Structural characterization of monellin in the alcohol-denatured state by NMR: evidence for beta-sheet to alpha-helix conversion.

Two-dimensional 1H NMR spectroscopy and hydrogen exchange methods have been used to characterize the alcohol-denatured state of monellin. Monellin is a sweet tasting protein composed of two chains. In the native state, the A-chain consists entirely of beta-structure, and the B-chain contains both alpha- and beta-structure. Upon addition of either 50% ethanol or 50% trifluoroethanol (TFE), the native structure of monellin is disrupted resulting in an alcohol-denatured state with properties different from those of the random coil state. In the alcohol-denatured state, the far-UV circular dichroism (CD) spectrum displays a higher helical content relative to the native state and the intensity of the near-UV CD signal is completely lost. One-dimensional NMR studies show that there are approximately 14 amide protons protected from exchange with solvent in the alcohol-denatured state and that large portions of the protein exchange at a rate that is comparable to the exchange rate of the protein in urea. Utilizing hydrogen exchange trapping techniques, the slowly exchanging residues are identified at pH 2.0 in 50% ethanol and 50% TFE (A10-A15, A18, A19, A21, A24, and A39) and are found to be clustered on one region of the A-chain. Preliminary 2D NMR assignments show that in the alcohol-denatured state the A-chain of monellin undergoes structural reorganization, with one strand of the native state beta-sheet on the A-chain (residues A17-A30) becoming an alpha-helix in the alcohol-denatured state. The secondary structure of the A-chain in the alcohol-denatured state is different from the native state structure, although the slowly exchanging residues are similar.