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

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. © 1992 Wiley‐Liss, Inc.