Cholesterol biosynthesis plants

Cholesterol is a soft waxy substance that is a steroidal alcohol or sterol. It is the most abundant steroid in the human body and is a component of every cell. Cholesterol is essential to life and most animals and many plants contain this compound. Cholesterol biosynthesis occurs primarily in the liver, but it may be produced in other organs. A number of other substances are synthesized from cholesterol including vitamin D, steroid hormones (including the sex hormones), and bile salts. Cholesterol resides mainly in cell membranes. 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

This enzyme is not only crucial in plants, recent studies have suggested that mevalonate kinase also plays an important role in the regulation of the cholesterol biosynthesis in mammals. The significance of MVK has been further highlighted by the implication... 

There is little doubt that the pathway from acetyl-CoA to squalene 2,3-oxide in plants is the same as that in animals and as it is detailed in the chapter on cholesterol biosynthesis it will not be reiterated here. 

See also Cholesterol Biosynthesis, Steroid Metabolism, Steroid Hormone Synthesis, Bile Acids, Lipid-Soluble Vitamins, Plant Hormones... 

Figure 1 Bulk drugs from natural sources Paclitaxel (antileukemic and antitumor) and lovastatin (inhibitor of cholesterol biosynthesis) are examples of the diverse and complex structures made by plant and microbial cell biosyntheses, respectively. In most instances of such compounds having desirable biological activities, their structural and chiral complexities make chemical synthesis not competitive with isolation from biosynthesis.

It is a plant sterol (phytosterol) present in canola oils and rape seed but is also found in some marine algae, diatoms and shellfish (oysters). It inhibits the enzyme A24-sterol reductase that is involved in mammalian cholesterol biosynthesis pemandez et al. Biochem J 336 109 2002], Brassino steroids have plant growth regulating activities [Anastasia et al. J C S Perkin 1379 7982.]... 

Experiments in which com, Zea mays, seeds produce obtusifoliol (18) from cycloeucalenol (19) upon germination in H20, the H-NMR spectra indicate significant incorporation of deuterium into the C-19 methyl group (Fig. 23.9). Experiments of this type provide some direct evidence for the intermediacy of these compounds in the biosynthesis of cholesterol in plants. 

Vitamin D is the sunshine vitamin . It was originally discovered as a crude mixture called vitamin Dj (no longer available as a supplement). Ergosterol, the plant equivalent of cholesterol, is converted to vitamin D2 by ultraviolet light. Vitamin D3 (cholecalciferol) is formed in the skin from 7-dehydrocholesterol (an intermediate in the cholesterol biosynthesis pathway) in the presence of ultraviolet light, which opens the B-ring of the steroid nucleus (Fig. 51.3). Cholecalciferol is successively hydroxylated first in the liver forming 25-hydroxycholecalciferol (25-HCC) and then in the kidney to form the most active form 1,25-dihydroxy cholecalciferol (1,25-DHCC), also known as calcitriol. 

In green plants, which contain little or no cholesterol, cydoartenol is the key intermediate in sterol biosynthesis.161-1623 As indicated in Fig. 22-6, step c, cydoartenol can be formed if the proton at C-9 is shifted (as a hydride ion) to displace the methyl group from C-8. A proton is lost from the adjacent methyl group to close the cyclopropane ring. There are still other ways in which squalene is cyclized,162/163/1633 including some that incorporate nitrogen atoms and form alkaloids.1631 One pathway leads to the hop-anoids. These triterpene derivatives function in bacterial membranes, probably much as cholesterol does in our membranes. The three-dimensional structure of a bacterial hopene synthase is known.164 1643 Like glucoamylase (Fig. 2-29) and farnesyl transferase, the enzyme has an (a,a)6-barrel structure in one domain and a somewhat similar barrel in a second domain. 

Cholesterol and the C29 sterols are used by plants as the starting materials for the biosynthesis of other steroids with the same number of carbons, such... 

Sterols and Cholesterol. Natural sterols are crystalline C76 C1(1 steroid alcohols containing an aliphatic side chain at C17. Sterols were first isolated as lionsaponifiable fractions of lipids from various plant and animal sources and have been identified in almost all types of living organisms. By far, the most common sterol in vertebrates is cholesterol (8). Cholesterol serves two principal functions in mammals. First, cholesterol plays a role in the structure and function of biological membranes.. Secondly, cholesterol serves as a central intermediate in the biosynthesis of many biologically active steroids, including bile acids, corticosteroids, and sex hormones. 

Plant Sterols. Sterols have been identified in almost all types of living organisms and can be isolated, in varying quantities, from many different plants. Similar to cholesterol, plant sterols have a structural and functional role in biological systems and serve as intermediates in the biosynthesis of an assortment of biologically active steroids. 

In the biosynthesis of vitamin D substances, precursors include cholesterol (skin + ultraviolet radiation) in animals ergosterol (algae, yeast + ultraviolet radiation), Intermediates in the biosynthesis include preergocaldferol, tachysterol, and 7-dehydrocholesterol. Provitamins in very small quantities are generated in the leaves, seeds, and shoots of plants. In animals, the production site is the skin. Target tissues in animals are bone, intestine, kidney, and liver. Storage sites in animals are liver and skin. 

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