Structure-taste relationships of the sweet protein monellin

Structure-taste relationship studies were carried out by chemically synthesizing monellin and its analogs. Replacement of the Asp(B7) by L-2-aminobutyric acid, Gly and D-Asp resulted in complete loss of sweetness. Replacement of Ile(B6) and Ile(B8) 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 [Abu(B7)]monellin showed no marked differences except for the region of the Asp(B7). Thus, the complete loss of sweetness in [Abu(B7)]monellin is caused by the lack of free beta-carboxyl group in the Asp(B7) and not by a result of major disruption in the overall 3-dimensional structure. These results suggested that the free beta-carboxyl group of the Asp(B7) would possibly bind to the receptor site through ionic bonding and trigger the sensation of intense sweet taste, and Ile(B6) and/or Ile(B8) would be involved in the hydrophobic interaction with the receptor site. Selectively labeled monellin was synthesized by the solid-phase method by incorporating N-15-labeled amino acids into 10 key residues including Asp(B7). Relaxation analysis shows that Asp(B7) is the most flexible of these 10 residues. The flexibility of the active site may be important for receptor binding.