Monday, June 22, 2015

Colorful Carotenoids


One of my professors once said (ok I lied, he actually says this all the time), “Plants are the world’s best biochemists.” And it is very true. Plants produce thousands of different types of phytochemicals, including pigments, toxins, hormones, signaling molecules, the list goes on and on. This is going to be the first in a series called “Best Biochemists” where I’ll highlight different plant produced chemicals. Today’s post is going to scratch the surface of the beautifully colored carotenoids.
Fall Leaves - personal photograph
Carotenoids are probably most well-known by the yellow-orange-red color they provide to fruits, vegetables and flowers, for example the orange in carrots, red in tomatos, or yellow in peaches. They are a huge family with over 600 naturally occurring carotenoids. Most of the coloration, fragrance, and even taste of many flowers and spices come from carotenoids. The bright oranges, reds, and yellows that paint the leaves of trees is due to a build-up of carotenoids as photosynthesis ceases. So where does all of this diversity come from? Biochemically, all carotenoids start as a 40 carbon chain called phytoene.

"Phytoene" by Edgar181. Own work. Licensed under Public Domain via Wikimedia Commons.
 Phytoene is colorless and requires 4 dehydrogenation (loss of hydrogen) and 2 isomerization (rearrangement) reactions to become lycopene, the first colored carotenoid. Lycopene is bright red and gets its name from Lycopersicum, the genus name of tomatoes. Most of the bright red fruits and vegetables we eat are high in lycopene, such as tomato, watermelon, pink grapefruit, red bell peppers, etc. Lycopene is an important branchpoint in carotenoid biosynthesis as in the next step, the ends of lycopene are cyclized (made into rings). These rings can be in several different configurations, the most famous of which is the beta ring, that forms both ends of beta-carotene.

"Beta-Carotin" by NEUROtiker. Own work. Licensed under Public Domain via Wikimedia Commons.
Carotenes are very important to humans as precursors of Vitamin A. Beta-carotene is the most important for this purpose as it is composed of 2 Vitamin A (retinol) molecules linked tail to tail. Alpha and gamma carotene both contain 1 Vitamin A linked to a different ring type (epsilon). Thus, for every molecule of beta-carotene we consume, 2 Vitamin A molecules are produced during digestion, while only 1 is produced by alpha and gamma-carotene. The most famous usage of Vitamin A is in our eye health and development. This is where the idea that eating carrots can increase your night vision originates.  

Obviously, plants do not require Vitamin A for their eye development, so why do plants produce carotenes? All of the carotenoids are produced inside the plastids of plant cells; the most famous plastid is the chloroplast. Chloroplasts are the site of photosynthesis within plants. Inside the photosynthetic apparatus, we find 12 beta-carotenes in photosystem II and 22 beta-carotenes in photosystem I. There they, and all the carotenoids, act as extra light receptors, collecting light at wavelengths that chlorophyll cannot. In addition to capturing light, beta-carotene is a powerful antioxidant. It quenches highly reactive oxygen species that are generated as a by-product of photosynthesis to protect the photosystems. For example, beta-carotene in photosystem II acts as a backdoor for the energy contained in singlet oxygen’s away from the reaction core D1/D2 proteins and into cytochrome b559 cycle electron transport so that the reaction core is not damaged and the energy is not lost.
Beta-carotene backdoor - my silly diagram work
Beta-carotene and alpha-carotene, while used in these forms, are also critical steps in the production of xanthophylls. Xanthophylls are oxygenated carotenoids and appear yellow in coloration. The most common xanthophyll is lutein, which is found in high concentrations in kale and spinach. Lutein is also important for our vision, in our retina it prevents oxidation of lipids and proteins. In plants, lutein quenches damaging reactive oxygen species produced during photosynthesis.

Light capture and antioxidant properties are only a few of the functions of carotenoids. All of the carotenoids can be cleaved at any double bond to produce a wide array of apocartenoids (less than 40 carbon chains) which are found in the fragrances, tastes, and colors of various spices in the world. The interconversion of several forms of xanthophylls play an important role in the state transitions of photosynthesis, a feature called non-photochemical quenching. Plant hormones, such as abscisic acid are formed from carotenoid precursors.

Clearly, carotenoids are too diverse of a group to be summed up in one post. Thus look for future Best Biochemists posts to delve into the apocarotenoids, xanthophyll state transition, and phytohormone formation.


References

Moise, A., Al-Babili, S., and Wurtzel, E., 2014. Mechanistic Aspects of Carotenoid Biosynthesis. Chemical Review 114(1):164-193.  http://pubs.acs.org/doi/abs/10.1021/cr400106-y (paywall :( )
Telfer, A. 2002. What is beta-carotene doing in the photosystem II reaction centre? Philosophical Transactions Royal Society of London B Biological Sciences 357(1426):143-139 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1693050/

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