This document discusses the biosynthesis of ascorbic acid (vitamin C) in plants. It begins with an introduction and overview of ascorbic acid, then describes its plant sources and occurrence. The major sections explain the biosynthesis of ascorbic acid in plants, including the L-galactose pathway that is dominant in plants. This pathway involves ten enzymatic steps converting D-glucose to L-ascorbic acid. The document concludes by noting the biological significance of ascorbic acid and listing several references for further information.

3. Introduction
Chemistry of ascorbic acid
Plant sources
Occurrence in plants
Biosynthesis
Biosynthesis of ascorbic acid in plants
Biosynthetic pathways of ascorbic acid in higher plants

4. Inversion pathway
Non-inversion pathway
L-galactose pathway
List of enzymes
Biological significance of ascorbicacid
Conclusion
References

5. Ascorbic acid also called vitamin C
Ascorbic acid is a naturally occurring organic compound
It is a white solid, but impure samples can appear yellowish
Plants are the primary source of vitamin C
Biosynthetically, it is produced both in plants and animals
There are several biosynthetic pathways of ascorbic acid in plants
The major pathway of plant ascorbic acid synthesis is through the L-galactose
pathway

6. Structurally, L-Ascorbic acid is one of the simplest vitamin
It is related to the C-6 sugars
There are three stereo isomers of ascorbic acid
i. L-Ascorbic acid
ii. D-Ascorbic acid
iii. D-erythro-ascorbic acid

8. Plants are generally a good source of ascorbic acid

9. Occurs in all plant tissues
Usually being higher in
 Photosynthetic cells
 Meristems
 Fruits
 It has been reported that AA mostly remain available in reduced form in leaves
and chloroplast
About 30 to 40% of the total ascorbic acid is in the chloroplast

10. 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 pathways
The essential elements for the biosynthesis are
 Precursor compounds
 Chemical energy e.g. ATP
 Catalytic enzymes
 Coenzymes e.g. NADH

11. Biosynthesis of ascorbic acid occur both in plants and animals
It is produced from glucose
There are many different pathways for ascorbic acid in plants
These pathways are derived from products found in glycolysis and other
pathways
The main biosynthetic pathway of ascorbic acid is L-galactose pathway
Different enzymes are take part in this pathway e.g. L-galactose dehydrogenase

12. 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

13. 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 acid
The 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

14. D-Gal
D-GalUA
(ester)
L-galactonic
acid
L-galactono-
1, 4 lactone
L-Ascorbic
Acid

15. 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-sorbosone
The oxidation of L-sorbosone then yields L-Ascorbic acid

17. This is the major biosynthetic pathway of ascorbic acid
This pathway involves ten enzymatic steps from D-glucose to L-ascorbic
acid
The pathway proceeds by the formation of various intermediates such as
GDP-d-mannose and L-galactose
Various enzymes catalyze the L-galactose pathway

18. 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

19.  L-galactose to L-galactono-1,4-lactone
 L-galactono-1,4-lactone to L-ascorbic acid
The initial three steps D-glucose to d-mannose-6-P are essential part of
the pathway
Hexokinase and phosphoglucose isomerase are well-established players
in glycolysis

20. The different enzymes taking part in L-galactose pathway are
1. Hexose phosphate isomerase
2. Phosphomannose isomerase
3. Phosphomannose mutase
4. GDP-D-mannose pyrophosphorylase

21. 5. GDP-D-mannose-3,5-epimerase
6. L-galactose-1-P phosphatase
7. L-galactose dehydrogenase
8. L-galactono-1,4-lactone dehydrogenase

23. The biological role of ascorbic acid is to act as a reducing agent
It donates electrons to various enzymatic and a few non-enzymatic
reactions
In plants, ascorbic acid play a role in
 Cell division
 Photosynthesis

24. There are many different biosynthesis pathways for ascorbic acid in plants
Most of these pathways are derived from products found in glycolysis and
other pathways
For example; inversion, non-inversion and L-galactose pathway
L-galactose biosynthetic pathway is the dominant pathway
Various types of enzymes are take part in biosynthetic pathways of
ascorbic acid

25.  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