Crystallographic Studies of the Methylamine Dehydrogenase from Methylobacterium extorquens AM1. Gilles Labesse, Zhi-wei Chen, Maria-Luisa Veisaga, F. Scott Mathews, Dept. of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO, USA; William S. McIntire, Molecular Biology Division, Dept. of Veterans Affairs Medical Center, San Francisco, CA, and Dept. of Biochemistry and Biophysics, and Anesthesia, San Fransisco, CA, USA.

The influence of monovalent cations on the enzymatic activity, structure and spectroscopic properties of methylamine dehydrogenase (MADH) from Methylobacterium extorquens AM1 is being investigated by high resolution X-ray crystallography.

MADH catalyzes the oxidation of methylamine in the periplasm of various methylotrophic bacteria to provide the organism with carbon and nitrogen. The crystal structures of two MADH, from Thiobacillus versutus and Paracoccus denitrificans have been solved showing a particular protein fold and a unique cofactor. The structure of the heavy subunit (about 400 residues) is composed of antiparallel [beta]-sheets and posseses a pseudo seven-fold symmetry. The light subunit (about 130 residues) comprises the tryptophan tryptophylquinone cofactor (TTQ) and seven disulphide bridges.

Crystal of MADH from M. extorquens AM1 were grown at low salt concentration in PEG4000 at acidic pH while published structures of other MADH were grown in high salt conditions. Data collection at room temperature to high resolution (1.7 Å) were obtained using a single crystal. The determination of the structure of MADH from AM1 is under way using molecular replacement.

The binding of monovalent cations binding to two different sites of the protein and the induced change on the enzyme activity, structure and crystal growth are under investigations. One of these cation binding sites is located in the active site. The high resolution structure of M. extorquens AM1 MADH crystals grown or soaked with various substrates and inhibitors will help us understand the mechanism of action of this enzyme.