The horse EE and human β1β1 alcohol dehydrogenase isoenzymes have almost identical protein backbone folding patterns and contain 2 tryptophans per subunit (Trp-15 and Trp-314). Tyr-286, which had been proposed to quench the fluorescence of Trp-314 by resonance energy transfer at alkaline pH in EE, is substituted by Cys in β1β1. The proposed role of Tyr-286 in pH-dependent quenching of EE is confirmed by our observation that tryptophan fluorescence of β1β1 is not substantially quenched at alkaline pH. Tyr-286 had also been implicated in the quenching of Trp-314 upon formation of the EE-NAD+-trifluoroethanol ternary complex. However, β1β1 exhibits the same extent of tryptophan fluorescence quenching as EE upon complexation, which strongly suggests that Tyr-286 is not involved in ternary complex quenching.
