Carotenoids structure function and biosynthesis

Carotenoid

Carotenoid is any of a large class of over 600 organic pigments, including the carotenes and xanthophylls, that are terpenoids. Carotenoids are lipid compounds that are universally present in nearly all higher plants, fungi, and bacteria.

They are usually red, orange, yellow or brown and are associated with chlorophylls. Along with chlorophyll b, the carotenoids are also called accessory pigments because they hand over the energy absorbed by them to chlorophyll a. Carotenoids have conjugate double bonds (—C=C—C=C—).

Occurrence:

In plants, carotenoids can occur in roots, stems, leaves, flowers, and fruits. Within a plant cell, carotenoids are found in the membranes of plastids, organelles surrounded by characteristic double membranes. Chloroplasts are the most important type of plastid and they synthesize and store carotenoids as well as perform photosynthesis.

Two of the best-known carotenoids are Beta-carotene and lycopene. Beta-carotene gives carrots, sweet potatoes, and other vegetables their orange color. Lycopene gives tomatoes their red color.

Properties

• They are nonnitrogenous widely distributed compound often involved as accessory pigments in photosynthesis.
• They are insoluble in water but soluble in a variety of nonpolar solvents.
• They are easily bleached by light or oxygen.
• They protect chlorophyll from photo damage. They do this by harmlessly dissipating excess light energy which they absorb as heat. In the absence of carotenoids, this excess light energy could destroy proteins, membranes, and other molecules.
• Chemically caretenoids are tetraterpenoids.
• Caretonoids with beta-inone ring have vitamin A like activity.
• All carotenoids are strong antioxydents.
• They absorb light energy for use in photosynthesis, They do this by transferring some of the light energy they absorb to chlorophylls, which then use this energy to drive photosynthesis.

Structure:-

 Chemists have identified about 500 different, naturally occurring carotenoids. Each consists of a long hydrocarbon chain with a 6-carbon ionone ring at each end. All carotenoids consist of 40 carbon atoms and are synthesized from eight 5-carbon isoprene subunits connected head-to-tail.

There are two general classes of carotenoids: carotenes and xanthophylls.

(a)Carotenes:-

• Molecular Formula : C₄₀H₅₆.     In Ps I
• Carotenes consist only of carbon and hydrogen atoms; beta-carotene is the most common carotene.
• Beta-carotene can be stored in the liverand converted to vitamin A as needed, thus making it a provitamin (i.e., a precursor to the vitamin). Vitamin A (also known as retinol) is a fat-soluble alcohol that plays a crucial role in vision; it is converted to a component of the light-sensitive pigment rodopsin present in the retina of the eyeThese are yellow coloured pigments.
• They are unsaturated fat soluble hydrocarbons.
• They do not contain oxygen.
• They absorb blue and green light and transmit yellow and red light.
• Examples,Alpha carotene,beta carotene and Lycopene.

(b)Xanthophylls:-

• Molecular Formula: C₄₀H₅₆. O₂      In Ps II
• Xanthophylls are also called carotelos.
• They are oxygen derivatives of caretonoids.
• Xanthophylls are often yellow, hence their class name.
• Lutein, zeaxanthin, cryptoxanthin, and astaxanthin are well-known xanthophylls
• Lutein and the other carotenoid pigments found in leaves are not obvious because of the presence of other pigments such as chlorophyll. Lutein and zeaxantin are found in kale, spinach, corn, alfalfa, broccoli, and egg yolks (Sims and Odle 2005)..

Biosynthesis of carotenes and xanthophylls.

These are as following steps are involved:

(i) Tail-to-tail addition of two units of geranylgeranyl pyrophosphate to form a 40C skeleton called phyton.

(ii) In a series of steps, phyton is dehydrogenated to form lycopene.

(iii) Lycopene undergoes cyclization.

(iv)Xanthophylls are derived from carotenes by oxidation.

Biochemical functions and important roles of Carotenoids:-

• The carotenoids perform functions in diverse processes including photosynthesis, phototropisms and protection against excessive light. Their role in photosynthesis appears to be secondary since tissues rich in carotenoids and lacking chlorophyll do not photosynthesize.
• Carotenes contribute a yellow or orange pigmentation to fruits such as apricots, root vegetables like carrots and sweet potatoes, and flowers such as dandelions and marigolds. The leafy greens broccoli and spinach are also good dietary sources, though the presence of carotene is visually masked by the green of chlorophyll molecules. Carotenes also give color to milk fat and egg yolks, and contribute to the ornamental hue of lobster
• It is also believed that light energy absorbed by them is transferred to chlorophyll and utilised in photosynthesis. When a plant is exposed to wavelengths of light absorbed exclusively by the carotenoids, a red fluorescence of chlorophyll a is observed.
• Action spectrum and absorption spectrum for the green alga (Ulva) shows that photosynthesis is appreciable in blue-green region at 480-500 nm wavelengths, indicating some transfer of energy from the carotenoids to chlorophyll.
• They also prevent destruction of chlorophyll from the degradative effects induced by excessive light and molecular oxygen (photooxidation).
• They may also act as photoreceptors for light causing phototropism
• In non-photosynthesizing organisms, carotenoids have been linked to oxidation-preventing mechanisms.
• Animals are incapable of synthesizing carotenoids, and must obtain them through their diet, yet they are common and often in ornamental features. It has been proposed that carotenoids are used in ornamental traits because, given their physiological and chemical properties.
• A wide range of carotenoids and other colorful compounds abound in the plant kingdom. The benefits for the plant in expending resources to produce these compounds are visible in their role in attracting insects for pollination and luring animals for seed

 Type # 3. PHYCOBILINS

• Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast.
• Phycobilins are always bounded with some water soluble proteins called phycobiliproteins.
• They occur only in Cyanobacteria and Rhodophyta.
• Phycobilins are usually found in organisms that are living in deep water for the efficient absorption of light.
• All phycobilins are strongly fluorescent.
• They are blue and red in colour.
• They emit red or orange light after fluorescent.
• There are two classes of Phycobilins.

(a)Phycocyanin

• They are blue colour pigments.
• They absorb green yellow and red light and transmit blue light.
• They are the princip[al pigment of blue green algae.

  (b)Phycoerythrin

• They are red colour pigments.
• They absorb blue, green and yellow light and transmit red light.
• Phycoerythrin present abundantly in the members of Rhodophyceae

 

Type # 4. Anthocyanin:-

Chemically, anthocyanins from the Greek anthos, a flower, and kyanos, dark blue) are flavonoids (flavan like).Anthocyanins are blue, red, or purple pigments found in plants, especially flowers, fruits, and tubers. In acidic condition, anthocyanin appears as red pigment while blue pigment anthocyanin exists in alkaline conditions.

Anthocyanin is considered as one of the flavonoids although it has a positive charge at the oxygen atom of the C-ring of basic flavonoid structure. It is also called the flavylium (2-phenylchromenylium) ion . The stability of anthocyanin is dependent on pH, light, temperature, and its structure

Occurrence/ Distribution:-

Anthocyanins responsible for the colors, red, purple, and blue, are in fruits and vegetables. Berries, currants, grapes, and some tropical fruits have high anthocyanins content. Red to purplish blue-colored leafy vegetables, grains, roots, and tubers are the edible vegetables that contain a high level of anthocyanins.

Among the anthocyanin pigments, cyanidin-3-glucoside is the major anthocyanin found in most of the plants. Anthocyanins are commonly found in flowers and the fruits of many plants. Most of the red, purple, and blue-colored flowers contained anthocyanins. Red flowers are red hibiscus, red rose, red pineapple sage, red clover, and pink blossom. These red flowers are edible. Blue (cornflower, blue chicory, and blue rosemary) and purple (purple mint, purple passion flower, purple sage, common violet, and lavender) flowers are the common edible flowers.

Some of these flowers have been traditionally used as folk medicine, as colorants, and as food. In addition to traditional usage, red, purple, and blue-colored fruits are commonly consumed for their beneficial effects. The colored pigments of anthocyanin from berries, blackcurrants, and other types of red to blue-colored fruits are strong antioxidants.

Moreover, anthocyanin-rich black carrot, red cabbage, and purple potato are potential functional foods that have been consumed for prevention of diseases.

Types of anthocyanin in plants:-

Anthocyanin is one of the subclasses of phenolic phytochemicals. Anthocyanin is in the form of glycoside while anthocyanidin is known as the aglycone. Anthocyanins are in the forms of anthocyanidin glycosides and acylated anthocyanins. The most common types of anthocyanidins are cyanidin, delphinidin, pelargonidin, peonidin, petunidin, and malvidin. Acylated anthocyanins are also detected in plants besides the typical anthocyanins.

Acylated anthocyanin is further divided into acrylated anthocyanin, coumaroylated anthocyanin, caffeoylated anthocyanin, and malonylated anthocyanin.Anthocyanin is derived from flavonol, and it has the basic structure of flavylium ion, that is a lack of a ketone oxygen at the 4-position . The empirical formula for flavylium ion of anthocyanin is C₁₅H₁₁O⁺ with a molecular weight of 207.24724 g/mol.

On the other hand, anthocyanins are the glycosylated form of anthocyanidins. The conjugated bonds of anthocyanins result in red, blue, and purple-colored plants.Cyanidin, delphinidin, pelargonidin, peonidin, malvidin, and petunidin are the most common anthocyanidins distributed in the plants. The distribution of these anthocyanidins in fruits and vegetables is 50%, 12%, 12%, 12%, 7%, and 7%, respectively. It is the major pigment in berries and other red-colored vegetables such as red sweet potato and purple corn.

Delphinidin has a chemical characteristic similar to most of the anthocyanidins. It appears as a blue-reddish or purple pigment in the plant. The blue hue of flowers is due to the delphinidin pigment .Pelargonidin differs from most of the anthocyanidins. In nature, it appears as red-colored pigment .Pelargonidin gives an orange hue to flowers and red to some of the fruits and berries.