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  Structure of a Quinohemoprotein Amine Dehydrogenase with an Uncommon Redox Cofactor and Highly Unusual Crosslinking



Recently, a number of unorthodox cofactors that are derived posttranslationally from their cognate proteins have been discovered, particularly in several redox enzymes. We determined the 2.05-Åcrystal structure of the quinohemoprotein amine dehydrogenase from Paracoccus denitrificans and identified a novel quinone cofactor, cysteine tryptophylquinone (CTQ) consisting of an orthoquinone-modified tryptophan side chain covalently linked to a nearby cysteine side chain, within an 82-residue subunit. The subunit is surrounded on three sides by a 489-residue, four-domain subunit that includes a diheme cytochrome c. Both subunits sit on the surface of a third subunit, a 337-residue 7-bladed β-propeller that forms part of the enzyme active site. The small catalytic subunit is internally crosslinked by three highly unusual covalent cysteine to aspartic or glutamic acid thioether linkages in addition to the cofactor crossbridge. Involvement of a 'Radical SAM Protein' encoded breaking into the enzyme gene is implicated for the biosynthesis of the unique catalytic subunit.

*Copyright (2001) National Academy of Sciences, U.S.A.


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