Shikimic Acid Pathway in plants
The shikimic acid pathway in plants is employed for the biogenesis of phenolic compounds, tannins, lignin and phenyl propane units of flavonoids and coumarins.
General biosynthesis pathway of secondary metabolites:
- Living plant consist of highly specialised organisation with distinct biochemical characteristics.
- The various organised structure in cells, for example chloroplast, mitochondria, play an important role in metabolic reaction.
- The growing seedling first of all develop rich foliage which produces large quantities of carbohydrate by photosynthesis.
- During a phase of growth and differentiation, most of the carbohydrate are metabolised to meet the energy requirement the metabolic process and also for building the cellular organisation.
- They are also stored in the cell for energy reserve as polysaccharide like starch.
During metabolism of starch or monosaccharide such as glucose, carbohydrates are broken down to glycerides, pyruvate and later to acetyl CoA, which is finally consumed in tricarboxylic acid cycle.
- During the course of this break down, glycerates and pyruvates give rise to aliphatic and aromatic amino acids which further produces protein.
Pyruvates are broken down to Acetyl CoA, which produces fixed oils and fatty through fatty acids.
- All these primary products are termed as primary metabolites because of their direct utilisation in biological system at the developmental stages of the plant.
- Once the biological necessities of carbohydrates, protein and fats are stored in the cells as starch grains, Aleuronic grains or fixed oils and fats respectively and represent food storage products.
- During biosynthesis reaction, many by-products of metabolism are produced.
- Many by-products are of useless to the plants and are deposited in various morphological parts of the plants.
- The by-products which do not have principal functions in the plants and which are secondary products of breakdown are called secondary metabolites.
- The production of secondary metabolites is mostly dependent on the genetic makeup which selectively excels in one or another fundamentals process involved in the biosynthesis of such metabolism.
- These genetically controlled fundamental process can be termed as basic metabolic pathways.
The secondary metabolites derived through one single or mixed pathways give rise to a vast array of compounds i.e., carbohydrates, proteins, fixed oils, gums and mucilage’s, tannins, resins, etc.
- These by-products of the plants are products of immense importance to humanity as food material, colorants, pesticides here, according to major biogenetic groups.
BIOSYNTHESIS OF CARBOHYDRATES
- Carbohydrates are products of photosynthesis, a biological process that converts light energy into chemical energy.
- The general process of photosynthesis can be described as;
CO2+h2o gives Sugar+o2 in presence of hv and green plants.
- All green plants and certain algae and bacteria have the capacity to synthesise Adenosine Tri-phosphate [ATP] and Nicotine Amide Adenine Dinucleotide Phosphate [NADPH].
- These compound mediates most of the biosynthetic reaction in plants. These are fundamentally 2 primary light facilitated processes in photosynthesis :
- (a) Absorption of light by chlorophyll or energy transmission to chlorophyll by other light absorbing pigments primary to production of ATP and NADPH.
- (b) Photolysis of water to produce oxygen and electrons which are transformed via carrier species and produce ATP and NADPH.
- These are two reactive molecules, which work as activating and reducing agents, respectively, during biosynthetic reactions.
- In the subsequent dark reaction carbon dioxide is reduced to produce four, five, six, and seven carbon sugars.
- The reactions were firstly given by Blackman and hence called Blackman reactions.
It is estimates that a quantity of about 4000 x 10(9) tones of co2 is fixed annually through photosynthesis was first given by Calvin which is termed as Calvin cycle…..
BIOSYNTHESIS OF GLYCOSIDES:
The glycosides are condensation products of sugar and the acceptor unit called as Aglycone.
The reaction occurs in two parts as given below;
firstly sugar phosphates bind with uredines triphosphate complex.
This sugar nucleotides complex reacts with acceptor units in the second reaction, which leads to glycoside production.
Once such glycosides are formed others specific enzymes may transfer another sugar unit in the later reaction , in which the glycosides formed in the previous reaction, works as an acceptor to provide di, tri, or tetra glycosides and so on by subsequent reactions.
- ACETATE_MEVALONATE PATHWAY: Since a long time, biochemistry were aware of the involment of acetic acid in the synthesis of cholesterol, squalene and rubber-like compounds.
- The discovery of acetyl co-enzyme a called ‘active acetate’ in 1950 further supported role of acetic acid in biogenetic pathways.
- Mevalonic acid was found to be connected with the acetate.
- Mevalonic acid further produced isopentyl pyrophosphate and its isomers Dimethylallyl pyrophosphate (DMAPP).
- These two main inter-mediates IPP and DAMPP set the active isoprene unit as a basic building block of isoprenoid compounds.
- Both the units yield geranyl pyrophosphate (C10-monoterpenes) which on further association with IPP produces farnesyl pyrophosphate (C15-sesquiterpenes).
Farnesyl pyrophosphate with one unit of IPP develops into geranyl-geranyl pyrophosphate (C20-diterpenes).
- The Farnesyl pyrophosphate multiplies with one more unit to produce squalene and its subsequent cyclisation gives rise to cyclo-pentano-perhydro-phenantherene skeleton containing steroidal compound such as cholesterols and others groups like triterpenoids.
- The acetate mevalonate pathway thus works through IPP DAMPP via squalene to produce two different skeleton containing compounds, namely steroids and terpenoids.
- It also produces a vast array of monoterpenoids, sesquiterpenoids, Diterpenoids, Carotenoids, polyphenols and also compounds like glycosides and alkaloids associated with other pathways.
ACETATE MALONATE PATHWAY
- Acetate Malonate Pathways operates functionally with involvement of Acyl Carrier protein (ACP) to yield fatty acids thioesters of ACP.
- These acyl thioesters forms the important intermediates in the fatty acid synthesis.
- These C2 Acetyl COA units At the later stage produce an even number of fatty acids from n -tetranoic (butyric) to n-ecosanoic (arachidonic acid).
- The Synthesis of fatty acids is thus explained by the reaction.
- Unsaturated fatty acids are produced by subsequent direct dehydrogenation of saturates fatty acids.
The ENZYMES play an important role in governing the position of newly introduced double bonds in the fatty acids.
• POLYACETATE-MALONATE PATHWAY:
Acetate pathway is additionally actively involved within the biosynthesis of ENDOCRIN, an intermediate anthraquinone group of compounds via polyketide formation.endocrin, after decarboxylation, yields emodin.
• Acetyl CoA first produces malonyl CoA which is further engaged within the chain lengthening by attachment of malonyl CoA with terminal acetate.
This pathway is termed as POLY-ACETATE-MALONATE PATHWAY which is produces a spread of anthraquinone derivatives in higher plants.
Shikimic acid pathway In Plants
• Shikimic acid pathway in plants is a crucial route for the biosynthesis of phenylpropanoid (C6-C3 Compounds) derivatives.
• Two significant starters for these compounds of abundant in nature are phospho-enol pyruvate a metabolite of glucose and a tetrose sugar erythrose 4-phosphate resulting via photo-synthesis.
• Both intermediates are condensed together to yield 2-keto 3-deoxy 7-phospho D-glucoheptanoic acid, 3-dehydroshikimic acid and shikimic acid 3-phosphate.
• The latter gets condensed with a unit of phosphoenopyruvate,to yield phenylpyruvic or p-hydroxy phenyl acid ,which successively undergo reductive amination to afford two important amino acids,phenylalanine and tyrosine respectively.
• Chorismic acid, through another chains of reaction produces anthranilic acid which provides rise to tryptophan.
• The shikimic acid pathway in plants is employed for the biogenesis of phenolic compounds,tannins, liganins and phenylpropane units of flavonoids and coumarins.
• Shikimic acid pathway in plants is given below;
• The shikimic acid pathway in plants starts with phosphoenol acid (PEPA).
The precursors for flavonoids are the unstable chalcones which are derived from the unstable shikimic acid pathway and acetate pathway.
The C6-C3 unit of phenylpropane formed by shikimate pathway undergoes building by the association of acetate units via malonyl CoA to supply polyketide.
A polyketide by ring closure gives rise to varied sorts of flavonoids derivatives.
AMINO ACID PATHWAYS
• Amino Acid are formed in plants and are found in Free State because the fundamental units of proteins.
• Some aromatic amino acids like phenylalanine, tyrosine and tryptophan are biosynthesized via shikimic acid pathway.
• The aliphatic and heterocyclic amino acids have different routes of bio-genesis.
• Nitrogen enters the metabolic reaction by reductive amination of alpha keto like Oxaloacetic and alpha ketoglutaric acids to afford the aliphatic amino acids like amino acid and glutaminic acid , respectively.
• Glutamic acid become a precursor for arginine, ornithine and proline.
3-Phosphoglyceric acid produces an aminoalkanoic acid , serine which may be readily converted to glycine.