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Formation of carboxylated and decarboxylated betalains in ripening grains of Chenopodium quinoa by a dual dioxygenase.

Chenopodium quinoa (quinoa) grains form part of the cultural heritage of Andean countries and have been recognized as a pseudo-cereal with high nutritional value and potential health benefits. Betalains are nitrogenous water-soluble pigments and bioactive molecules that contribute to these health promoting properties. Betalains are restricted in plants to the order Caryophyllales where quinoa belongs. A new family of betalains has been discovered as unconventional decarboxylated pigments here described to accumulate in ripening quinoa grains of fluorescent nature, and putatively based on a dopamine-cleaving activity. This paper describes for the first time the purification and molecular and functional characterization of a 4,5-dopamine-extradiol-dioxygenase enzyme from plants. It is a monomeric protein with a molecular mass of 34.5 kDa characterized by chromatography, electrophoresis and TOF mass spectrometry. In quinoa the key enzyme is dual, demonstrated to be involved in a square-shaped biosynthetic pathway towards the formation of carboxylated and decarboxylated pigments. Enzyme kinetic properties are characterized in the production of 6-decarboxy-betalamic acid and DOPA-derived betalamic acid, the two structural units of plant pigment in nature. The profile of multiple betalains present in quinoa grains has been reproduced in one-pot bioreactors containing the novel enzyme and two competing substrates.