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1. INTRODUCTION
1.1. Background
The Kreb’s cycle is a series of chemical reactions used by all aerobic organisms to generate energy
through
the oxidation of acetate derived
from
carbohydrates, fats and proteins into carbon
dioxide and chemical energy in the form of adenosine triphosphate(ATP). 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. 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. 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.
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.
1.2. Objectives:

To know the importance of TCA Cycle in energy production

To know about the steps involved in TCA cycle
2. Literature Review
2.1. Discovery and Evolution of TCA Cycle
Several of the components and reactions of the citric acid cycle were established in the 1930s by
the research of the Nobel laureate Albert Szent-Györgyi, for which he received the Nobel Prize in
1937 for his discoveries pertaining to fumaric acid, a key component of the cycle. The citric acid
cycle itself was finally identified in 1937 by Hans Adolf Krebswhile at the University of Sheffield,
for which he received the Nobel Prize for Physiology or Medicine in 1953.
Components of the TCA cycle were derived from anaerobic bacteria, and the TCA cycle itself may
have evolved more than once. Theoretically there are several alternatives to the TCA cycle;
however, the TCA cycle appears to be the most efficient. If several TCA alternatives had evolved
independently, they all appear to have converged to the TCA cycle.
2.2. Overview of TCA Cycle
It plays a starring role in both the process of energy production and biosynthesis. The cycle finishes
the sugar-breaking job started in glycolysis and fuels the production of ATP in the process. It is
also a central hub in biosynthetic reactions, pro viding intermediates that are used to build amino
acids and other molecules.
TCA cycle involves 10 different reactions and they are as follows:
i.
Citrate synthesis from oxaloacetate
The condensation of acetyl CoA with oxaloacetate forms citrate. The enzyme catalyzing
this reaction is citrate synthase.
ii.
Conversion of citrate into aconitate
The citrate formed in the first step is a bit too stable, so the second step moves an oxygen
atom to create a more reactive isocitrate molecule. Aconitase performs this isomerization
reaction, with the assistance of an iron-sulfur cluster.
iii.
Formation of isocitrate
Aconitate is converted into isocitrate by the enzyme aconitase, but here one molecule of
H2O is added to the aconitate.
iv.
Conversion of oxalosuccinate to alpha keto glutarate
Some enzyme, isocitrate dehydrogenase catalyses the conversion of oxalosuccinate into
alphaketo glutrate. In this reaction, decarboxylation of oxalosuccinate occurs in the
presence of Mn2+.
v.
Conversion of oxalosuccinate to alpha keto glutrate
Same enzyme, isocitrate dehydrogenase catalyses the conversion of oxalosuccinate into
alphaketo glutrate. In this reaction decarboxylation of oxalosuccinate occurs in the
presence of Mn2+.
vi.
Formation of succinyl CoA
This is the onl.y step in the cycle where ATP is made directly. The bond between succinate
and coenzyme A is particularly unstable, and provides the energy needed to build a
molecule of ATP. In mitochondria, the enzyme actually creates GTP in the reaction, which
is converted to ATP by the enzyme nucleoside diphosphate kinase
vii.
Formation of succinate
Succinate dehydrogenase, found in the membrane, links its citric acid cycle task directly to
the electron trans-port chain. It extracts hydrogen atoms from succinate and transfers them
first to the carrier FAD and finally to the mobile electron carrier ubiquinone.
viii.
Formation of fumarese
The eight step is performed by fumarase, which adds a water molecule to get everything
ready for the final step of the cycle.
ix.
Conversion of fumarate to malate
The enzyme fumarase catalysis the addition of water to fumarate to give malate.
x.
Formation of oxaloacetate
Finally, oxaloacetate is regenerated by the enzyme malate dehydrogenase. NADH is
produced in this reaction. This oxaloacetate can again combine with acetyl CoA for next
cycle.
2.3. Overall Reaction
(1) Acetyl-CoA + Oxaloacetate + H2O → Citrate + CoA-SH + H+
(2) Citrate → Isocitrate
(3) Isocitrate + NAD+ → etoglutarate + CO2 + NADH
(4) a-Ketoglutarate + NAD+ + CoA-SH → Succinyl-CoA + CO2 + NADH
(5) Succinyl-CoA + Pi + GDP → Succinate + GTP + CoA-SH
(6) Succinate + E-FAD → Fumarate + E-FADH2
(7) Fumarate + H2O → L-Malate
(8) L-Malate + NAD+ → Oxaloacetate + NADH + H+
Acetyl-CoA + 2H2O + 3NAD+ + Pi + GDP + FAD → 2CO2 + 3NADH + GTP + CoA-SH +
FADH2 + 2H+
3. Yield of ATP
At this point the yield of ATP is 4 moles per mole of Glucose as it passes through the Krebs
cycle






This is not much more than the 2 moles which would have been produced from
glycolysis
However, NADH and FADH2 are energy rich molecules. They are the electron acceptors
(oxidizing agents) for the carbon in glucose. Glucose is being "burned" in the TCA cycle,
but not by oxygen but by these cofactors, to produce CO2.
The oxidation of NADH and FADH2 is highly exergonic and is coupled with the
production of ATP from ADP
Oxidation of 1 mole NADH produces 3 moles ATP
Oxidation of 1 mole FADH2 produces 2 moles ATP
Thus total ATP yield = (10 x 3) + (2 x 2) + 4 = 38 moles ATP per mole Glucose
Conclusion
The citric acid cycle is the third step in carbohydrate catabolism (the breakdown of sugars).
Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule).
In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA by
decarboxylation and enters the citric acid cycle. In protein catabolism, proteins are broken down
by proteases into their constituent amino acids. The carbon backbone of these amino acids can
become a source of energy by being converted to acetyl-CoA and entering into the citric acid cycle.
In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the
liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and
glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue,
fatty acids are broken down through a process known as beta oxidation, which results in acetylCoA, which can be used in the citric acid cycle. Beta oxidation of fatty acids with an odd number
of methylene bridges produces propionyl CoA, which is then converted into succinyl-CoA and fed
into the citric acid cycle. The total energy gained from the complete breakdown of one molecule
of glucose by glycolysis, the citric acid cycle, and oxidative phosphorylation equals about 30 ATP
molecules, in eukaryotes. The citric acid cycle is called an amphibolic pathway because it
participates in both catabolism and anabolism.
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