Role of redox reaction in plant metabolism and examples of oxidation and reduction reaction in plants, plant Biochemistry II

1. PLANT BIOCHEMISTRY II
Role of Redox agents and plant metabolism

2. Metabolism
•"The sum of total chemical activities of all cells
."
• Plant metabolism is the set of complex of p
hysical and chemical events of photosynthesis, r
espiration and the synthesis and degradation of
organic compounds.

3. • Photosynthesis produces the substrates fo
r respiration and the starting organic co
mpounds used as building blocks for sub
sequent biosynthesis of nucleic acids, a
mino acids, and proteins, carbohydrates
and organic acids, lipids, and natural pro
ducts.

4. Types of metabolism

6. Types of metabolic pathway
1.Catabolic pathway
• Involved in the breakdown of large molecule.
• Commonly involving oxidation reactions.
• Theses pathways are exothermic(release energy).
• e.g; respiration.

7. 2 . Anabolic pathway
Involved in the synthesis of large and more complex compounds from
small molecules.
Theses pathways are endothermic.
 E.g photosynthesis.

8. Biological oxidation reactions
Biological oxidation is that which occur in biological system to produce energy.
 Oxidarion can occur by :
 Addition of oxygen (less common).
Removal of hydrogen ( common).
Removal of electron (most common).
Electrons are not stable in free state.so, there removal from substance (oxidation) must be acco
mpained by another substance (reduction).
Hence the reaction is called oxidation -reduction reaction or redox reaction.
And the invovled enzymes are called oxido- reductases enzymes.

10. Transfer of electrons
Transfer of electrons can take place by 4 different ways
1. Directly as electrons
2. As hydrogen atom (H)
3. AS an hydride ion (:H-) electron donor which
has 2 electron ..which occur in case of NAD+ linked
dehydrogenases
4 . Though as direct combination with oxygen.

11. Enzymes invovled in redox Reaction are called as
oxidoreductases which include :
 Oxidases
 Dehydrogenases
 Hydroperoxidases
 Oxygenases.
Oxidases and dehydrogenases are involved in respi
ration, Hydroperoxidases neutralizes free radicals
& Oxygenases are involved in biotransformation re
actions.
Oxidation is often synonymous with dehydrogenati
on and many enzymes catalyze oxidation reactions
are called dehydrogenases

12. Redox potential
• It is the affinity of a substance to accept electrons i.e; it is
the potential of a substance to become reduced.
• hydrogen has lowest redox potential (-0.42 volt) while
oxygen has the highest redox potential (+0.82 volt).
• The redox potential of all substances lie between the
hydrogen and oxygen.
• Electrons are transferred from substances with low redox
potential to substance with higher redox potential .This
transfer of electrons is an energy yielding process and the
amount of energy liberated depends upon the redox
potential difference between the electron donor and
acceptor.

14. Biological oxidation reactions
•Reducing equivalent (proton and electron) derived from oxidations of substrate
s (cellular respiration) are transported to the electron transporters/ carriers.
Electron carries
Hydro soluble coenzymes
Nicotina amide coenzymes
Flavin coenzymes
lipid soluble coenzymes( quinones & plasto quinones)
Fe-S proteins
Cytochromes.

15. Electron carries
Nicotinamide coenzymes
- Nicotinamide adenine dinucleotide(
NAD+).
- Nicotinamide adenine dinucleotide p
hosphate(NADP+)
• These are the coenzymes of enzym
e dehydrogenases
• NAD+ and NADP+ are differ only in
the presence or absence of phosph
ate group on C-2 of moiety.
• NAD+ and NADP+ can be reduced t
o generate NADH and NADPH resp
ectively.

16. Nicotinamide coenzyme......
• They are derived from niacin (nicotin acid)(B3) _ to provide
nicotin amide portion.
• They transfer hydride (:H-) to and from the reactants.
• The reduced coenzymes NADH and NADPH then serves a
s reductants in other reactions.
• In Glycolysis and citric acid cycle NAD+ oxidizes glucose a
nd releases energy which is then transfers to NAD+ by red
uction to NADH.
• NAD+ is act as coenzymes for various enzymes like
Pyruvate dehydrogenases ,glyceraldehyde phosphate
Dehydrogenase(in glycolysis) and 4 dehydrogenations
In citric cycle..

17. Nicotinamide adenine dinucleotide(NAD+).

18. Oxidative decarboxylation
(Pyruvate dehydrogenases)
• In the oxidation process electron along with protons
are transferred from pyruvate to NAD+.
• Energy released by oxidation of pyruvate is
transferred to NADH.

19. (Glyceraldehyde-3-phosphate
dehydrogenases)
• Redox reaction.
• Glyceraldehyde-3-phosphate
is oxidized .
• NAD+ is reduced to NADH.
• This occur when a hydride
(:H-) is transferred from G-3-
P 's carbonyl carbon to
NAD+.
• This reduction requires
energy which comes from
G-3-P.

20. 4 dehydrogenations in citric cycle.

22. • They are forms of riboflavin (vita.B2).
• They contain ribitol and a flavin (isoalloxazine ring).
• The ribitol is linked to phoshate in FMN.
• FAD is formed from FMN by transfer of an AMP from ATP.
• 2 coenzymes : flavin mono nucleotide (FMN).
And flavin adenine dinucleotide(FAD).
• The isoalloxazine ring is reduced by accepting one or two hydrogen atoms
from reduced substrate.
• Flavin coenzymes are very tightly bound or covalently bound to
flavoprotein.
• The redox potential of isoalloxazine ring depend upon specific flavoprotein.

25. Biological functions and importance
FAD and FMN
 The enzymes that use flavin coenzymes are called Flavoproteins .
 FAD has more positive reduction potential than NAD+.
 Is very strong oxidizing agent..
 FAD act as coenzyme for various enzymes like alpha ketoglutarate
dehydrogenase , succinate dehydrogenase, lipid metabolites,
Xanthanin dehydrogenase and acyl CoA dehydrogenases.
• It is involved in electron transport chain and play role in production
of ATP.
• The reduced coenzyme FADH2 contributes to oxidative
phosphorylation in the mitochondria.FADH2 is re-oxidized to FAD
which make it possible to produce 1.5 equivalent of ATP.

26. • Beta oxidation of fatty acids
Redox flavoproteins that not covalently bind to FAD like
Acetyl-coA dehydrogenases which are involved in beta ox
idation of fatty
• Mitochondrial Glycerol 3- dehydrogenase
It catalyses the conversion of glycerol 3 phosphate to DH
AP in mitochondria.
It is essential for carrying reducing equivalents from
cytosol to mitochondria.

27. Alpha- ketoglutarate dehydrogenase
complex
• It catalyzes the oxidative decarboxylation of alpha
-ketoglutarate to succinyle CoA .
• FAD is present in dihydrolipoyldehydrogenase of
alpha-ketuglutarate dehydrogenase complex.

28. Cytochromes.
• Cytochromes are proteins.
• Cytochromes are electron carriers
containing heme prosthetic group s
which functions as one electron
carriers.
• The heme iron is involved in one
electron transfers involving the Fe2+
and Fe3+ oxidation states.
• Heme in the 3 classes of
cytochromes (a ,b ,c)differ in the
substituents on the porphyrin ring.
• Some cytochromes (b, c1, a, a3)are
part of large integral membrane
protein complexes.
• Cytochrome c is a small water
soluble protein.

30. Fe-s(iron sulfur proteins)
• These are non - heme iron-sulfur protein in which iron is
tetrahedrally coordinated by four cysteine.
• These are participate in one electron transfer involving the Fe2+and
Fe3+.
• Involved in electron transport chain of respiration.

31. Ubiquinones
• Co enzyme Q (CoQ ,Q or ubiquinone) is lipid soluble
.
• It is soluble electron carrier in the hydrophobic
bilipid layer of inner mitochondrial membrane .
• The only electron carrier not bound to a protein.
• It can accept 1 or 2 e-.

32. Respiratory chain
Electron transport chain (ETC)
• The ETC is a series of membrane bound electron carriers
arranged in stepwise of increasing redox potential .
• it collects reducing equivalents( H-atoms and electron).
• It is also known as redox chain or respiratory chain.
• Embedded in the inner mitochondrial membrane
• Electrons from NADH and FADHH2 are transferred to
complexes of the ETC.
• Each complex transfers the electron to the next complex
in chain.

34. Complexes of ETC.
• Complex l NADH dehydrogenase ( NADH -Ubiquinone oxido-
reductase)
It is flavoprotein that contain FMN as well as Fes protein as co enzyme
transfers hydrogen from NADH +H to ubiquinone.
• Complex ll: Succinate dehydrogenases(succinate -ubiquinine
oxidoreductase)
It is flavoprotein that contain FAD as well as Fe S protein as coenzyme it
transfers H-atoms from succinate to ubiquinones.
• Complexlll : ubiquinol dehydrogenase ( ubiquinol cytochrome
oxidoreductases )
It transfers electron from ubiquinol to cytochrome using cytochrome b and
cytochrome c1 as coenzyme.

35. Complex lV: cytochrome oxidase
(cytochrome- oxygen oxidoreductases)
It transfers electrons from cytochrome C
to oxygen.It needs cyt a and cyt a3 as co
enzyme.
In this way hydrogen atoms are
successively transferred through the
respiratory chain to oxygen to produce
water and energy.

37. Redox reactions in photosynthesis

38.  It is not a single simple step process but is a complex
one that is completed by a series of reactions.
1- The light dependent reaction( Light reaction).
2- The light independent reaction ( dark reaction).
Light dependent phase ( oxidation phase).
 Photolysis
 Phase of photosynthesis during which light energy i
s absorbed by chlorophyll and other photosynthetic
pigments.
 It is then converted into chemical energy .Due to this
energy conversion ,reducing and assimilating power i
n the form of NADPH2(NADPH H+) and ATP are prod
uced .
 Products of light reaction : ATP , NADPH.

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