Structure and dynamic studies by NMR of the potent sweet protein monellin and a non‐sweet analog

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Abstract
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 β‐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.