Highly probable active site of the sweet protein monellin

The sweet protein monellin consists of two noncovalently associated polypeptide chains, the A chain of 44 amino acid residues and the B chain of 50 residues. Synthetic monellin is 4000 times as sweet as sucrose on a weight basis, and the native conformation is essential for the sweet taste. Knowledge of the active site of monellin will provide important information on the mode of interaction between sweeteners and their receptors. If the replacement of a certain amino acid residue in monellin removes the sweet taste, while the native conformation is retained, it may be concluded that the position replaced is the active site. Our previous replacement studies on Asp residues in the A chain did not remove the sweet taste. The B chain contains two Asp residues at positions 7 and 21, which were replaced by Asn. [Asn B21 ] Monellin and [Asn B7 ]monellin were 7000 and 20 times sweeter than sucrose, respectively. The low potency of the [Asn B7 ]monellin indicates that ASp B7 participates in binding with the receptor. ASp B7 was then replaced by Abu. [Abu B7 ]Monellin was devoid of sweetness, and retained the native conformation. ASp B7 is located at the surface of the molecule (Ogata et al.). These results suggest that Asp 7 in the B chain is the highly probable active site of monellin.