Sweet proteins are a class of proteins with the ability to elicit a sweet sensation in humans upon interaction with sweet taste receptor T1R2/T1R3. Single-chain Monellin, MNEI, is among the sweetest proteins known and it could replace sugar in many food and beverage recipes. Nonetheless, its use is limited by low stability and high aggregation propensity at neutral pH. To solve this inconvenience, we designed a new construct of MNEI, dubbed Mut9, which led to gains in both sweetness and stability. Mut9 showed an extraordinary stability in acidic and neutral environments, where we observed a melting temperature over 20 °C higher than that of MNEI. In addition, Mut9 resulted twice as sweet than MNEI. Both proteins were extensively characterized by biophysical and sensory analyses. Notably, Mut9 preserved its structure and function even after 10 min boiling, with the greatest differences being observed at pH 6.8, where it remained folded and sweet, whereas MNEI lost its structure and function. Finally, we performed a 6-month shelf-life assessment, and the data confirmed the greater stability of the new construct in a wide range of conditions. These data prove that Mut9 has an even greater potential for food and beverage applications than MNEI.

The crystal structures can reveal detailed information about the overall structure, active site structure, and functional mechanism of enzymes. This study focused on the crystallization of 3-hexulose-6-phosphate synthase from Methylomonas aminofaciens 77a, to produce higher resolution crystals for precise structural characterization. 3-Hexulose-6-phosphate synthase is from Methylomonas aminofaciens 77a (EC 4.1.2.43). It belongs to the orotidine 5'-monophosphate decarboxylase superfamily, and acts as a key enzyme for a ribulose-monophosphate cycle of formaldehyde fixation and detoxification. 3-Hexulose-6-phosphate synthase catalyzes the aldol condensation of formaldehyde with D-ribulose-5-phosphate. For the maximum activity, 3-hexulose-6-phosphate synthase requires Mg2+ or Mn2+ as ligands. MaHPS crystallized at the concentration of 7 mg/mL and conditions consisting of 0.2 M MgCl2, 18% PEG 3350 at pH = 7.0.