Multiple mutations of the critical amino acid residues for the sweetness of the sweet-tasting protein, brazzein

We have previously identified critical residues important for sweetness of the sweet protein brazzein by site-directed mutagenesis (Yoon, Kong, Jo, & Kong, 2011). In order to elucidate the interaction mechanisms of brazzein with the sweet taste receptor, we made multiple mutations of three residues (His31 in loop 30–33, Glu36 in β-strand III, and Glu41 in loop 40–43). We found that all double mutations (H31R/E36D, H31R/E41A and E36D/E41A) made the molecules sweeter than des-pE1M-brazzein and three single mutants. Moreover, the triple mutation (H31R/E36D/E41A) made the molecule significantly sweeter than three double mutants. These results strongly support the hypothesis that brazzein binds to the multisite surface of the sweet taste receptor. Our findings also suggest that mutations reducing the overall negative charge and/or increasing the positive charge favour sweet-tasting protein potency.