Probing the surface of a sweet protein: NMR study of MNEI with a paramagnetic probe

The design of safe sweeteners is very important for people who are affected by diabetes, hyperlipemia, and caries and other diseases that are linked t o the consumption of sugars, Sweet proteins, which are found in several tro pical plants, are many times sweeter than sucrose on a molar basis. A good understanding of their structure-function relationship can complement tradi tional SAR studies on small molecular weight sweeteners and thus help in th e design of safe sweeteners, However, there is virtually no sequence homolo gy and very little structural similarity among known sweet proteins. Studie s on mutants of monellin, the best characterized of sweet proteins, proved not decisive in the localization of the main interaction points of monellin with its receptor. Accordingly, we resorted to an unbiased approach to res trict the search of likely areas of interaction on the surface of a typical sweet protein. It has been recently shown chat an accurate survey of the s urface of proteins by appropriate paramagnetic probes may locate interactio n points on protein surface. Here we report the survey of the surface of MN EI, a single chain monellin, by means of a paramagnetic probe, and a direct assessment of bound water based on an application of ePHOGSY, an NMR exper iment that is ideally suited to detect interactions of small ligands to a p rotein. Detailed surface mapping reveals the presence, on the surface of MN EI, of interaction points that include residues previously predicted by ELI SA tests and by mutagenesis.