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.

Brief Biochemistry of Melittin, a Toxin from Bee Venom

One protein structure that I have been wire modelling quite a bit is melittin, a toxin from honey bee (Apis mellifica) venom (examples below):

Melittin is a 26 amino acid peptide made of two alpha helical sections with a nonpolar N-terminus and a polar C-terminus. This structure resembles a bent rod most likely due to proline-14 causing helix destabilization. Given its structure, it is most often studied as a water-soluble tetramer when isolated at high concentrations in bee venom and a monomer at the lowest concentrations known for cell membrane disruption.

To elucidate, the toxicity of melittin on exposed cells comes from biochemical activities that are hardly mentioned in general discussion. As general knowledge, the most prevalent cause of accidental fatalities from bee venom stings results from allergic reactions in hypersensitive people. However, this lethal reaction is the result of phospholipase A2 and in some cases hyaluronidase. There are a myriad of other toxins in bee venom, such as melittin, that are weakly allergenic, yet still cause biochemical aberrations.

Specifically, melittin has the ability to inhibit some Ca(2+)/calmodulin kinases and ion transport pumps such as NA(+)/K(+) ATPase, thus increasing the cell membrane permeability to ions. In addition, negatively charged membrane lipids are an attractant to melittin, thus favoring melittin incorporation into the membrane leading to cell lysis.

Given the aberrant changes in cell structure from this protein, several complex mechanisms have been observed in bees to prevent autolysis by melittin. This protein is derived from a prepromelittin precursor that underwent a 21-amino acid signal peptide cleavage to form promelittin. Further processing occurs after promelittin is secreted into the bee venom sac to protect the bee from the damaging lytic effects of melittin.

P&S

References

Strong, P. N. & Wadsworth, J. D. F. (2000). Chapter 9:  Pharmacologically Active Peptides and Proteins from Bee Venom. In Rochat, H. & Marie-France Martin-Eauclaire M. (Ed. 1) Animal Toxins: Facts and Protocols. (127-151). Basel Switzerland:  Birkhäuser Verlag.

Terwilliger, T. C. & Eisenberg, D. (1981). “The Structure of Melittin” The Journal of Biological Chemistry, 257(11), 6016-22.

Yang, S. & Carrasquer, G. (1997). “Effect of melittin on ion transport across cell membranes”. Zhongguo Yao Li Xue Bao, 18 (1), 3–5.