Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix. In its natural form, monellin is composed of the two chains shown above, but the protein is unstable at high temperatures or at extremes of pH. To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker. This modified version of the protein has been studied using NMR and X-ray diffraction. In addition to its secondary structure, four stably bound sulfate ions were located on the monellin protein, three on the concave face of the protein and one on the convex face of the protein. The sulfate ion on the convex face of the protein is of particular interest because it lies adjacent to a patch of positive surface potential, which may be important in electrostatic interactions with the negative T1R2-T1R3 sweet taste protein receptor.
Sweetness properties
Monellin is perceived as sweet by humans and some Old World primates, but is not preferred by other mammals. The relative sweetness of monellin varies from 800 to 2000 times sweeter than sucrose, depending on the sweet reference against which it is assessed. It is reported to be 1500-2000 times sweeter than a 7% sucrose solution on a weight basis and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis. Monellin has a slow onset of sweetness and lingering aftertaste. Like miraculin, monellin's sweetness is pH-dependent; the protein is tasteless below pH 2 and above pH 9. Blending the sweet protein with bulk and/or intense sweeteners reduces the persistent sweetness and shows a synergistic sweet effect. Heat over 50 °C at low pH denatures monellin proteins, causing a loss of the sweetness. So far, five high-intensity sweet proteins have been reported: monellin, thaumatin, pentadin, mabinlin and brazzein.
As a sweetener
Monellin can be useful for sweetening some foods and drinks, as it is a protein readily soluble in water due to its hydrophilic properties. However, it may have limited application because it denatures under high temperature conditions, which makes it unsuitable for processed food. It may be relevant as noncarbohydrate tabletop sweetener, especially for individuals such as diabetics who must control their sugar intake.
In addition, monellin is costly to extract from the fruit and the plant is difficult to grow. Alternative production such as chemical synthesis and expression in micro-organisms are being investigated. For instance, monellin has been expressed successfully in yeast and synthesised by solid-phase method. The synthetic monellin produce by yeast was found to be 4000 times sweeter than sucrose when compared to 0.6% sugar solution.
