Structure and dynamic studies by NMR of the potent sweet protein monellin and a non-sweet analog. Evidence on the importance of residue AspB7 for sweet taste.

Monellin, an intensely sweet protein and a non-sweet analog in which the AspB7 in monellin has been replaced with AbuB7 were studied by NMR. The results of our investigations show that the 3-dimensional structure of these two proteins are very similar indicating that the lack of the beta-carboxyl group in the AbuB7 analog is responsible for the loss of sweet potency. Selectively labeled monellin was prepared by solid-phase peptide synthesis by incorporating 15N-labeled amino acids into 10 key positions including AspB7. The internal mobility of these 10 key residues in monellin was estimated by the method of model-free analyses and our NMR studies show that AspB7 is the most flexible of these 10 residues. The flexibility of the AspB7 side chain may be important for receptor binding.