Cofactors made from constitutive amino acids in proteins are now known to be relatively common. A number of these involve the generation of quinone cofactors, such as topaquinone in the copper-containing amine oxidases, and lysine tyrosylquinone in lysyl oxidase. The biogenesis of the quinone cofactor tryptophan tryptophylquinone (TTQ) in methylamine dehydrogenase (MADH) involves the post-translational modification of two constitutive Trp residues (Trpβ57 and Trpβ108 in Paracoccus denitrificans MADH). The modifications for generating TTQ are the addition of two oxygens to the indole ring of Trpβ57 and the formation of a covalent cross-link between Cε3 of Trpβ57 and Cδ1 of Trpβ108. The order in which these events occur is unknown. To investigate the role Trpβ108 may play in this process, this residue was mutated to both a His (βW108H) and a Cys (βW108C) residue. For each mutant, the majority of the protein that was isolated was inactive and exhibited weaker subunit-subunit interactions than native MADH. Analysis by mass spectrometry suggested that the inactive protein was a biosynthetic intermediate with only one oxygen atom incorporated into Trpβ57 and no cross-link with residue β108. However, in each mutant preparation, a small percentage of the mutant enzyme was active and appears to possess a functional tryptophylquinone cofactor. In the c0ase of βW108C, this cofactor may be identical to cysteine tryptophylquinone, recently described in the bacterial quinohemoprotein amine dehydrogenase. In βW108H, the active cofactor is presumably a histidine tryptophylquinone, which has not been previously described, and represents the synthesis of a novel quinone protein cofactor.