Crystallization and preliminary X-ray crystallographic analysis of MxaJ, a component of the methanol-oxidizing system operon from the marine bacterium Methylophaga aminisulfidivorans MPT.

The methanol-oxidizing system (mox) is essential for methylotrophic bacteria to extract energy during the oxidoreduction reaction and consists of a series of electron-transfer proteins encoded by the mox operon. One of the key enzymes is the α₂β₂ methanol dehydrogenase complex (type I MDH), which converts methanol to formaldehyde during the 2e⁻ transfer through the prosthetic group pyrroloquinoline quinone. MxaJ, a product of mxaJ of the mox operon, is a component of the MDH complex and enhances the methanol-converting activity of the MDH complex. However, the exact functional mechanism of MxaJ in the complex is not clearly known. To investigate the functional role of MxaJ in MDH activity, an attempt was made to determine its crystal structure. Diffraction data were collected from a selenomethionine-substituted crystal to 1.92 Å resolution at the peak wavelength. The crystal belonged to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 37.127, b = 63.761, c = 99.246 Å. The asymmetric unit contained one MxaJ molecule with a calculated Matthews coefficient of 2.11 Ų Da⁻¹ and a solvent content of 41.7%. Three-dimensional structure determination of the MxaJ protein is currently in progress by the single-wavelength anomalous dispersion technique and model building.