Nootkatone-a biotechnological challenge

Due to its pleasant grapefruit-like aroma and various further interesting molecular characteristics, (+)-nootkatone represents a highly sought-after specialty chemical. (+)-Nootkatone is accumulated in its producer plants in trace amounts only, and the demand of the food, cosmetics and pharmaceutical industry is currently predominantly met by chemical syntheses. These typically require environmentally critical reagents, catalysts and solvents, and the final product must not be marketed as a natural flavour compound. Both the market pull and the technological push have thus inspired biotechnologists to open up more attractive routes towards natural (+)-nootkatone. The multifaceted approaches for the de novo biosynthesis or the biotransformation of the precursor (+)-valencene to (+)-nootkatone are reviewed. Whole-cell systems of bacteria, filamentous fungi and plants, cell extracts or purified enzymes have been employed. A prominent biocatalytic route is the allylic oxidation of (+)-valencene. It allows the production of natural (+)-nootkatone in high yields under mild reaction conditions. The first sequence data of (+)-valencene-converting activities have just become known.