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Catabolism:
the third stage
Intermediary Oxidative
Metabolism
The TCA Cycle or citric acid cycle or
Krebs Cycle
Stages of catabolism
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1st stage: Large molecules in food are broken
down into smaller units. Preparation stage
without capture of energy.
• Proteins -> amino acids,
• Polysaccharides -> monosaccharides
(glucose, ...)
• Fats -> glycerol, fatty acids.
2nd stage: Molecules are degraded to simple
units that play a central role in metabolism.
Most of them are converted into the acetyl
unit of acetyl CoA. Some ATP is generated in
this anaerobic stage, but amount is small
compared with 3rd stage.
3rd stage: ATP is produced from the complete
oxidation of the acetyl unit of acetyl CoA.
Acetyl CoA brings acetyl units into the citric
acid cycle, where they are completely
oxidized to CO2. Four pairs of electrons are
transferred (three to NAD+ and one to FAD)
for each acetyl group that is oxidized. Then,
a proton gradient is generated as electrons
flow from the reduced forms of these carriers
to O2, and this gradient is used to synthesize
ATP.
Third stage
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Anaerobic generation of ATP from glucose
via glycolysis is inefficient:
• Endproducts still contain energy that
could
be
released
by further
oxidation.
The addition of an oxygen requiring stage
(stage 3, oxidative metabolism) resulted in a
more powerful means of extracting energy
from food molecules.
The third stage of catabolism consists of the
TCA cycle (or citric acid cycle or Krebs
cycle) and the process of oxidative
phosphorylation.
In citric acid cycle, acetyl groups from acetyl
CoA are oxidized to produce CO2 and NADH
In oxidative phosphorylation, NADH reacts
with O2 to produce ATP and H2O via a
complicated series of steps involving
mitochondrial electron transport.
From pyruvate to acetyl-CoA
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Pyruvate
is
transported
into
the
mitochondrial matrix by a pyruvate/OHantiport.
In the matrix, pyruvate is oxidatively
decarboxylated
by
the
pyruvate
dehydrogenase complex to form acetyl CoA.
Acetyl CoA is then fed into the TCA cycle.
Other sources of Acetyl CoA
Beta oxidation of fatty acids
• Each FA molecule undergoes an initial activation
step to become a fatty acyl-CoA – this is
energized by hydrolysis of an ATP to AMP
• After activation, a repeating cycle of 4 reactions
splits off acetyl Co-A until the end of the fatty
acid is reached
The activation step
Fatty acids: β-oxidation
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Fatty acids in cytosol are bound to CoA.
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Transport into mitochondria
1st oxidation at β C and transformation of
FAD to FADH2.
Hydration of double bond.
Oxidation and transformation of NAD+ to
NADH + H+.
Acetyl CoA splits off and rest of chain is
bound to another CoA.
… until fatty acid is at its end.
Special cases are unsaturated fatty acids
and fatty acids with odd numbers of C
atoms.
Fatty acid oxidation takes also place in
peroxisomes, but no ATP generation.
Acetyl-CoA back to cytosol (synthesis)
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The beta oxidation spiral
Mitochondria do beta-oxidation
• Beta oxidation takes place inside mitochondria –
so without mitochondria a tissue cannot
metabolize fat for energy.
Ketone bodies and ketoacidosis
• High rates of fat oxidation tend to leave us with an
overload of acetylCoA, which the liver converts to
acetate, acetone, acetoacetate, and beta-OH butyrate.
These are the so-called ketone bodies that appear in
the blood, sweat and breath of individuals that are
starving, or suffering from untreated diabetes mellitus.
Some of them are acids, so their appearance in the
blood causes ketoacidosis. Some of them are smelly,
so they can be detected by a practitioner without the
use of chemical analysis.
Odd vs even-numbered fatty acids
• Since fatty acids are taken apart (and also
assembled) in 2-C pieces, some cells find it
harder to deal with odd-numbered fatty acids –
they are unusual in mammals but common in
plants and marine organisms. The tail-end of an
odd-numbered fatty acid turns out to be 3-C
propionyl-CoA, which is decarboxylated in a
multistep process to form pyruvate. So, we can
eat oysters, after all.
Amino acids: protein is broken down to amino acids
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Proteins are digested to amino acids that are delivered to the cells.
Amino acids: transamination and deamination
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Transamination: an amino acid transfers its amino group to an α-keto acid. The amino acid
becomes an α-keto acid and the α-keto acid becomes an amino acid:
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In this way amino groups are collected in a few types of amino acids (often glutamate).
Oxidative deamination: an amino acid (often glutamate) is oxidized and deaminated with
liberation of ammonium:
Amino acids: carbon skeletons enter catabolic pathways
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The carbon skeletons of amino acids enter at different points into catabolic pathways.
TCA cycle: an overview
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Primary function of TCA cycle is to oxidize acetyl groups that enter the cycle as acetyl CoA
molecules (coming from pyruvate and fatty acids)
4-C compound condenses with 2-C acetyl unit to yield 6-C tricarboxylic acid
6-C compound is oxidatively decarboxylated to yield a 5-C compound
5-C compound is oxidatively decarboxylated to yield a 4-C compound
In subsequent reactions another NADH is produced as well as 1 FADH2 and a high energy
phosphate (GTP)
The electron carriers (NADH and FADH2) yield 9 ATP when they are oxidized by O2 in the electron
transport chain.
Feeding acetyl CoA into the cycle
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Enzyme: citrate synthase
CoA-SH is liberated to be again used for transfers
Citrate is isomerized
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Enzyme: aconitase
For later oxidative decarboxylation the tertiary OH-group is not at the right place
Dehydration results in cis-Aconitate
Hydration results in Isocitrate with the OH-group at C2.
Oxidative decarboxylation I
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Enzyme: Isocitrate dehydrogenase
Oxidative decarboxylation II
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Enzyme: a-ketoglutarate dehydrogenase
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Reaction resembles oxidative decarboxylation of pyruvate
Generation of GTP
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Enzyme: Succinyl CoA synthetase
Cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a purine
nucleoside diphosphate, usually GDP.
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This is a substrate-level phosphorylation
Phosphorylating GDP is equavalent energetically to phosphorylating ADP
GTP is not just ATP in another form - it is involved in signal transduction – a large number of
intracellular signals are GTP-binding proteins
Regeneration of oxaloacetate
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A methylene group (CH2) is converted into a carbonyl group (C = O) in three steps:
• an oxidation
• a hydration
• a second oxidation
Oxaloacetate is regenerated for another round of the cycle
More energy is extracted in the form of FADH2 and NADH.
Summary of TCA cycle (again)
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2 CO2 split off
3 NADH + H+ and 1 FADH2 generated
1 GTP generated
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No oxygen involved, but TCA cycle is
only active in the presence of
oxygen! Why?
Summary of TCA cycle
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Almost all reactions have negative ΔG.