Methylamine dehydrogenase (MADH) is a soluble periplasmic α₂β₂ heterotetrameric enzyme, present in many methylotropic bacteria, that catalyzes the oxidation of methylamine to formaldehyde and ammonia. It is upregulated by the presence of substrate, and enables bacterial growth on methylamine as the sole carbon, nitrogen and energy source. The redox center is trytophan tryptophylquinone (TTQ) which is composed of two cross-linked Trp residues that are posttranslational modified through the addition of two oxygens to form an O-quinone. Amicyanin (a type I blue copper protein) is the redox partner of MADH and it is also induced in the presence of methylamine. TTQ is a chromophore, and has distinct spectral features in the UV/visible, which define specific electronic states of the cofactor during catalytic turnover. Through a novel combination of single crystal kinetics, UV/visible microspectrophotometry and X-ray crystallography of the holo- (with Cu) and apo- (without Cu) complexes of MADH with amicyanin, different catalytic intermediates have been trapped in crystals, and their structures determined. In this oral presentation, the structural features of O-quinone, N-quinol and N-semiquinone catalytic intermediates will be discussed in terms of the current model for catalysis and electron transfer.