Chemo-enzymatic Synthesis of γ-Glutamyl S-alk(en)yl cysteines from garlic and onion with kokumi activity

The use of monosodium glutamate (MSG) in synergistic combination with the sodium salts of guanosine monophosphate (GMP) and inosine monophosphate (IMP) as flavor enhancers is well-established and industrially implemented. The japanese term umami (="delicious") refers to the characteristic taste of MSG, actually recognized as a fifth basic taste, and is commonly used even in the western countries. However, other substances supplement and modify, in part, the flavor of foods, enhancing mouthfulness, continuity, roundness and balance. The japanese term kokumi refers to all these flavor sensations. Kokumi is different from umami, in that it is not considered as a basic taste. -Glutamyl derivatives of natural or modified amino acids constitute a broad class of compounds and a lot of them show kokumi properties. Sulfur-containing substances are often the candidate of kokumi taste. Among them, organosulfur metabolites of various Allium species, in particular -glutamyl derivatives of S-substituted cysteines, have been found to give characteristic kokumi flavor (Vermeulen et al., 2005). In continuing our work in the field of the chemoreception of taste (Cairoli et. al., 2008), we developed a chemoenzymatic synthesis of -glutamyl-S-alk(en)yl cysteines (2) and their sulfoxides (3), which is based on the use of the enzyme -glutamyl-transpeptidase (GGT) as a catalyst to tranfer the -glutamyl group from glutathione (GSH) to S-substituted cysteines (1). Oxidation of the sulfur atom in 2 can be readily achieved with hydrogen peroxide in water at room temperature. This synthesis is straightforward, involves readily accessible starting materials and requires no protective groups. It is carried out at room temperature using water as the only reaction solvent.