Biochemistry of Plant Phenolics
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Shikimic Acid Pathway in Biosynthesis of Phenolic Compounds
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The chemistry and biochemistry of phenolic glycosides. Shirley M. The first page of this article is displayed as the abstract. You have access to this article. Please wait while we load your content Something went wrong. Try again?
Cited by. Back to tab navigation Download options Please wait Article type: Review Article. DOI: Another general characteristic of secondary metabolism is that found in a specific organism, or groups of organisms, and is an expression of the individuality of species [ 2 ].
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The secondary metabolism provides chemical diversity to organic molecules with low molecular weight that are related by the respective pathways; such organic molecules are called secondary metabolites. These organic molecules isolated from terrestrial plants are the most studied, and their syntheses have an important role in the protection against pathogens, unfavorable temperature and pH, saline stress, heavy metal stress, and UVB and UVA radiation [ 3 ].
Secondary metabolism reflects plant environments more closely than primary metabolism [ 4 ]. Phenolic compounds are biosynthesized by the shikimate pathway and are abundant in plants. The shikimate pathway, in plants, is localized in the chloroplast.
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These aromatic molecules have important roles, as pigments, antioxidants, signaling agents, electron transport, communication, the structural element lignan, and as a defense mechanism [ 6 ], Figure 1. The seven steps of the shikimate pathway and the metabolites for branch point are described in this chapter, as factors that induce the synthesis of phenolic compounds in plants.
Phenolic compound biosynthesis promoted by biotic and abiotic stresses e. The shikimate biosynthesis pathway provides precursors for aromatic molecules in bacteria, fungi, apicomplexan, and plants, but not in animals [ 2 , 7 ]. Shikimic acid is named after the highly toxic Japanese shikimi Illicium anisatum flower from which it was first isolated [ 8 ].
This biochemical pathway is a major link between primary and secondary metabolism in higher plants [ 6 ]. In microorganisms, the shikimate pathway produces aromatic amino acids L-phenylalanine L-Phe , L-tyrosine L-Tyr , and L-tryptophan L-Trp , molecular building blocks for protein biosynthesis [ 9 ]. But in plants, these aromatic amino acids are not only crucial components of protein biosynthesis; they also serve as precursors for diverse secondary metabolites that are important for plant growth [ 10 ].
These secondary metabolites are called phenolic compounds and are synthesized when needed by the plant [ 11 ]. These molecules play an important role in the adaptation of plants to their ecosystem, and their study advances biochemical techniques and molecular biology [3, Bourgaud].
Plant Phenolics and Human Health : Biochemistry, Nutrition and Pharmacology
The principal aromatic phenolic compounds synthesized from L-Phe and L-Tyr are cinnamic acids and esters, coumarins, phenylpropenes, chromones C 6 -C 3 , stilbenes, anthraquinones C 6 -C 2 -C 6 , chalcones, flavonoids, isoflavonoids, neoflavonoids C 6 -C 3 -C 6 , and their dimers and trimers, respectively C 6 -C 3 -C 6 2,3 , lignans, neolignans C 6 -C 3 2 , lignans C 6 -C 3 n , aromatic polyketides, or diphenylheptanoids C 6 -C 7 -C 6 [ 12 ]. L-Trp is a precursor of alkaloids in the secondary metabolism [ 2 ].
Additionally, diverse hydroxybenzoic acids and aromatic aldehydes C 6 -C 1 are biosynthesized via branch points in the shikimate pathway, Figure 2. Phenolic compounds biosynthesized from the shikimate pathway have structural versatility. The shikimic and chorismic acids are the common precursors for the synthesis of L-Phe, L-Tyr, and L-Trp and diverse phenolic compounds. The shikimate pathway consists of seven sequential enzymatic steps and begins with an aldol-type condensation of two phosphorylated active compounds, the phosphoenolpyruvic acid PEP , from the glycolytic pathway, and the carbohydrate D-erythrosephosphate, from the pentose phosphate cycle, to give 3-deoxy-D- arabino -heptulosonic acid 7-phosphate DAHP , Figure 3.
Flavonoids as important molecules of plant interactions with the environment. Behavioural Processes. Sato F, Minami H. Weixing Sun, Miller Jack M. Applied and Environmental Microbiology. Abd El-Baky Hanaa H.
The shikimate pathway has special characteristics that are present only in bacteria, fungi, and plants. The absence of the pathway in all other organisms provides the enzymes catalyzing these reactions with potentially useful targets for the development of antibacterial agents and herbicides. For example, 5- enol pyruvylshikimate 3-phosphate synthase EPSP-synthase catalyzes the transfer of the enolpyruvyl carboxyvinyl moiety from PEP to shikimic acid 3-phosphate S3P [ 6 ]. In the second reaction step, DAHP loses phosphate Pi ; the enolic-type product is cyclized through a second aldol-type reaction to produce 3-dehydroquinic acid DHQ.
The 3-dehydroquinate synthase DHQS catalyzes this cyclization in the shikimate pathway.
Biochemistry of Plant Phenolics | SpringerLink
The DHQ dehydrates to produce 3-dehydroshikimic acid DHS 3-dehydroquinate dehydratase ; this compound has a conjugated double carbon-carbon, Figure 3. The protocatechuic and the gallic acids C 6 -C 1 are produced by branch-point reactions from DHS [ 2 ]. The fifth section of the pathway is the activation of shikimic acid with adenosine triphosphate ATP shikimate kinase, SK to make shikimic acid 3-phosphate S3P.
The sixth chemical reaction is the addition of PEP to S3P to generate 5- enol pyruvylshikimic acid 3-phosphate; the enzyme that catalyzes this reaction step, 5-e nol pyruvylshikimate 3-phosphate synthase EPSPS , has been extensively studied. The reason for this interest is because glyphosate [ N - phosphonomethyl glycine] is a powerful inhibitor of EPSPS [ 2 ], so glyphosate has been used as a broad-spectrum systemic herbicide.
It is an organophosphorus molecule, phosphonic acid, and glycine derivative that has a similar molecular structure to PEP, Figure 4. The last reaction step of the shikimate pathway is the production of chorismic acid from catalytic action on the chorismate synthase CS. This reaction is a 1,4- trans elimination of Pi, to yield the conjugated molecule, chorismic acid, Figure 3.
The first reaction of the shikimate pathway is an aldol-type condensation of PEP and carbohydrate erythroseP, to give 3-deoxy-D- arabino -heptulosonic acid 7-phosphate DAHP , Figures 3 and 5. The second reaction of the shikimate pathway is an intramolecular aldol-type reaction cyclization, where the enol C6-C7 of DAHP nucleophilically attacks the carbonyl group C2 , to produce a six-member cycle, the 3-dehydroquinic acid DHQ , Figures 3 and 6.
The reduction reaction of DHQ leads to quinic acid at this branch point in the shikimate pathway. Biosynthesis of Gossypol.
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Heinstein, R. Widmaier, P. Wegner, J. Metabolism and Degradation of Phenolic Compounds in Plants. Metabolism of the Aromatic Amino Acids by Fungi. Oxidases in Aromatic Metabolism. Vegetable Tannins. The Physiology of Phenolic Compounds in Plants. Phenolic Substances and Plant Disease.
Phenolic Compounds in Plants of Pharmaceutical Interest.