Formation of W(3)A(1) electron-transferring flavoprotein (ETF) hydroquinone in the trimethylamine dehydrogenase center dot ETF protein complex

The electron-transferring flavoprotein (ETF) from Methylophilus methylotrop hus (sp. W(3)A(1)) exhibits unusual oxidation-reduction properties and can only be reduced to the level of the semiquinone under most circumstances (i ncluding turnover with its physiological reductant, trimethylamine dehydrog enase (TMADH), or reaction with strong reducing reagents such as sodium dit hionite). In the present study, we demonstrate that ETF can be reduced full y to its hydroquinone form both enzymatically and chemically when it is in complex with TMADH. Quantitative titration of the TMADH ETF protein complex with sodium dithionite shows that a total of five electrons are taken up b y the system, indicating that full reduction of ETF occurs within the compl ex. The results indicate that the oxidation-reduction properties of ETF are perturbed upon binding to TMADH, a conclusion further supported by the obs ervation of a spectral change upon formation of the TMADH ETF complex that is due to a change in the environment of the FAD of ETF. The results are di scussed in the context of ETF undergoing a conformational change during for mation of the TMADH ETF electron transfer complex, which modulates the spec tral and oxidation-reduction properties of ETF such that full reduction of the protein can take place.