Here is your short essay on Polyisoprenic lipids

1. Polyisoprenic hydrocarbons

A very large number of compounds present in plants are formed by the polymerization of isoprene units; e. g. rubber is formed by the condensation of thousands of isoprene units.

Similarly the phytol chain, which is a part of the structure of chlorophyll pigment, is an acyclic diterpene.

Some lipids can be considered as derivatives of isoprene (steroids, carotenoids).

2. Sterols and steroids

These compounds are derived from a polycyclic ring cyclopentanophenanthrene. The stereochemistry and nomenclature problems of steroids are very complex.

Steroids

Steroids represent a very large and varied family of compounds. Their biological activities are very diverse and it is found that often, small variations of the structure nature of the substituent result in major modifications of biological activity.

A- Sterols

This is a very important group of lipids found in practically all eukaryotes. M than hundred sterols are known of which we may mention cholesterol, which is t principal sterol of vertebrates.

B- Derivatives of sterols

a- Bile acids-

The two main bile acids are cholic acid and deosycholic acid. Their solubility in aqueous medium is extremely low. They are found in the bile, conjugated with glycine

b- Steroid hormone

Testicular hormones, called male sex hormones. (Ex. Testosterone, androsterone) ovarian and Placentary hormones, called female sex hormones. (Ex. Estrogen hormones) are steroid hormones.

Hormones of the adrenal cortex (Ex. Corticosterone, aldosterone) are also steroidal hormones

c- Hormones of Insects-

A large group of sterol derivatives is represented by the insects “population” hormones. These compounds are analogous to the steroid hormones of mammals. The most important is ‘ecdysone’. The other compounds differ by the number and position of hydroxyl groups.

d- Vitamin D-

There are several very similar compounds having the same vitamin action. Vitamin D is necessary for the proper formation of bones and teeth because it controls the phospho-ealeium mf tabolism. This vitamin D is actually the precursor of biologically active compounds, which are derivatives, hydroxylated in position 24 or 25.

3. Carotenoids

A- Carotenes

They are isoprene derivatives. They contain a large number of conjugated double bonds that give them a coloration ranging from yellow to red.

B- Xanthophylls

These pigments derive from carotenes by oxidation and have hydroxyl groups on the rings.

C. Vitamin A

Compounds with vitamin A activity can be formed from carotenes. Vitamin A has several roles. It influences the growth of the animal and protects epithelial tissues.

A derivative of vitamin A, retinoic acid is a modulator of cellular growth. Its role in the protection of epitheliums seems to be correlated with the fact that retinal derivatives (retinal pyrophosphorly monosaccharide) are, like dolichols, important intermediates in the synthesis of some glycoprotein.

Its best-known role is that of a co-factor in the process of vision. Vitamin A binds opsin, a protein of the retina, to give the visual pigment called rhodopsin.

4. Isoprenic chain quinones A- Vitamin E

These are compounds, which can be transformed into hydroquinone by hydrolytic cleavage and oxidized-reversibly to quinines. This enables them to act as antioxidants and to prevent in particular, the oxidation of unsaturated fatty acids. Some tocopherols having very similar structures are known.

Besides its role as biological antioxidant, vitamin E has other functions. Vitamin E deficiency causes a series of disorders, which are often specific for each type of animal, sterility in rat, neurological disorders in the chicken etc.

B- Ubiquinones and plastoquinones

As indicated by their name, ubiquinones are universally distributed. They are particularly found in animal and plant mitochondria. They play an important role in the electron transport chain. One of the most frequent is ubiquinone ₅₀ or coenz Q ₅₀. Plastoquinone has a very similar structure and participates in electron transport in chloroplast.

C- Vitamin K

It is a naphtoquinone with a chain of 4 isoprene units called phytyl residue, grafted on it. Phylloquinone is present in plant where it plays a role of electron acceptor processes related to photosynthesis. Menaquinones, or vitamins K_a, differ from vital K₁ by the number of isoprene units of the side chain (there may be up to 10 units), the number of double bonds present in this chain.

They are found in bacteria. Vita; K₂ is the active form a vitamin K in mammals (phylloquinone is converted into menaquinone in the liver). Vitamin K is also the coenzyme of an enzyme catalyzing the carboxylation of glutamic residues of proteins.

The carboxylation is for example necessary for the activation of a serum factor permitting the synthesis of prothrombin, a substance indispensable for blood clotting. This explains the anti-hemorragic action of vitamin K.