Protein which contains at least one tryptophan tryptophylquinone (TTQ) cross-link modification. TTQ, the cofactor of methylamine dehydrogenase (MADH), is formed by oxidation of the indole ring of a tryptophan to form tryptophylquinone followed by covalent cross-linking with another tryptophan residue. MADH converts primary amines to their corresponding aldehydes plus ammonia. During the catalytic cycle, TTQ mediates electron transfer from the substrate to a copper protein, amicyanin. These electrons are transferred to the respiratory chain via a C-type cytochrome.