Tetraterpenes and Carotenoids
Tetraterpenes and carotenoids
Tetraterpenes and carotenoids are found in all plants, bacteria, and fungi. They are probably the world's most widely distributed pigments.
Carotenoids are pigments in birds, fish, insects, and other animals. They are also common pigments in plants.
About 500 carotenoids are known; more than 150 have been reported from plants.
They are C40 compounds. Most carotenoids are highly unsaturated. Because of this, they are difficult to isolate, purify and characterize. They are both heat and light sensitive.
Although the color of most free carotenoids ranges from yellow to orange or red, they form protein-carotenoid complexes that are highly variable in color
Biosynthesis
Tetraterpenes are synthesized from DOXP precursors in plants and some bacteria. Those carotenoids involved in photosynthesis are formed in chloroplasts. Others are formed in plastids. Carotenoids formed in fungi are presumably formed from mevalonic acid precursors.
Carotenoids are formed by a head-to-head condensation of geranylgeranyl pyrophosphate or diphosphate (GGPP). There is no NADPH requirement, as in biosynthesis of squalene.
Prephytoene OPP appears to be an intermediate in some cases, but not in others.
The enzyme system may be a "channelled" process.
Formation of phytoenes
In the second major step of carotenoid biosynthesis, inorganic pyrophosphate is expelled and
a proton is eliminated. Both Z- and E-15,15'-phytoene are formed. Z-15,
15'-phytoene is the precursor of most other carotenoids. In some bacteria E-15,15'-phytoene
is the dominant form.
Acyclic carotenoids
In tomatoes, Z-15,15'-phytoene gives rise to a series of acyclic carotenoids including
Z-phytofluene, E-phytofluene, neurosporene, and lycopene. Prolycopene is the
main carotenoid produced in some other plants.
Alicyclic carotenoids
Lycopene undergoes cyclization to form b- and
a-carotene. Cyclization probably requires the (7Z, 9Z, 7'Z, 9'Z)-
isomer of lycopene.
Oxygenated carotenoids
Oxygenated carotenoids are widespread in plants. These colored substances are often called
xanthophylls. Rearrangement of simple compounds such as zeaxanthin or lutein lead to the formation
of pigments such as capsanthin.
Biological activity
Carotenoids are colored and absorb light in the visible range of the spectrum. The exact wavelength depends on several factors.
The leaves of all green plants contain the same major carotenoids: β-carotene, lutein, violaxanthin, and neoxanthin. These compounds are located in the chloroplast grana. The carotenoids are involved in light harvesting and as antioxidants.
Three carotenoids, violaxanthin, antheraxanthin, and zeaxanthin undergo rapid, light-induced changes in concentration. These interconversions are called the "xanthophyll" cycle. This seems to protect the photosynthetic apparatus against excess light.
Carotenoids are involved in vision in animals. Vitamin A (retinol) is an essential precursor for normal growth and vision. Some insects, such as bees, have modified visual pigments and can "see" in the UV range, whereas mammals cannot.
E-Retinoic acid is involved in developmental processes in animals.
The pigments of many animals, egg yolks, bird feathers, fish, amphibian, and reptile skins is attributable to the rpesence of carotenoids and/or protein-carotenoid complexes. The pink color of flamingos is produced by ketocarotenoids. The blue color of lobsters is crustacyanin (a carotenoid) and a protein. These complexes are readily denatured by heat.
The pigmentation of many flowers and fruits is produced by carotenoids and protein-carotenoid complexes. These serve as synomones, compounds that benefit both the receiving and emitting organism.
Carotenoids also are used as human food pigments. The carotenoids of achiote or annato, Bixa
orellana, are used for cooking in many parts of the world, but also are margarine pigments.
β-Carotene also is widely used as a pigment.
Metabolites of carotenoids
Cleaved products derived from carotenoids are common in nature. Some of these are difficult to distinguish from sesquiterpenes, diterpenes, and triterpenes, except by careful biosynthetic studies. Some volatile derivatives are essential oil components.
(+)-S-Abscisic acid, a plant growth inhibitor, often involved in dormancy problems, is derived from carotenoid pathways. This compound is a factor in bud and seed dormancy. This compound is apparently synthesized as a sesquiterpene in some fungi.
Trisporic acids
Compounds such as trisporic acids play a pheromonal role in the reproduction of fungi of the order Mucorales. Although the interactions are very complex, in general hyphae of the two mating types each contain some of the enzymes needed convert the precursors into the active metabolites. Only when the two mating types are both present are the active compounds formed.
Z-15,15'-phytoene biosynthesis
Oxidation of violaxanthin to zeaxanthin
Retinol (Vitamin A), retinoic acid and related compounds
β-Carotene, zeaxanthin, and oxygenated tetraterpenes
Cleavage products or metabolically altered tetraterpenes
Lecture Slides
Plants with Tetraterpenes and
Carotenoids
© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant
Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
