Biosynthetic pathway of ascorbic acid in higher plants....slides
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Transcript of Biosynthetic pathway of ascorbic acid in higher plants....slides
Biosynthetic Pathway of Ascorbic Acid in Higher Plants
By Muhammad Asif Gondal
ContentsIntroduction Chemistry of ascorbic acidPlant sourcesOccurrence in plantsBiosynthesisBiosynthesis of ascorbic acid in plantsBiosynthetic pathways of ascorbic acid in higher plants
Inversion pathwayNon-inversion pathwayL-galactose pathwayList of enzymesBiological significance of ascorbicacidConclusionReferences
IntroductionAscorbic acid also called vitamin CAscorbic acid is a naturally occurring organic compoundIt is a white solid, but impure samples can appear yellowishPlants are the primary source of vitamin CBiosynthetically, it is produced both in plants and animalsThere are several biosynthetic pathways of ascorbic acid in plantsThe major pathway of plant ascorbic acid synthesis is through the L-galactose
pathway
Chemistry of Ascorbic AcidStructurally, L-Ascorbic acid is one of the simplest vitaminIt is related to the C-6 sugarsThere are three stereo isomers of ascorbic acid
i. L-Ascorbic acid
ii. D-Ascorbic acid
iii. D-erythro-ascorbic acid
Plant sources
Plants are generally a good source of ascorbic acid• 28 mg/100 g • 53 mg/100 g
• 10 mg/100g• 6 mg/100 g
Mango
Lemon
ApricotApple
Occurrence in Plants
Occurs in all plant tissuesUsually being higher in Photosynthetic cells Meristems Fruits It has been reported that AA mostly remain available in reduced form in leaves
and chloroplastAbout 30 to 40% of the total ascorbic acid is in the chloroplast
Biosynthesis
Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms
This process often consists of metabolic pathwaysThe essential elements for the biosynthesis are Precursor compounds Chemical energy e.g. ATP Catalytic enzymes Coenzymes e.g. NADH
Biosynthesis of Ascorbic Acid in Plants
Biosynthesis of ascorbic acid occur both in plants and animals It is produced from glucoseThere are many different pathways for ascorbic acid in plantsThese pathways are derived from products found in glycolysis and other
pathwaysThe main biosynthetic pathway of ascorbic acid is L-galactose pathwayDifferent enzymes are take part in this pathway e.g. L-galactose dehydrogenase
Biosynthetic Pathways of Ascorbic Acid in Higher Plants
Evidences for the ascorbic acid-biosynthesis pathways so far proposed for higher plants are inconclusive
However, three pathways have been suggested
i. Inversion Pathway
ii. Non-inversion Pathway
iii. L-galactose pathway
Inversion Pathway
This pathway suggests that the precursor of ascorbic acid is L-galactono-1, 4-lactone (GAL) (derivative of D-galactose)
This pathway is supported by rapid conversion of GAL to ascorbic acidThe enzyme that catalyzes this reaction is L-galactono-1, 4-lactone
dehydrogenase (GAL dehydrogenase )GAL dehydrogenase catalyzes L-galactono-1, 4-lactone (GAL) oxidation to
ascorbic acid
D-Gal D-GalUA (ester)
L-galactonic acid
L-galactono-1,
4 lactoneL-Ascorbic
Acid
Non-inversion Pathway
This biosynthetic rout of ascorbic acid route from D-glucose was proposed by Loewos et al. in 1990
In this pathway, D-glucose is oxidized to D-glucosone by the activity of pyranose-2-oxidase
D-glucosone is then epimerizes to give L-sorbosoneThe oxidation of L-sorbosone then yields L-Ascorbic acid
D-glucose
D-glucosone
L-Galactose Pathway
This is the major biosynthetic pathway of ascorbic acidThis pathway involves ten enzymatic steps from D-glucose to L-ascorbic
acidThe pathway proceeds by the formation of various intermediates such as
GDP-d-mannose and L-galactoseVarious enzymes catalyze the L-galactose pathway
The first six steps of the L-galactose pathway are used to synthesize activated nucleotide sugars that are also precursors of cell wall polysaccharides and glycoproteins
The pathway to L-ascorbic acid biosynthesis then consists of the sequential conversion of the followings
Five different enzymes convert D-glucose-6-phosphate (D-glucose-6-P) to GDP-D- mannose and GDP-L-galactose-1-p
L-galactose-1-P to L-galactose
L-galactose to L-galactono-1,4-lactone L-galactono-1,4-lactone to L-ascorbic acidThe initial three steps D-glucose to d-mannose-6-P are essential part of
the pathwayHexokinase and phosphoglucose isomerase are well-established players
in glycolysis
List of Enzymes
The different enzymes taking part in L-galactose pathway are1. Hexose phosphate isomerase2. Phosphomannose isomerase3. Phosphomannose mutase4. GDP-D-mannose pyrophosphorylase
5. GDP-D-mannose-3,5-epimerase6. L-galactose-1-P phosphatase7. L-galactose dehydrogenase8. L-galactono-1,4-lactone dehydrogenase
Biological Significance of Ascorbic AcidThe biological role of ascorbic acid is to act as a reducing agentIt donates electrons to various enzymatic and a few non-enzymatic
reactionsIn plants, ascorbic acid play a role in Cell division Photosynthesis
Summary
There are many different biosynthesis pathways for ascorbic acid in plantsMost of these pathways are derived from products found in glycolysis and
other pathwaysFor example; inversion, non-inversion and L-galactose pathwayL-galactose biosynthetic pathway is the dominant pathwayVarious types of enzymes are take part in biosynthetic pathways of
ascorbic acid
References
W.H. Schopeer, “Plants and Vitamins” Peter N. Campbell, “Biochemistry” A. Philip, “Analytical Biochemistry” DavidL. Nelson, Michael M.cox “Lehninger principles of Biochemistry 3rd
edition” http://www.ncbi.nlm.nih.gov/pmc/articles http://www.nature.com/nature/journal/v393/n6683/full/393365a0.html http://www.sciencedirect.com/science/article/pii/S0083672901610082