Carotenoids: Structure, Types, and the Chemistry of Carotenes and Xanthophylls
libre
Carotenoids are terpenoid compounds that chromoplasts are able to accumulate in large quantities. Most carotenoids belong to the tetraterpenes, containing 40 carbon atoms per molecule (C40 compounds), which is why these pigments dominate the coloration of many flowers, fruits and storage organs.
Each carotenoid molecule is built from eight isoprene units. The molecule forms through a "tail-to-tail" linkage of two fragments, each of which consists of four isoprene residues joined "head to head." As a result, the two central methyl groups sit in the 1,6-position relative to one another, while the remaining non-terminal methyl groups occupy the 1,5-position (Figure 1).
Figure 1 — Scheme of the linkage of isoprene residues in the central part of carotenoid molecules.
What are the general structural features of carotenoids?
All carotenoids can formally be derived from the acyclic compound lycopene (Figure 2) through a series of reactions. These transformations include hydrogenation, dehydrogenation, cyclization, insertion of oxygen at various positions, migration of double bonds, migration of methyl groups, chain elongation and chain shortening.
Figure 2 — Structure of lycopene.
Carotenoids fall into two broad chemical classes depending on whether they contain oxygen:
- Carotenes — carotenoids made up exclusively of carbon and hydrogen atoms. This group includes lycopene, phytoene, phytofluene, the α-, β-, γ-, δ-, ζ- and ε-carotenes, neurosporene, and the α- and β-zeacarotenes (Figure 4).
- Xanthophylls — carotenoids that contain oxygen. The overwhelming majority of carotenoids known today are xanthophylls (Figure 4).
Retrocarotenoids are carotenoids in which the single and double bonds are shifted by one position. The xanthophyll-group pigment eschscholtzxanthin is one example of a retrocarotenoid.
Figure 3 — Structural formulas of chromoplast carotenes.
Besides the C40 carotenoids, plants commonly contain derivatives with fewer than 40 carbon atoms, known as apocarotenoids; examples include β-citraurin and crocetin. Fungi and bacteria additionally possess C45 and C50 carotenoids that have not been found in higher plants. The conjugated double bonds in the carotenoid backbone can give rise to cis–trans isomerism.
Most naturally occurring carotenoids exist in the trans form. Nevertheless, cis isomers of certain carotenoids have also been detected in living organisms, including plants — for example cis-phytoene, cis-phytofluene and prolycopene (the cis isomer of lycopene). The cyclic structures in many carotenoids contain asymmetric carbon atoms, which gives rise to numerous stereoisomers as well.
Chrysanthemaxanthin and flavoxanthin illustrate this stereochemical diversity: they share the same structural formula but differ in the spatial orientation of their side groups.
Figure 4 — Structural formulas of chromoplast xanthophylls.
Carotenoids occur either in the free state or esterified with fatty acids, acetate or carbohydrates. Xanthophyll esters with palmitic, stearic, myristic and lauric acids, as well as with acetate, have been found in the ray florets of the annual sunflower, while most of the crocetin — the most abundant pigment in saffron petals — is esterified with gentiobiose and glucose in various combinations.
Where do carotenoids occur and how are they localized in the cell?
In photosynthesizing tissues carotenoids are localized mainly in the grana of chloroplasts, probably in the form of chromoproteins. Complexes of proteins with violaxanthin and β-carotene have been identified, for instance. When chloroplast proteins are solubilized with a detergent, centrifugation can separate them into two main fractions — light and heavy — which correspond to photosystems I and II.
Carotenoids are distributed unevenly between these two fractions. Photosystem I is enriched in β-carotene, whereas xanthophylls predominate in photosystem II. In etiolated seedlings the pigments are localized in etioplasts, and it should be noted that the dominant pigments of etioplasts in etiolated seedlings differ from those of mature-leaf chloroplasts.
The principal xanthophylls of common-bean etioplasts are flavoxanthin and chrysanthemaxanthin, which are absent from green leaves. Conversely, the etioplasts lack neoxanthin, the most abundant pigment in the leaves of adult plants. In flower petals, carotenoids are localized in chromoplasts.
The structural site of carotenoid accumulation within chromoplasts varies markedly from species to species:
- In the chromoplasts of the yellow daffodil, carotenoids accumulate mainly in numerous concentric membranes.
- In the corona plastids of the snow-white daffodil, β-carotene occurs as crystals located in the intrathylakoid space.
- In the chromoplasts of cultivated chrysanthemum, common Spanish broom, tulip, Scotch broom (Sarothamnus scoparius) and many other plants, carotenoids are localized in osmiophilic plastoglobules.
In the petals of marsh marigold, carotenoids are found not only in chromoplasts but also in chloroplasts, while in the flowers of some plants carotenoids are absent altogether.
In the chromoplasts of tulip flowers, carotenoids are localized in osmiophilic plastoglobules.
Unlike the pigments of photosynthesizing tissues, the xanthophylls in flower chromoplasts are esterified with palmitic, stearic, myristic or lauric acids. Carotenoids esterified with acetate and carbohydrates have also been detected. The ripe fruits of many plants owe their color to the presence of one carotenoid or another.
As in flowers, the carotenoids of fruits are localized in chromoplasts, which develop from chloroplasts during ripening. In some cases — for example in the fruits of lily of the valley — chromoplasts form from proplastids instead.
In the chromoplasts of the red fruits of annual pepper, common pumpkin, rugosa rose and certain other plants, carotenoids are localized in osmiophilic plastoglobules and in tubular structures. In the yellow, orange and white varieties of annual pepper, carotenoids accumulate in the form of crystalline structures.
The xanthophylls in fruits, as in flowers, are largely esterified. Carotenoids are also widespread in the underground organs of carrot and sweet potato, although it should be noted that the color of some Asian carrot varieties is due to anthocyanins. In orange carrot cultivars, 90–95% of the carotenoids are carotenes.
Among these the most abundant are α-, β- and γ-carotene and lycopene, while γ-carotene, ζ-carotene, neurosporene, phytoene and phytofluene occur only in trace amounts. Xanthophylls make up only 5–10% of the total carotenoid content of orange carrots, but their proportion rises to 75–93% in yellow carrot varieties and to no less than 95% in white carrots.
The principal pigment of sweet potato (Ipomea batatas edulis) is β-carotene. In carrot the pigments are localized in crystalline-type chromoplasts, whose structure has been studied in detail. Carotenoids have also been found in the seeds, anthers, stamens and pollen of various plants. In the spadix appendages of Typhonium divaricatum and Arum they have been shown to be localized in chromoplasts. The carotenoid composition of chromoplasts is highly distinctive and differs substantially from the pigment composition of chloroplasts.
Although the main carotenoids of most chromoplasts are also present in the chloroplasts of photosynthesizing tissues, their quantitative ratios in these organelles differ. At the same time, the chromoplasts of some plants contain specific carotenoids that are absent from chloroplasts. Capsanthin — one of the predominant pigments of ripe tomatoes — occurs only in chromoplasts, for example.
Moreover, capsanthin is a species-specific pigment, since it has so far not been detected in any other plant. As noted earlier, the bulk of plant carotenoids is localized in plastids. However, carotenoids have also been identified in non-plastid structural components of plant cells.
Many green algae, in particular, accumulate large quantities of carotenoids under unfavorable growth conditions — usually during nitrogen starvation — in intracellular deposits without bounding membranes and in lipid vacuoles. S. Brown and J. Prebble, taking special precautions to inhibit lipases and polyphenol oxidases, found that the distribution of carotene among fractions during differential centrifugation of a cauliflower homogenate in a sucrose density gradient coincided with the distribution of succinate dehydrogenase — an enzyme that serves as a mitochondrial marker.
On the basis of these experiments the authors concluded that mitochondria contain carotenoids. Similar conclusions were drawn from experiments with potato tubers, where carotenoids were also detected in other fractions, in particular the "light" membrane fraction and the microsomes. However, the amount of pigment in the non-plastid fractions was negligible, which somewhat complicates the interpretation of these results.
Carotenoids have also been found in fungi, bacteria and animals. Unlike higher plants, algae, bacteria and fungi, animals cannot synthesize these compounds themselves; they obtain them from food and then modify them into specific "animal carotenoids" by means of dedicated enzyme systems.
