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Also known as the TCA cycle
or Krebs cycle
CITRIC ACID CYCLE
Group 4
:: Dote, Nikko :: Villaver, Stephanie :: Quitorio, Freah
:: Abella, Tiffany :: Enriquez, Paola Jane :: Dipaling, Nio Albert:: Habana, Carolyn
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is a series of chemical reactions used
by all aerobic organisms to generateenergy through the oxidization of
acetate derived from carbohydrates,
fats and proteins into carbon dioxide.a series of enzyme-catalyzed
chemical reactions that form a key
part of aerobic respiration in cells
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In addition,
the cycle provides precursors includingcertain amino acids as well as the reducing
agent NADH that is used in numerous
biochemical reactions Its central importance to many biochemical
pathways suggests that it was one of the
earliest established components of cellularmetabolism and may have originated
abiogenically.
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Origin of the cycles name
The name of this metabolic pathway is
derived from citric acid (a type of
tricarboxylic acid) that is first
consumed and then regenerated by
this sequence of reactions to complete
the cycle.
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Evolution
Components of the TCA cycle were derivedfrom anaerobic bacteria, and the TCA cycle
itself may have evolved more than once.
Theoretically there are several alternativesto the TCA cycle, however the TCA cycle
appears to be the most efficient. If several
TCA alternatives had independentlyevolved, they all appear to have converged
onto the canonical TCA cycle.
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Definition of terms
Pyruvate-
Pyruvate is the end product of glycolysis,
which is converted into acetyl coA that entersthe Krebs cycle when there
is sufficient oxygen available. When
the oxygen is insufficient, pyruvate is brokendown anaerobically,
creating lactate in animals (including humans)
and ethanol in plants.
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Acetyl-CoA
Acetyl coenzyme A oracetyl-CoA is animportant molecule in metabolism, used in
many biochemical reactions. Its main function
is to convey the carbon atoms within
the acetyl group to the citric acid cycle (Krebs
cycle) to be oxidized for energy production.
Definition of terms
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FAD - flavin adenine dinucleotide (FAD)
In biochemistry, is a redox cofactor involved in
several important reactions in metabolism. FAD
can exist in two different redox states, which itconverts between by accepting or donating
electrons. The molecule consists of a
riboflavin moiety (vitamin B2) bound to the
phosphate group of an ADP moleculeFAD can be reduced to FADH2, whereby it accepts
two hydrogen atoms (a net gain of two electrons):
Definition of terms
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NADH- Nicotinamide adenine dinucleotide, - abbreviated NAD+, is a coenzyme found in all
living cells.
- In metabolism, NAD+ is involved in redox
reactions, carrying electrons from one reaction toanother.
- The coenzyme is, therefore, found in two forms in
cells: NAD+ is an oxidizing agent it accepts
electrons from other molecules and becomesreduced. This reaction forms NADH, which can then
be used as a reducing agent to donate electrons.
These electron transfer reactions are the main
function of NAD+.
Definition of terms
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The cycleoverview
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1. a two carbon organic product acetate in the form of acetyl-
CoA is produced through catabolism of sugars, fats, and
proteins.
2. Acetyl-CoA along with two equivalents of water (H2O) isconsumed by the citric acid cycle producing two
equivalents of carbon dioxide (CO2) and one equivalent of
HS-CoA.
3. one complete turn of the cycle converts three equivalentsof nicotinamide adenine dinucleotide (NAD+) into three
equivalents of reduced NAD+ (NADH), one equivalent of
ubiquinone (Q) into one equivalent of reduced ubiquinone
(QH2), and one equivalent each of guanosine diphosphate
(GDP) and inorganic phosphate (Pi) into one equivalent ofguanosine triphosphate (GTP).
4. The NADH and QH2 generated by the citric acid cycle are
in turn used by the oxidative phosphorylation pathway to
generate energy-rich adenosine triphosphate (ATP).
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One of the primary sources of acetyl-CoA is sugars
that are broken down by glycolysis to produce pyruvate
that in turn is decarboxylated by the enzyme pyruvate
dehydrogenase generating acetyl-CoA according to the
following reaction scheme:
CH3C(=O)C(=O)O (pyruvate) + HSCoA + NAD+
CH3C(=O)SCoA (acetyl-CoA) + NADH + CO2
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The product of this reaction,
acetyl-CoA, is the starting point for
the citric acid cycle.
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During glycolysis, glucose isbroken down to pyruvate.
A two-carbon fragment ofpyruvate is used to form acetyl-CoA. The acetyl CoAenters the Krebs Cycle, which occurs in the mitochondrion.
During the conversion ofpyruvate to acetyl-CoA, carbon dioxide (CO2) is produced and a
molecule of NADH is formed
The two carbon acetyl portion of the acetyl-CoA is transferred to a
4-carbon molecule, producing a six-carbon compound. The CoA
carrier molecule is released.
Carbon dioxide is then released from the 6-carbon molecule, forming a 5-
carbon compound. In this step, hydrogen is removed and transferred to
NAD+ to form NADH
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Finally the 4-carbon molecule is further oxidized and the hydrogens that
are removed are used to form NADH and FADH2. These reactionsregenerate the 4-carbon molecule that initially reacts with acetyl-CoA.
Each glucose molecule is broken down into two pyruvate molecules
during glycolysis. Then each pyruvate is converted to acetyl-CoA, whichenters the Krebs Cycle.
Thus, for each glucose molecule, the Krebs cycle must complete two
circuits to completely break dwon the two pyruvate molecules.
Next, a second oxidation and decarboxylation occurs. Again, NADH and carbon
diocide are produced. In addtition, a molecule of ATP is produced. As a result ofthese reactions, a 4-carbon molecule is formed in the Krebs Cycle.
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Simplified
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2 only per cylce
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The greatly simplified cycle below starts withpyruvate, which is the end product of gylcolysis, thefirst step of all types of cell respiration.
Since it is well known that ATP is the energy currencythat is used for nearly every task in the human body
and other organisms, it may seem surprising that solittle ATP is produced directly by the TCA cycle. Butthe TCA cycle as shown here represents the processtaking place in the matrix of the mitochondria usingthe reactions with NAD+ and FAD which carry highenergy electrons to the electron transport systemsinthe inner mitochondrial membrane. The reactionsthere are responsible for the production of most of the
ATP yield by chemiosmosis.
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The Krebs cycle, also called the citric acid
cycle, is a fundamental metabolic pathwayinvolving eight enzymes essential for energy
production through aerobic respiration, and,
like glycolysis, arose early in evolution
The Krebs cycle takes place in mitochondriawhere it oxidizes acetyl-CoA, releasing carbon
dioxide and extracting energy primarily as the
reduced high-energy electron carriers NADH
and FADH2. NADH and FADH2 transferchemical energy from metabolic intermediates
to the electron transport chain to create a
different form of energy, a gradient of protons
across the inner mitochondrial membrane.
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The Krebs cycle, like other metabolic pathways,
is tightly regulated to efficiently meet the needsof the cell and the organis
Some Krebs cycle enzymes require non-protein
cofactors for activity, such as thiamine, vitamin
B1. Insufficient quantities of this vitamin in thediet leads to decreased activity of pyruvate
dehydrogenase and alpha-ketoglutarate
dehydrogenase, and a decrease in the ability of
the Krebs cycle to meet metabolic demands,causing the disease beriberi.
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Kreb's cycle is important in the processes
of cellular respiration as well as theprocessing of fats and other molecules for
the harvesting of chemical energy
Fats are broken down in a process calledbeta oxidation and enter the Kreb's cycle
as acetyl groups.