Brazzein, a Small, Sweet Protein: Effects of Mutations on its Structure, Dynamics and Functional Properties

The demand for non-calorigenic protein-based sweeteners with favorable taste properties is high. The optimal design of such sweeteners requires knowledge about structure–function relationships and the identification of chemical entities that trigger the sweetness response. Among the known, naturally occurring, sweet-tasting proteins, brazzein has properties that make it particularly attractive as a potential economic sweetener. It is highly stable over wide temperature and pH ranges and has taste properties that resemble those of carbohydrate sweeteners. Brazzein is a single polypeptide of 54 standard amino acids and contains no carbohydrate. The brazzein protein originally was purified from the fruit of Pentadiplandra brazzeana, a climbing vine that grows in Gabon, Zaire and Cameroon (Ming and Hellekant, 1994). The fruit is consumed by the local population and is prized for its sweetness. Pure brazzein has been shown to elicit sweetness responses in humans by taste trials and in a non-human primate (rhesus monkey) as determined from electrophysiological recordings of signals from the chorda tympani (van der Wel et al., 1989). Recently receptor proteins have been discovered that are responsive to sweet ligands. The human receptor appears to be a heterodimer of two conventional seven-transmembrane-helix Gcoupled type receptors (T1R2/T1R3) but with unusually large ectodomains ((Nelson et al., 2001; Li et al., 2002); P. Jiang and M. Max, personal communication). Different sweet proteins interact with and activate the same heterodimeric receptor in somewhat different ways and tentative models for such interactions have been elaborated ((Temussi, 2002); M. Max, personal communication). The discovery of the sweet taste heteroreceptor opens up exciting new avenues for research on the mechanism of action of sweet substances. Brazzein is an excellent candidate for experimental investigations of the chemical and structural requirements for extracellular triggering of a sweet response in humans and for understanding the mechanism of the signal transduction.