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Pure Appl. Chem., 2002, Vol. 74, No. 7, pp. 1235-1242

Structure­taste relationships of the sweet protein monellin

Masanori Kohmura*, Toshimi Mizukoshi, Noriki Nio, Ei-ichiro Suzuki and Yasuo Ariyoshi

Central Research Laboratories, Ajinomoto Co., Inc., 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki, 210-8681 Japan

Abstract: Structure­taste relationship studies were carried out by chemically synthesizing monellin and its analogs. Replacement of the AspB7 by l-2-aminobutyric acid, Gly and d-Asp resulted in complete loss of sweetness. Replacement of IleB6 and IleB8 by different amino acids resulted in a significant decrease of sweetness, or complete loss of sweetness. Comparison of short- and long-range nuclear Overhauser effects (NOEs) and chemical shifts between monellin and [AbuB7]monellin showed no marked differences except for the region of the AspB7. Thus, the complete loss of sweetness in [AbuB7]monellin is caused by the lack of free β-carboxyl group in the AspB7 and not by a result of major disruption in the overall 3-dimensional structure. These results suggested that the free β-carboxyl group of the AspB7 would possibly bind to the receptor site through ionic bonding and trigger the sensation of intense sweet taste, and IleB6 and/or IleB8 would be involved in the hydrophobic interaction with the receptor site. Selectively labeled monellin was synthesized by the solid-phase method by incorporating 15N-labeled amino acids into 10 key residues including AspB7. Relaxation analysis shows that AspB7 is the most flexible of these 10 residues. The flexibility of the active site may be important for receptor binding.