Enzyme discovery thought to be step toward creating vitamin A-rich rice

24 Jul 2017 --- An important step in the journey toward adding vitamin A to rice has been made by biochemists from the University of Freiburg who have explained in detail the structure of an enzyme that supplies the carotenoid beta-carotene. As carotenoids have been found in the leaves of the rice plant, this leaves the door open for a future vitamin A-rich rice that could help with malnutrition issues.

Carotenoids are plant pigments that are responsible for the bright red, yellow and orange colors in many fruits and vegetables, including their namesake the carrot. Beta-carotene is one of the carotenoids that can be converted to vitamin A and are called pro-vitamin A carotenoids. 

Thanks to its strong color, beta-carotene is used in the food industry in soft drinks, yogurts and other foods and is known as food coloring E160. Rice, which is the most important basic nutrient in Asia, has no beta-carotene in its kernel, but there are carotenoids in the leaves of the rice plant. The University of Freiburg biochemists note that these long, fat-soluble pigments are used by the plant not only in photosynthesis, during which the plant generates energy and oxygen but in other processes as well.

One of the first precursors to beta-carotene is phytoene, reports the university’s news release. It is colorless and not water-soluble and can be found in the lipid bilayer of plastid organelles – in other words, in the outer layer of these closed cellular compartments – which is involved in many processes including photosynthesis. Here, the enzyme phytoene desaturase (PDS) transforms the phytoene into the next intermediate of synthesis, which already has a yellowish color.

Researchers in the labs of Prof. Dr. Peter Beyer at the Faculty of Biology and Prof. Dr. Oliver Einsle at the Faculty of Chemistry and Pharmacy of the University of Freiburg have successfully found the three-dimensional structure of phytoene desaturase in rice, along with the mechanism of phytoene transformation. To achieve this, Dr. Sandra Gemmecker and Anton Brausemann had to isolate the highly pure enzyme and crystalize it so they could get a three-dimensional structural image through diffraction experiments with X-rays.

Since the early 20th century, scientists worldwide have been working to reveal the exact mechanisms of carotene synthesis. This has proven difficult, however, due to the complicated composition of various enzyme complexes and because of their relatively low numbers in plant cells.

The researchers were able to elucidate the structure of the PDS enzyme when they added a molecule of norflurazon, an herbicide developed in the 1970s, during the isolation of PDS. Due to the presence of norflurazon, PDS is deactivated and thus no longer available to the growing plant, causing it to bleach and die. Knowledge of this bleaching herbicide within the enzyme can, therefore, be useful to researchers when developing new agents in the future. It is now also possible to induce specific changes in the sequence of the enzyme and use biotechnological procedures to give crops an advantage over weeds.

This new information on carotene synthesis is very welcome because the lack of vitamin A is a major cause of health problems worldwide and can lead to blindness and even death. This is an especially serious problem in threshold or third-world countries, the university’s news release says, because children in these countries are likely to suffer from a lack of vitamin A or its precursor beta-carotene due to malnourishment.

The University of Freiburg researchers’ vitamin A discovery is not the only recent good news when it comes to rice with added nutritional benefits. Researchers in China have recently genetically engineered a purple rice rich in antioxidants, which could decrease the risk of certain cancers, cardiovascular disease, diabetes and other chronic disorders. 

Meanwhile, organically grown rice has been in the news recently for the benefits of rice bran, the outer covering of rice grain. A study published in the open access journal Rice declares it a rich source of proteins, fats, minerals and micronutrients such as B vitamins.