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Study of Amide-proton Exchange of Escherichia coliMelibiose Permease by Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy

The accessibility ofEscherichia coli melibiose permease to aqueous solvent was studied following hydrogen-deuterium exchange kinetics monitored by attenuated total reflection-Fourier transform infrared spectroscopy under four distinct conditions where MelB forms different complexes with its substrates (H+, Na+, melibiose). Analysis of the amide II band upon2H2O exposure discloses a significant sugar protection of the protein against aqueous solvent, resulting in an 8% less exchange of the corresponding H+·melibiose·MelB complex compared with the protein in the absence of sugar. Investigation of the amide I exchange reveals clear substrate effects on β-sheet accessibility, with the complex H+·melibiose·MelB being the most protected state against exchange, followed by Na+·melibiose·MelB. Although of smaller magnitude, similar changes in α-helices plus non-ordered structures are detected. Finally, no differences are observed when analyzing reverse turn structures. The results suggest that sugar binding induces a remarkable compactness of the carrier's structure, affecting mainly β-sheet domains of the transporter, which, according to secondary structure predictions, may include cytoplasmic loops 4–5 and 10–11. A possible catalytic role of these two loops in the functioning of MelB is hypothesized.