cations and pH influence the UV-visible absorption spectrum of the tryptophan tryptophylquinone (TTQ) enzyme, aromatic amine dehydrogenase (AADH). Little spectral perturbation was observed when pH was varied in the absence of univalent cations. The addition of alkali metal univalent cations (K+, Na+, Li+, Rb+ or Cs+) to oxidized AADH caused significant changes in its absorption spectrum. The apparent Kd for each cation, determined from titrations of the spectral perturbation, decreased with increasing pH. Transient kinetic studies involving rapid mixing of AADH with cations and pH jump revealed that the rate of the cation-induced spectral changes initially decreased with increasing cation concentration to a minimum value, then increased with increasing cation concentration. A kinetic model was developed to fit these data, determine the true pH-independent Kd values for K+ and Na+, and explain the pH-dependence of the apparent Kd. A chemical reaction mechanism, based on the kinetic data, is presented in which the metallic univalent cation facilitates the chemical modification of the TTQ prosthetic group to form an hydroxide adduct which gives rise to the spectral change. Addition of NH4(+)/NH3 to AADH caused changes in the absorption spectrum which were very different form those caused by addition of the metallic univalent cations. The kinetics of the reaction induced by addition of NH4+/NH3 were also different, being simple saturation kinetics. Another reaction mechanism is proposed for the NH4+/NH3-induced spectral change that involves nucleophilic addition of the unprotonated NH3 to TTQ. The general relevance of these data and models to the physiological reactions of TTQ-dependent enzymes and to the roles of univalent cations in modulating enzyme activity are discussed.