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