Studies on solution NMR structure of brazzein - Secondary structure and molecular scaffold

Brazzein is a sweet-tasting protein isolated from the fruit of West African plant Pentadiplandra brazzeana Baillon. It is the smallest and the most water-soluble sweet protein discovered so far and is highly thermostable. The proton NMR study of brazzein at 600 MHz (pH 3.5, 300 K) is presented. The complete sequence specific assignments of the individual backbone and sidechain proton resonances were achieved using through-bond and through-space connectivities obtained from standard two-dimensional NMR techniques. The secondary structure of brazzein contains one α-helix (residues 21-29), one short 3₁₀-helix (residues 14-17), two strands of antiparallel β-sheet (residues 34-39, 44-50) and probably a third strand (residues 5-7) near the N-terminus. A comparative analysis found that brazzein shares a so-called 'cysteine-slabilized alpha-beta' (CSαβ) motif with scorpion neurotoxins, insect defensins and plant γ-thionins. The significance of this multi-function motif, the possible active sites and the structural basis of thermostability were discussed.