Showing posts with label GLYCOSIDES. Show all posts
Showing posts with label GLYCOSIDES. Show all posts
Sunday, April 15, 2012
Friday, April 13, 2012
4 PROFILE OF GLYCOSIDES IN NATURAL PLANT SOURCES
The variety of glycosides occurring in natural plant sources are so numerous that it is not quite possible to include all of them in one chapter in the present context. Hence, it is thought worthwhile to summarize them in the following Table 4.6:
Table 4.6 Summary of Glycosides in Natural Plant Sources
3.8 Biosynthesis of Saponin Glycosides
Saponins are usually of two types, namely: first, the steroidal saponins which essentially have a spiroketal side chain, and secondaly, the triterpenoid saponins. It has been proved that labelled acetate and mevalonate are duly incorporated into spiroketal steroids as well as pentacyclic triterpenoids. It is, however, pertinent to mention here that the major pathway adopted by both types of sapogenins is more or less identical and that it involves head to tail coupling of various acetate units. It is asumed that a branching takes place most probably after the formation of the triterpenoid hydrocarbon squalene, that ultimately leads to the spiroketal steroids in one direction and to the pentacyclic triterpenoids in the other as shown below:
4.3.7 Biosynthesis of Thioglycosides
The seeds of a number of plants belonging to the mustard family comprise of glycosides, the aglycones of which are invariably isothiocyanates. Hence, these glycosides are also known either as thioglycosides or as glucosinolates and mostly form a group of bound toxins.
It has been observed that the aglycone portions of thioglycosides may largely consist of either aliphatic or aromatic derivatives. It is established experimentally that the carboxy-labelled acetate is being inorporated in the allyl moiety of Sinigrin usually present in Brassica juncea.
4.3.6 Biosynthesis of Cyanogenetic Glycosides
Cyanogenesis is the ability of living organisms that exist freely in the higher plants but instead it is released from the cyanogenetic precursors as a result of the enzymatic action. Now, it has been well established that these precursors are normally glycosides of hydroxynitriles (or cyanohydrins). Once the cellular integrity of a cyanophoric plant tissue is disrupted, the cyanogenetic glycosides in
turn are brought in contact with the respective catabolic enzymes that helps to hydrolyze the glycosides and ultimately give rise to the formation of hydroxynitriles.
There are two important cyanogenetic glycosides, namely: dhurrin nad prunasin which are present in a variety of plant families and genera as stated below*:
The two amino acids Tyrosine and Phenylalanine are considered to have derived from the
Shikimic Acid Pathway and Phenylalanine Pathway as depicted below:
(a) Shikimic acid Pathway: It is based on studies carried out with E. coli as follows:
(b) Phenylalamine Pathway: It starts with Shikimic acid as the starting material as shown under:
(c) Tyrosine from Phenylalanine: It is obtained by the oxidation of phenylalanine.
(d) Dhurrin from Tyrosine: It is obtained from tyrosine as shown below:
(e) Prunasin (or Prulaurasin) from Phenylalanine: It is obtained from phenylalanine as given below: It has been established that labelled shikimic acid and labelled tyrosine were equally effective precursors of the hydroxylated aglycone of Dhurrin, a cyanogenetic glycoside produced by Sorghum vulgare Linn. belonging to family Gramineae.
Likewise, introducing labelled phenylalanine to young cherry laurel plants ie; Prunus
laurocerasus, has proved that the amino acid acts as a precursor of Prunasin in young peach seedlings.
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* Seigler, D.S., Prog, Phytochem., 4, 83-120, 1977.
Seigler, D.S., Rev Latinoam Quim, 12, 39-48, 1981.
3.5 Biosynthesis of Coumarin and Furanocoumarin Glycosides
It has been established experimentally that by using grafts of Melilotus alba on Trigonella foenum graecum, practically no coumarin is formed in the shoots of M. alba; therefore, it may be inferred that the roots are absolutely essential for coumarin synthesis, perhaps since they provided an important precursor (Reppel and Wagenbreth, 1958)**. However, more recently it has been shown with the aid of reciprocal grafting experiments involving parsnip (Pastinacia sativa) that furanocoumarinsin this species are usually generated in the fruits where they accumulate, and of course, no evidence for translocation could be observed (Beyrish, 1967)***.
There are two experimentally demonstrated pathways whereby natural products incorporating the bezopyran nucleus are usually formed, namely:
(a) In 3- and 4-phenylcoumarins, the aromatic components of this nucleus is derived from
polyketide, wherein the 3 aliphatic carbon atoms and the phenyl substitute is found to originate from shikimic acid via a phenylpropanoid intermediate, and
(b) The coumarin originates via the shikimate-chorismate pathway leading to phenylpyruvic acid, from which arise L-phenylalanine by transmination and trans-cinnamic acid in turn by the action of phenylalanine ammonia lyase. Generally, two types of furanocoumarins are recognised, namely: Linear furanocoumarin and Angular furanocoumarin as shown below:
In (A), the furan ring is fused at C-6 & C-7 positions of the benzopyran nucleus (eg., psoralens), whereas in (B) the fusion is between C-7 and C-8. However, the latter is less widely distributed than the former.
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* Kreuzaler, F., and K., Hahlbrock, Phytochemistry, 12, 1149-1152, 1973.
** Reppel L., and D. Wagenbreth, Flora (Jena), 146, 212-227, 1958.
*** Beyrish, T., Planta Med. 15, 306-310, 1967.
3.4 Biosynthesis of Flavonoid Glycosides
Recently, both extensive and intensive research at the enzymatic level has more or less confirmed the original hypothetical steps postulated for the incorporation of acetate and phenylalanine into flavonoids. In fact, studies related to enzymology and regulation of flavone and flavonol glycoside biosynthesis has unfolded many further details of the individual reactions. It has been reported that more than 20 different flavonoid glycosides occurrig in irradiated parsley cells are based on only three flavone and three flavonol aglycones all of which have essentially very similar substitution modes (Kreuzaler and Hahlbrock*, 1973). The chemcial structures of the aglycones and their probable reactions are as shown here under.
It may be observed that except for the characteristic C-3 hydroxyl moiety of flavonols the six aglycones essentially differ only with respect to substitution at C-3' position.
Another school of thought suggests that the flavonoid glycoside aglycones may be obtained from the major pathways ultimately leading to the synthesis of aromatic compounds in the biological systems, namely:
(a) Acetate Pathway, and
(b) Shikimic Acid Pathway
It has been observed that one 6-carbon fragment of the C6-C3-C6 compounds derived from the acetate pathways gets combined with the remaining 9 carbon fragment ontained from the shikimic (phenyl propanoid) pathway as stated below:
Explanation
1. The C6–C3segment, perhaps in the oxidation form of a cinnamic acid molecule, gets combined with three molecules of acetate to yield first a C15 chalcone moiety an intermediate and subsequently the flavanone residue.
2. The simultaneous introduction of removal of OH moieties from the aromatic rings B and A gives rise to the production of a good number of derivatives,
3. Flavonoids are first formaed by the introduction of the hydroxy group at position 3, whereas dehydrogenation at positions 2 and 3 results in the formation of flavonols, and
4. Evidently, the simultaneous occurance of a variety of glycosides having the same aglycone in a specific plant species strongly supports the well established hypothesis that glycosylation usually takes place at a late stage of flavonoid biosynthesis.
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* Reinhard, E., In Tissue Culture and Plant Science—1974, (H.E. Street Ed.), Academic Press, New York, pp.
433-459, 1974.
3.3 Biosynthesis of Steroid Glycosides
Biotransformation of steroids and cardiac glycosides (e.g., gitoxin, digitoxin) by plant cell cultures have been studied extensively and have been reviewed by Reinhard* (1974), Stohs and Rosenberg** (1975), Stohs*** (1977), and Furuya**** (1978).
However, in general the steroidal aglycones of cardioactive glycosides may be assumed to have formed as a broad based overall mechanism of steroid biogenesis as shown below:
Acetate -> Mevalonate -> Isopentenyl Pyrophosphate -> Squalene æÆSteroid
The steroidal molecule is considered to have generated with the head to tail linkage of several acetate units.
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* Reinhard, E., In Tissue Culture and Plant Science—1974, (H.E. Street Ed.), Academic Press, New York, pp.
433-459, 1974.
** Stohs S.J., and H. Rosenburg., Lloydia, 38, 181-194, 1975.
*** Stohs, S.J., ‘In plant Tissue Culture and its Biotechnological Applications’, (W. Barz, E. Reinhard and M.H.
Zenk, eds), Springer-Verlag, New York, pp. 142-150, 1977.
**** Fuarya, T., In ‘Frontiers of Plant Tissue Culture’, (T.a. Thorpe-ed.) The Boostore, University of Calgary, Alberta,
Canada, pp. 191-200, 1978.
3.1 Biosynthesis of Anthracene Glycosides
The biosynthesis of anthracene glycosides may be considered under the following two heads, namely:
(a) Emodin and Other Related Derivatives: The indepth knowledge with regard to the biosynthesis of anthracene aglycones has been duly established from an elaborated study with microorganisms, specifically Penicillium islandicum as shown below:
In this particular instance, an intermediate poly β-ketomethylene acid is assumed to have formed from 8 acetate units which on being subjected to intramolecular condensation gives rise to anthraquinones i.e.; emodin and other related derivatives.
(b) Alizarin: Another metabolic pathway for the formation of anthraquinone is established and recognised through the shikimic acid—mevalonic acid mediators as could be seen functional in certain plants belonging to the family Rubiaceae as given below:
The biosynthesis of alizarin reveals that the ring A in alizarin molecule has been derived from the shikimic acid, whereas the ring C in alizarin has been incorporated by the mevalonic acid component.
3 BIOSYNTHESIS OF GLYCOSIDES
Generally, the naturally occurring living plant could be regarded as the most sophisticated and meticulously designed biosynthetic laboratory not only confined to the primary metabolites such as: Amino acids, carbohydrates, terpenes, fatty acids which are mostly consumed as a source of edible food material by human beings, but also for a plethora of secondary metabolites of enormous pharmaceutical significance, for instance: glycosides, flavonoids, alkaloids, essential oils and the like. Interestingly, such naturally found chemical substances which specifically attribute
plant drugs their marked and pronounced therapeutic activities are collectively termed as ‘phytopharmaceuticals’. Therefore, a higher plant is nothing but an intricate solar energised biochemical reactor that is exclusively responsible for the mass production of primary as well as secondary metabolites from air, water, minerals and sunlight - a source of UV radiations.
However, the primary metabolites are more or less widely distributed in nature practically in all organisms that are essentially required for the overall growth as well as physiological development by virtue of their basic cell metabolism. Nevertheless, the secondary metabolites are biosyntheticallyengineered products solely derived from the primary ones and are confined in their distribution strategically ie; being restricted to a particular taxononic group. These products may be regarded either as various chemical adaptations to environmental stresses or they may be considered as nature’s protective, defensive or offensive chemical entities against the host of microorganism, fungi, insects
and higher herbivorous predators.
Thus, with regard to cellular economic cognizance the secondary products are mostly tedious to form and subsequently accumulate, and hence, invariably show up in the plant kingdom in relatively much lesser amounts in comparison to the primary metabolites. The secondary metabolites are also regarded to be as the waste products of the plant metabolic processes.
The different biosynthetic reactions taking place in the plant cells are based on certain enzymes. In fact, it is the control of enzymatic activity on the plant metabolism which ultimately governs a specific biosynthetic pathway. In general, the enzymatic reactionbs in plants ae reversible. Under the influence of specific enzymes the secondary metabolites are either synthesized or hydrolysed in plants.
The biosynthetic pathways in plants may be duly elucidated and extensively studied by the aid of isotopically labelled precursors. Nowadays, with the advent of ‘tracer technology’, it is a lot easier to introduce isotopes into the anticipated precursors of plant metabolites and employed as specific ‘markers’ in the elaborated biogenetic experiments. It is now quite possible to unfold the mysteries of biosynthetic pathways with the use of radioactive carbon (14C), hydrogen (3H), sulphur (35S) and phosphorus (32P).
The biosynthesis of different categories of glycosides shall be discussed briefly in the sections that follow.
Thursday, April 12, 2012
2.11.4 Nucleosides (Nucleic Acids)
These naturally occurring substances are of prime biological importance and essentially possess three vital components namely: first, a sugar moiety e.g.; ribose or 2-desoxyribose; secondly, a purine or pyrimidine base e.g.; adenine, guanine and cytosine; and thirdly, a phosphoric acid. A base-sugar unit is known as a nucleoside, whereas a base-sugar phosphoric acid unit is known as nucleotide. An example of a nucleotide and a nucleoside is given here under:
Nucleotide : An odenylic acid unit of RNA
Nucleoside : Adenosine with the adenine as the heterocyclic base.
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