Three‐dimensional structure of the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans determined by molecular replacement at 2.8 Å resolution

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Abstract
The three‐dimensional structure of the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans (PD‐MADH) has been determined at 2.8 Å resolution by the molecular replacement method combined with map averaging procedures, using data collected from an area detector. The structure of methylamine dehydrogenase from Thio‐bacillus versutus, which contains an “X‐ray” sequence, was used as the starting search model. MADH consists of 2 heavy (H) and 2 light (L) subunits related by a molecular 2‐fold axis. The H subunit is folded into seven four‐stranded β‐segments, forming a disk‐shaped structure, arranged with pseudo‐7‐fold symmetry. A 31‐residue elongated tail exists at the N‐terminus of the H subunit in MADH from T. versutus but is partially digested in this crystal form of MADH from P. denitrificans, leaving the H subunit about 18 residues shorter. Each L subunit contains 127 residues arranged into 10 β‐strands connected by turns. The active site of the enzyme is located in the L subunit and is accessible via a hydrophobic channel between the H and L subunits. The redox cofactor of MADH, tryptophan tryptophylquinone is highly unusual. It is formed from two co‐valently linked tryptophan side chains at positions 57 and 107 of the L subunit, one of which contains an orthoquinone.

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