Determinants of sweetness in proteins: a topological approach

Sweet taste in mammals is accounted for by a single receptor that shares homology with a metabotropic glutamate receptor. Most sweeteners are small molecular weight molecules that interact with small cavities in the so-called Venus Flytrap domains of the sweet receptor. The mechanism of action of larger molecules such as sweet proteins is, however, more difficult to interpret. The first and still the only general mechanism proposed for the action of sweet proteins, the "wedge model," hypothesizes that proteins bind to an external binding site of the active conformation of the sweet receptor. Here, I have extended the concept that inspired the wedge model using a combination of structural analysis, bioinformatics tools, and a relatively large dataset of mutations of the two most extensively studied sweet proteins, monellin and brazzein. I show here that it is possible to single out, among the ensemble yielded by low-resolution docking, a unique complex that satisfies simple topological constraints. These models of the complexes of monellin and brazzein are fully consistent with experimental evidence, thus providing predicting power for further validation of the wedge model.     

Optimization of fermentation conditions for the expression of sweet-tasting protein brazzein in Lactococcus lactis

Aims: To improve the production of sweet‐tasting protein brazzein in Lactococcus lactis using controlled fermentation conditions. Methods and Results: The nisin‐controlled expression system was used for brazzein expression. The concentration of nisin for induction and the optical density (OD) at induction were therefore optimized, together with growth conditions (medium composition, pH, aerobic growth in the presence of hemin). Brazzein was assayed with ELISA on Ni‐NTA plates and Western blot. Use of the M‐17 medium, containing 2·5% glucose, anaerobic growth at pH 5·9 and induction with 40 ng ml^?1 nisin at OD 3·0 led to an approx. 17‐fold increase in brazzein per cell production compared to non‐optimized starting conditions. Aerobic growth in the presence of hemin did not increase the production. Conclusions: Considerable increase in brazzein per cell production was obtained at optimized fermentation conditions. Significance and Impact of the Study: Optimized growth conditions could be used in application of brazzein expression in L. lactis. The importance of pH and OD at induction contributes to the body of knowledge of optimal recombinant protein expression in L. lactis. The new assay for brazzein quantification was introduced.