Introduction
§ Carotenoids (carotenes, tetraterpenoids) are biogenetically GGPP derived widely distributed and highly conjugated coloring pigments which absorb light between 400 and 500 nm.
§ The color of carotenoids ranges from deep red to light yellow and sometimes even extends to purple, depending on the environment of the extended conjugated system.
§ The name carotene is derived from carrots (edible roots) Daucus carota (Umbelliferae) in which these polyene pigments were first found by Wackenroder in 1832, and the word termination “ene” was proposed by Hofmann in 1866 for unsaturated compounds.
§ Carotenes impart yellow to reddish color to carrots.
§ They are widespread and distributed in flowers (daffodils, marigold), fruits (orange, tomato, pumpkin, red pepper or paprika), algae, fungi, photosynthetic bacteria, fall-coloration of deciduous plants, and also in animals— imparting natural coloration to birds, reptiles, amphibians, fishes, and various invertebrates.
§ During the ripening of fruits and bright yellow-orange coloration of leaves in autumn (fall color), the chlorophyll pigments break down in chloroplast thylakoid membrane, and the otherwise masked color of carotenoids starts revealing their brilliant bright yellow to orange colors.
§ Further, during ripening biosynthesis of some new carotenoids also occurs.
§ Chlorophylls are the primary light harvesting pigments, and carotenoids are important accessory light-harvesting molecules that transfer energy to the reaction center during photosynthesis.
§ They protect plants from the damage caused by oxygen especially during fall, when chlorophyll starts degrading and is not able to absorb light energy. Hence, carotenes minimize or stop the photooxidative damage.
§ They quench the triplet excited state of photosynthesizers as well as the excited singlet state of oxygen. Thus, plants lacking carotenoids are damaged and killed quickly on exposure to light and oxygen, compared to the plants having their presence.
§ The light harvesting molecules chlorophyll a, chlorophyll b, and carotenoids remain arranged in the highly organized way around the reaction center these accessory pigments increase the efficiency of the system by transferring the energy to the reaction center through a mechanism of resonance energy transfer.
§ Similar light harvesting combination also exists in photosynthetic bacteria as energy donors.
§ Carotenoids being fat soluble are also known as lipochromes or chromolipids.
Structures of Carotenoids (chemistry)
§ Carotenoids belong to the category of tetraterpenoids (i.e. they contain 40 carbon atoms, being built from four terpene units each containing 10 carbon atoms).
§ Structurally, carotenoids take the form of a polyene hydrocarbon chain which is sometimes terminated by rings, and may or may not have additional oxygen atoms attached.
§ Currently more than 800 structurally unique natural carotenoids are known, each of which can form further cis-trans isomers.
§ Carotenoids containing oxygen functions such as OH, OMe, epoxy, carboxy, aldehyde, etc. are collectively known as xanthophylls.
§ pocarotenoids are structurally close to carotenoids but contain carbon atoms less than C40.
§ Dietary vitamin A deficiency is a severe nutritional problem for children in developing and underdeveloped countries.
§ β-Carotene and lycopene are precursors of vitamin A.
§ Carotenoids with C45, C50 units are also known. Structures of a few familiar carotenoids.
Fig.1 Structures of Carotenoids
Chemistry of vision: role of vitamin A
§ β-carotene is converted to vitamin A1 (retinol) in our liver. Vitamin A1 is a fat-soluble vitamin found in animal products, e.g. eggs, dairy products, livers and kidneys.
§ It is oxidized to an aldehyde called all-trans-retinal, and then isomerized to produce 11-cis-retinal, which is the light-sensitive pigment present in the visual systems of all living beings
§ Rod and cone cells are the light sensitive receptor cells in the retina of the human eye. About three million rod cells are responsible for our vision in dim light, whereas the hundred million cone cells are responsible for our vision in the bright light and for the perception of bright colours. In the rod cells, 11-cis-retinal is converted to rhodopsin.
§ When light strikes the rod cells, isomerization of the C-11/C-12 double bond takes place, and trans-rhodopsin (metarhodopsin II) is formed.
§ This cis–trans isomerization is accompanied by an alteration in molecular geometry, which generates a nerve impulse to be sent to the brain, resulting in the perception of vision.
§ Metarhodopsin II is recycled back to rhodopsin by a multi-step sequence that involves the cleavage to all-trans-retinal and cis–trans isomerization back to 11-cis-retinal.
§ A deficiency of vitamin A leads to vision defects, e.g. night blindness.
§ Vitamin A is quite unstable and sensitive to oxidation and light. Excessive intake of vitamin, however, can lead to adverse effects, e.g. pathological changes in the skin, hair loss, blurred vision and headaches.
Biosynthetic Pathway
§ Carotenoids are essential for diverse processes in plant biology, ranging from photosynthesis, photomorphogenesis, free radical detoxification, lipid peroxidation, to synthesis of plant hormone abscisic acid.
§ The carotenoid biosynthetic pathway is well known, enzymes involved have been identified, and genes encoding the enzymes have been cloned.
§ Nevertheless, the processes that regulate their biosynthesis and accumulation are complex and poorly understood. The first dedicated step in carotenoid biosynthesis is the ubiquitous C20 isoprenoid precursor geranylgeranyl pyrophosphate (GGPP).
§ Phytoene synthase (PSY) condenses two GGPP molecules in a head to tail manner to form phytoene (C40)
§ Phytoene contains three conjugated double bonds as a 15-cis geometric isomer. Desaturation reactions extend the series of conjugated double bonds.
§ Phytoene desaturase (PDS) introduces a double bond at the 90 of the phytoene molecule to create 15, 90-di cis-phytofluene, and another double bond at the 9 position forms 9, 15, 90-tricis-ζ-carotene. Seven conjugated double bonds give this molecule a characteristic yellow/green color.
§ Lycopene with eleven conjugated double bonds forms the chromophore that imparts red color toripe tomato fruit. Lycopene is then stepwise cyclized to β-carotene and α-carotene by β-cyclase (LCY-B, CYC-B) and e-cyclase (LCY-E) enzymes, respectively.
Fig. 2 Carotenoid biosynthesis pathway in plants
(Enzymes (with abbreviations) indicated are isopentenyl pyrophosphate isomerase (IPI), geranylgeranyl pyrophosphate synthase (GGPS), phytoene synthase (PSY), phytoene desaturase (PDS), zeta-carotene desaturase (ZDS), carotenoid isomerase (CRTISO), lycopene beta-cyclase (LCYB), lycopene epsilon-cyclase (LCYE), beta-ring carotene hydroxylase (CHXB), epsilon-ring carotene hydroxylase (CHXE), zeaxanthin epoxidase)
Note: [Structures is important in Biosynthesis Pathway]
Pharmacological activity of carotenoids
§ Natural carotenoids are precursors of vitamin A and also important photosynthetic light-harvesting pigments.
§ β-Carotene is commercially important as a food coloring agent.
§ In 1995 commercial preparation of β-carotene to the scale of 500 tons per year was planned
β-Carotene
§ It is a weak antioxidant, but prove to be strong against singlet oxygen.
§ The supplements may enrich LDL-cholesterol-b-carotene content without affecting other
§ It may significantly boost the activity of Natural Killer (NK) immune cells.
§ It may appreciably cause stimulation for the DNA-repair enzymes.
§ It definitely provides distinctly better cornea protection against the harmful UV-radiation (from sun-rays or UV-tubes) in comparison to lycopene.
α-Carotene
§ It has proved to be almost tenfold more anti-carcinogenic in comparison to b-carotene.
§ It distinctly increases the release of immunogenic IL-1 and TNF-a.
Photosynthesis
Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water.
This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.
Photosynthesis may be summarised by the word equation:
carbon dioxide + water
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glucose + oxygen
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Significance of photosynthesis
§ Green plants possess the green pigment, chlorophyll which can capture, transform, translocate and store energy which is readily available for all forms of life on this planet.
§ Photosynthesis is a process in which light energy is converted into chemical energy.
§ Except green plants no other organism can directly utilise solar energy, hence they are dependent on green pants for their survival.
§ Green plants can prepare organic food from simple inorganic elements (autotrophic) while all other organisms cannot prepare their own food and are called heterotrophic.
§ During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all other organisms.
§ Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids and other organic molecules.
§ Plants and plant products are the major food sources of almost all other organisms of the earth.
§ Fossil fuels like coal, gas, oil etc. represent the photosynthetic products of the plants belonging to early geological periods