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 sp ectroscopic properties of methylamine dehydrogenase (MADH). Here, we r eport the structure determination of this tryptophan tryptophylquinone -containing enzyme from Methylobacterium extorquens AM1 by high resolu tion x-ray crystallography (1.75 Angstrom). This first MADH crystal st ructure at low ionic strength is compared with the high resolution str ucture of the related MADH from Paracoccus denitrificans recently repo rted. We also describe the first structures (at 1.95 to 2.15 Angstrom resolution) of an MADH in the substrate-reduced form and in the presen ce of trimethylamine and of cesium, two competitive inhibitors. Polari zed absorption microspectrophotometry was performed on single crystals under various redox, pH, and salt conditions. The results show that t he enzyme is catalytically active in the crystal and that the cations cause the same spectral perturbations as are observed in solution. The se studies lead us to propose a model for the entrance and binding of the substrate in the active site.