Electrostatic environment of the tryptophylquinone cofactor in methylamine dehydrogenase: Evidence from resonance Raman spectroscopy of model compounds

Methylamine dehydrogenase (MADH) utilizes its endogenous tryptophan tryptophylquinone (TTQ) as a cofactor in enzymatic catalysis, with the C6 carbonyl of the quinone implicated as the site of attack by substrates and other nucleophiles. Resonance Raman (RR) spectroscopy provides an ideal method for investigating the state of this carbonyl group whose C=O stretch is distinct from other vibrational modes of the cofactor and is readily identified by its shift to lower energy in H₂¹⁸O. In a series of indole 6,7-quinone models for TTQ, the in-phase stretching vibration of the two C=O groups occurs at 1650 cm^-1 in nonpolar solvents and shifts to 1638 cm^-1 in H₂O. The absorption maximum undergoes an analogous shift from 400 to 425 nm. The spectral properties of the indole quinones in H₂O approach the corresponding values in Thiobacillus versutus MADH (C=O stretch at 1612 cm^{- 1}, λ(max) at 440 nm) and are indicative of strong hydrogen bonding of the C=O and NH groups of the cofactor in the native enzyme. Addition of monovalent cations (NH4⁺, Cs⁺, and (CH₃)₃NH⁺] to MADH causes further increases in the λ(max) and decreases in the frequency of the C=O stretch [1590 cm^-1m with (CH₃)₃NH⁺]. This implies a strong electrostatic interaction between monovalent cations and a carbonyl oxygen (most likely at C6) in TTQ. The fact that these cations behave as competitive inhibitors of the methylamine substrate suggests that methylamine binds to the same location in the enzyme prior to its covalent reaction with the cofactor. Addition of monovalent cations to the one-electron-reduced semiquinone form of MADH results in RR spectral shifts for a number of vibrational modes of the cofactor. Thus, the ability of monovalent cations to promote and stabilize the formation of the semiquinone intermediate is also due to their direct electrostatic interaction with the TTQ cofactor.