Structure of a quinohemoprotein amine dehydrogenase with an uncommon redoxcofactor and highly unusual crosslinking

The crystal structure of the heterotrimeric quinohemoprotein amine dehydrog enase from Paracoccus denitrificans has been determined at 2.05-Angstrom re solution. Within an 82-residue subunit is contained an unusual redox cofact or, cysteine tryptophylquinone (CTQ), consisting of an orthoquinone-modifie d tryptophan side chain covalently linked to a nearby cysteine side chain. The subunit is surrounded on three sides by a 489-residue, four-domain subu nit that includes a diheme cytochrome c. Both subunits sit on the surface o f a third subunit, a 337-residue seven-bladed beta -propeller that forms pa rt of the enzyme active site. The small catalytic subunit is internally cro sslinked by three highly unusual covalent cysteine to aspartic or glutamic acid thioether linkages in addition to the cofactor crossbridge. The cataly tic function of the enzyme as well as the biosynthesis of the unusual catal ytic subunit is discussed.