defination-intro-TCA-glyoxylate cycle

1. TCA CYCLE AND
GLYOXYLATE CYCLE

2. CONTENTS
 Definition
 Introduction
 TCA Cycle
 Glyoxylate Cycle

3.  DEFINITION
• TCA cycle : The tricarboxylic acid cycle
(TCA cycle) is a series of enzyme-catalyzed
chemical reactions that form a key part of
aerobic respiration in cells.
• This cycle is also called the Krebs cycle and
the citric acid cycle.

4. INTRODUCTION
• The citric acid cycle – also known as the
tricarboxylic acid (TCA) cycle or the Krebs cycle – is
a series of chemical reactions used by all aerobic
organisms to generate energy through the
oxidation of acetyl-CoA derived from
carbohydrates, fats and proteins into carbon
dioxide and chemical energy in the form of
guanosine triphosphate (GTP).
• In addition, the cycle provides precursors of certain
amino acids as well as the reducing agent NADH
that is used in numerous other biochemical
reactions.

5. • Its central importance to many biochemical
pathways suggests that it was one of the
earliest established components of cellular
metabolism and may have originated
abiogenically.
• The name of this metabolic pathway is derived
from citric acid (a type of tricarboxylic acid)
that is consumed and then regenerated by this
sequence of reactions to complete the cycle.

6. •In addition, the cycle consumes acetate (in the
form of acetyl-CoA) and water, reduces NAD+ to
NADH, and produces carbon dioxide as a waste
byproduct. The NADH generated by the TCA cycle
is fed into the oxidative phosphorylation
(electron transport) pathway.
• The net result of these two closely linked
pathways is the oxidation of nutrients to produce
usable chemical energy in the form of ATP.

7. • In eukaryotic cells, the citric acid cycle occurs
in the matrix of the mitochondrion. In
prokaryotic cells, such as bacteria which lack
mitochondria, the TCA reaction sequence is
performed in the cytosol with the proton
gradient for ATP production being across the
cell's surface (plasma membrane) rather than
the inner membrane of the mitochondrion

8. FIG : TCA cycle

9. Reaction of Citric Acid Cycle
1. Citrate synthase: Formation of Citroyl Co-A
intermediate.

10.  Aconitase: This enzyme catalyses the
the isomerization reaction by removing
and then adding back the water (H and
OH) tocis-aconitate in at different
posiotions.

11.  Isocitrate dehydrogenase: There
are two isoforms of these enzyme,
AOne uses NAD+ and other uses
NADP+ as electron accepter.

12. Ketoglutarate dehydrogenase: This is a
complex of defferent enzymatic
activities similar to the pyruvate
dyhrogenase

13. 5.Succinyl coA synthatse: Sccinyl CoA,
like Acetyl CoA has a thioester bond
with very negative free energy of
hydrolysis.

14. 6. Succinate Dehydrogenase: Oxidation of
succinate to fumarate. This is the only citric
acid cycle enzyme that is tightly bound to
the inner mitochondrial membrane.
It is an FAD dependent enzyme.

15. 7.Fumarase: Hydration of fumarate to
malate: it is a highly
stereospecificenyme.
Cis-Malate the cis form of fumarate is
not recognized by this enzyme.

16. 8. L-Malate dehydrogenase: Oxidation
of malate to oxalocetate.
It is an NAD+ dependent enzyme.

17. Glyoxylate cycle

18. Definition-
• Glyoxylate cycle : The glyoxylate cycle, a
variation of the tricarboxylic acid cycle, is an
anabolic pathway occurring in plants, bacteria,
protists, and fungi. The glyoxylate cycle centers
on the conversion of acetyl-CoA to succinate
for the synthesis of carbohydrates.

19. INTRODUCTION
• The glyoxylate cycle, a variation of the
tricarboxylic acid cycle, is an anabolic pathway
occurring in plants, bacteria, protists, and fungi.
The glyoxylate cycle centers on the conversion
of acetyl-CoA to succinate for the synthesis of
carbohydrates.
• In microorganisms, the glyoxylate cycle allows
cells to utilize simple carbon compounds as a
carbon source when complex sources such as
glucose are not available.

20. Fig : Glyoxylate cycle

21. • The glycoxylate cycle utilizes five of the eight
enzymes associated with the tricarboxylic acid
cycle: citrate synthase, aconitase, succinate
dehydrogenase, fumarase, and malate
dehydrogena.
• The two cycles differ in that in the glycoxylate
cycle, isocitrate is converted into glyoxylate and
succinate by ICL instead of into -ketoglutarate.

22. • This bypasses the decarboxylation steps that
place in the TCA cycle, allowing simple carbon
compounds to be used in the later synthesis of
macromolecules, including glucose.
• Glyoxylate is subsequently combined with
acetyl-CoA to produce malate, catalyzed by
MS.
• Malate is also formed in parallel from
succinate by the action of succinate
dehydrogenase and fumarase.

23. Reference
• Plant physiology
v.verma
• A textbook of plant physiology, biochemistry
and biotechnology
S.k.Verma
Mohit verma
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