Brief Biochemistry of Brazzein, a Sweet Tasting Protein

This protein was quite fun in modelling considering its secondary structures. I will admit though that I have a newfound appreciation for molecular chaperones, the proteins that assist with non-covalently folding other proteins. Below is an example of my brazzein wire model:

Derived from the berries of a West African plant, the Pentadiplandra brazzeana Baillon, brazzein is one of the sweetest and the smallest of the sweet tasting proteins. In fact, this 54 amino acid long protein can be 500 to 2000 times sweeter than sucrose. The presence of PyrE (pyroglutamic acid) at the N-terminus of brazzein is known to cap sweetness at the lower aformentioned levels, thus cleaving PyrE increases sweetness. So far, studies have shown that only humans and Old World primates can taste brazzein’s sweetness. This occurrence is best explained by the activation of human sweet receptors, heterodimeric G-protein coupled receptors (GPCRs), by brazzein.

The structure of brazzein plays a critical role in sweetness. As with most other proteins found in nature, the most common stereoisomer of this protein is the L-enantiomer. The D-enantiomer, or the mirrored image, can be prepared by synthesizing brazzein with the fluoren-9-yl-methoxycarbonyl (Fmoc) solid-phase method, a pepetide synthesis technique originally developed by Robert Bruce Merrifield. Interestingly enough,  D-brazzein has no sweetness and was in fact tasteless most likely due to minimal to no human taste receptor binding. The counterpart L-brazzein is quite a hardy protein with exceptional heat and pH stability maintaining its sweetness at a high of 98°C for two hours in a pH range of 2.5-8. This stability is mainly credited by its four intramolecular disulfide bonds and no free sulfhydryl groups.

Ironically, the brazzein fold comprising of one bent alpha helix and three strands of antiparallel beta-sheets shares the same Scorpion-toxin like domain as some small potent scorpion toxins such as TsKapa, a potassium channel blocker. A structural resemblance is also found in plant gamma-thionins and defensins yet this sweet protein has no published harmful side effects when consumed.

P&S

References

Hellekant, G. & Danilova, V. (2005). Brazzein a Small, Sweet Protein: Discovery and Physiological Overview. Chemical Senses, 30(suppl 1), i88-i89.

Assadi-Porter, F. M., Maillet, E. L., Radek, J. T., Quijada, J., Markley, J. L. & Max, M. (2010). Key amino acid residues involved in multi-point binding interactions between brazzein, a sweet protein, and the T1R2-T1R3 human sweet receptor. Journal of Molecular Biology, 398(4), 584–599.

Caldwell, J.E,, Abildgaard, F., Dzakula, Z., Ming, D., Hellekant, G. & Markley, J.L. (1998). Solution structure of the thermostable sweet-tasting protein brazzein. Natural Structural Biology, 5(6), 427-31.

Izawa, H., Ota, M., Kohmura, M. & Ariyoshi, Y. (1996). Synthesis and characterization of the sweet protein brazzein. Biopolymers, 39(1), 95-101.