CHAPTER 4: THE CITRIC ACID AND OXIDATIVE PHOSPHORYLATION ppt

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CHAPTER 4: THE CITRIC ACID AND
OXIDATIVE PHOSPHORYLATION
INTERNATIONAL UNIVERSITY
SCHOOL OF BIOTECHNOLOGY
BIOCHEMISTRY

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Learning objectives
1. To understand the intermediates in CAC
2. The ATPs produce in CAC
3. The CO2 is released in CAC
4. The electrons are transferred in the electron
transport chain
5. The oxidative phosphorylation


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Content
Citric acid cycle
Introduction
Cellular location
Catabolism
Anabolism & catabolism
Sources of acetyl-CoA
Fatty acid –aminoacid- monosaccharides

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Content
Reactants & products

Cyclical reaction pathway
Fate of acetyl CoA carbon
regulation: inhibition
Energetics
Anaerobic
Anaplerotic reactions
Oxidative phoshorylation
Introduction
Mitochondrial anatomy
Shuttle system
Introduction to the transport chain
Comlex I- comlex II- comlex III and Cytochrome C- comlex IV

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Citric Acid Cycle
INTRODUCTION
The citric acid cycle is a central metabolic pathway that
completes the oxidative degradation of fatty acids,
amino acids, and monosaccharides. During aerobic
catabolism, these biomolecules are broken down to
smaller molecules that ultimately contribute to a cell’s
energetic or molecular needs.

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FIG. 01: Citric acid cycle is the central metabolic pathway

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INTRODUCTION
Early metabolic steps, including glycolysis and the
activity of the pyruvate dehydrogenase complex, yield a

two-carbon fragment called an acetyl group, which is
linked to a large cofactor known as coenzyme A (or
CoA). It is during the citric acid cycle that acetyl-CoA is
oxidized to the waste product, carbon dioxide, along
with the reduction of the cofactors NAD+ and
ubiquinone.

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FIG. 02: Early catabolic pathway

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FIG. 03: Citric acid cycle is the central metabolic pathway

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INTRODUCTION
The citric acid cycle serves two main purposes:
1.To increase the cell’s ATP-producing potential by
generating a reduced electron carriers such as
NADH and reduced ubiquinone; and
2.To provide the cell with a variety of metabolic
precursors.

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FIG. 04: Main purposes of CAC

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INTRODUCTION
Be able to describe the sources of acetyl groups that
enter the citric acid cycle;
Trace the conversion of substrates to products

through each of the citric acid cycle’s eight reactions
and understand how flux through the cycle is
regulated;
Understand the energetic output of the citric acid
cycle;

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INTRODUCTION
Describe the role of the reduced electron carriers and
their role in coupling the citric acid cycle to
downstream reactions that produce ATP;
Describe the amphibolic character of the citric acid
cycle; and Understand the reactions that replenish
citric acid cycle intermediates.

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CELLULAR LOCATION

Both prokaryotic and eukaryotic cells use the citric
acid cycle to help meet their energetic and molecular
needs. In respiring prokaryotes, the citric acid cycle
takes place in the cytosol. In eukaryotic cells, such
as the cells of the human body, the cycle takes place
within the mitochondrial matrix.

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FIG. 05: CAC can be found in both prokaryote and eukaryote

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FIG. 06: Matrix is where the CAC happens

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CATABOLISM
The reactions of the citric acid cycle oxidize acetyl-
CoA’s acetyl group to two molecules of carbon
dioxide. During the reaction cycle, electrons are
transferred from acetyl-CoA to electron carriers.
Once an electron carrier accepts an electron, it is
referred to as “reduced.” Ultimately, reduced electron
carriers participate in downstream reaction pathways
that generate ATP, the energy currency of the cell.

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CATABOLISM
Note that one high-energy nucleoside triphosphate is
generated directly from the reaction cycle.
Because acetyl-CoA is broken down to smaller
molecules during the citric acid cycle, the citric acid
cycle is described as catabolic.

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FIG. 07: The products from CAC

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ANABOLISM AND CATABOLISM

In addition to catabolizing molecules to meet cellular
energetic needs, the citric acid cycle is key in
supplying various biochemical pathways with
precursors needed for synthesizing molecules.

Reactions that involve “building” molecules from
smaller parts are referred to as anabolic. Anabolic
reactions use citric acid cycle intermediates as
precursors for fatty acid, amino acid, and
carbohydrate synthesis. These anabolic processes
may also require reduced cofactors.

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FIG. 08: Intermediates in CAC

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FIG. 09: Intermediates in CAC are the precursors for biosynthesis

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ANABOLISM AND CATABOLISM
Many citric acid cycle intermediates serve the cell as
both reaction precursors and reaction products. For
example, a-ketoglutarate may act as a precursor for
amino acids in an anabolic pathway, or it may be
catabolized to carbon dioxide during the reactions of
the citric acid cycle. As such, the citric acid cycle is
neither purely anabolic nor purely catabolic.
Reactions that possess this dual character of
building and degrading molecules are considered
amphibolic. Amphi is a Greek prefix meaning both.

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FIG. 10: The CAC is amphibolic

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SOURCES OF ACETYL-CoA
The skeleton drawings of the monosaccharide glucose,
the fatty acid palmitic acid, and the amino acids lysine
and glutamate are depicted. These molecules are
degraded to a common compound called acetyl-CoA,
the initiator of the citric acid cycle. Select the various
molecules to learn how each compound ultimately
enters the citric acid cycle as acetyl-CoA. Then consider
how the efficiency of metabolism would change if a
common product of carbohydrate, fatty acid, and amino
acid catabolism did not exist.