Dissimilar sweet proteins from plants: Oddities or normal components?

The fruits of a few tropical plants contain intensely sweet proteins. Their common property points to a protein family. Generally, proteins belonging to the same family share similar folds, similar sequences and, at least in part, similar function but sweet proteins constitute an exception to this rule. Apart from sharing the rather unusual taste function, they show no obvious similarities either in their sequences or in three-dimensional structures.In this review we describe the nature, structure and mechanism of action of the best known sweet tasting proteins, including two taste modifying proteins. Sweet proteins stand out among sweet molecules because their volume is not compatible with an interaction with orthosteric active sites of the sweet taste receptor. The best explanation of their mechanism of action is the interaction with the external surface of the sweet taste receptor, according to a model that has been named “wedge model”. It is hypothesized that this mode of action may be related to the ability of other members of their protein families to inhibit different enzymes.

► Some tropical plants contain proteins several orders of magnitude sweeter than sugars.
► Their taste is explained by a wedge model as external interaction with the receptor.
► Most sweet proteins belong to families whose members are enzyme inhibitors.
► A large surface of interaction is consistent with the wedge model for sweet proteins.
► Their mechanism of action has deepened our understanding of the sweet taste receptor.