Aerobic Metabolism: The Citric Acid Cycle Khadijah Hanim Abdul Rahman School of Bioprocess Eng, UniMAP Sem 1, 2011/2012 Week 14: 15/12/2011
Transcript
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Khadijah Hanim Abdul Rahman School of Bioprocess Eng, UniMAP
Sem 1, 2011/2012 Week 14: 15/12/2011
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1. The citric acid cycle (also known as the tricarboxylic acid
cycle, the TCA cycle, or the Krebs cycle) is a series of chemical
reactions of central importance in all living cells that utilize
oxygen as part of cellular respiration.chemical reactions
cellsoxygencellular respiration 2. In aerobic organisms, the citric
acid cycle is part of a metabolic pathway involved in the chemical
conversion of carbohydrates, fats and proteins into carbon dioxide
and water to generate a form of usable energy.aerobic
organismsmetabolic pathwaycarbohydratesfatsproteinscarbon
dioxidewater
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1. It is the second of three metabolic pathways that are
involved in fuel molecule catabolism and ATP production, the other
two being glycolysis and oxidative phosphorylation.fuel
moleculecatabolismATP glycolysisoxidative phosphorylation 2. The
citric acid cycle also provides precursors for many compounds such
as certain amino acids, and some of its reactions are therefore
important even in cells performing fermentation.amino
acidsfermentation
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Facultative anaerobes and obligate aerobes that use O 2 to
generate energy, employ the following biochemical processes: - TCA
cycle - Electron transport pathway - oxidation phosphorylation In
eukaryotes- occur in mitochondrion
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TCA cycle- metabolic pathway, 2 carbons fragments derived from
organic fuel molecules are oxidized to form CO 2 and the coenzymes
NAD + and FAD are reduced to form NADH and FADH 2. The electron
transport chain (ETC)- mechanism in which electrons are transferred
from reduced coenzymes to an acceptor, O 2. In oxidative
phosphorylation- energy released by ETC is captured in a form of a
proton gradient that drives the synthesis of ATP, the energy
currency of living organisms.
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In living organisms, both energy- capturing and energy
releasing processes consist primarily of redox reactions. In redox
reactions electrons move between an electron donor and an electron
acceptor.
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TCA cycle- series of biochemical reactions aerobic organisms
use to release chemical energy stored in the 2- carbon acetyl group
in acetyl-CoA. Acetyl-CoA composed of an acetyl group derived from
the breakdown of carbohydrates, lipids and some amino acid that is
linked to the acyl carrier molecule Coenzyme A.
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Acetyl-CoA is synthesized from pyruvate Acetyl-CoA- also the
product of fatty acid catabolism and certain reactions in amino
acid metabolism. Its main function is to convey the carbon atoms
within the acetyl group to the citric acid cycle to be oxidized for
energy production.carbon atomsacetylcitric acid cycleoxidized In
TCA cycle- the carbon atoms are oxidized to CO 2 and the
high-energy electrons are transferred to NAD + and FAD to form the
reduced coenzymes NADH and FADH 2.
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In the first reaction- a 2-carbon acetyl group condenses with a
4-carbon molecules (oxaloacetate) to form a 6-carbon molecule
(citrate). During the subsequent 7 reactions, in which 2 CO 2
molecules are produced and 4 pairs of electrons are removed from
carbon compounds, citrate is reconverted to oxaloacetate. In one
step in the cycle, substrate-level phosphorylation, high-energy
molecule GTP is produced.
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The net reaction for TCA cycle as follows: Acetyl-CoA + 3NAD +
+ FAD + GTP + P i + 2H 2 O 2CO 2 + 3NADH + FADH 2 + oASH + GTP + 3H
+ In addition to its role in energy production, TCA cycle plays
important role. Its cycle intermediates are substrates in variety
of biosynthetic reactions.
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Pyruvate is transported out from cytosol into the mitochondrial
matrix where it converted to acetyl-CoA Pyruvate acetyl-CoA- in a
series of reactions catalyzed by pyruvate dehydrogenase complex
enzyme The net reaction, an oxidative decarboxylation as follows:
Pyruvate + NAD + + CoASH Acetyl-CoA + NADH + CO 2 + H 2 O + H
+
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The pyruvate dehydogenase enzyme complex contains 3 enzymes
activities: - Pyruvate dehydrogenase - Dihydrolipoyl transacetylase
- Dihydrolipoyl dehydrogenase TPP (Thiamine pyrophosphate), FAD,
NAD+ and lipoic acid are the required co-enzymes for acetyl-CoA
synthesis from pyruvate.
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The main function of acetyl-CoA is to convey the carbon atoms
within the acetyl group to the citric acid cycle to be oxidized for
energy production.carbonatoms acetylcitric acid cycle oxidized
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in each turn of the cycle, acetyl-CoA from the glycolytic
pathway/fatty acid catabolism enters and 2 fully oxidized carbon
molecules leave as CO 2. 3 molecules of NAD + and 1 molecule of FAD
are produced 1 molecule of GTP (interconvertible with ATP) is
generated in a substrate-level phosphorylation reaction.
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TCA cycle is composed of eight reactions that occur in 2
stages: 1) The 2-carbon of acetyl group of acetyl- CoA enters the
cycle by reacting with a 4-carbon compound oxaloacetate. 2 molecule
of CO 2 are subsequenly released. 2) Oxaloacetate is regenerated so
it can react with another acetyl-CoA
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TCA cycle begins with the condensation of acetyl-CoA with
oxaloacetate to form citrate:
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citrate which contains a tertiary alcohol is reversibly
converted to isocitrate by an enzyme aconitase. During
isomerization reaction, an intermediate- cis-aconitate is formed by
dehydration. The carbon-carbon double bond of cis- aconitate is
rehydrated to form isocitrate.
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isocitrate dehydrogenase catalyzes oxidative decarboxylation of
isocitrate to form -ketoglutarate: There are two different forms of
isocitrate dehydrogenase, one requiring NAD + as electron acceptor
and the other requiring NADP +. The overall reactions catalyzed by
the two isozymes are otherwise identical. The NAD-dependent enzyme
is found in the mitochondrial matrix and serves in the citric acid
cycle to produce a-ketoglutarate. T he NADP-dependent isozyme is
found in both the mitochondrial matrix and the cytosol. It may
function primarily in the generation of NADPH, which is essential
for reductive anabolic reactions.
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The conversion of - ketoglutarate to succinyl-CoA is catalyzed
by the enzyme activities in the dehydrogenase complex: -
ketoglutarate dehydrogenase.
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The cleavage of the high-energy thioester bond of succinyl-CoA
to form succinate, catalyzed by succinate thiokinase is coupled in
mammals to the substrate-level phosphorylation of GDP.
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Succinate dehydrogenase catalyzes the oxidation of succinate to
form fumarate: Succinate dehydrogenase is a flavoprotein using FAD
to drive the oxidation of succinate to fumarate.
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Fumarate is converted to L-malate in a reversible
stereospecific hydration reaction catalyzed by fumarase:
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Finally, oxaloacetate is regenerated with the oxidation of
L-malate:
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The citric acid cycle begins with the condensation of
acetyl-CoA molecule with oxaloacetate to form citrate, which is
eventually reconverted to oxaloacetate. During this process, - 2
molecules of CO 2 - 3 molecules of NADH - 1 molecule of FADH 2 - 1
molecule of GTP
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Many of the enzymes in the TCA cycle are regulated by negative
feedback from ATP when the energy charge of the cell is
high.negative feedbackenergy charge Such enzymes include the
pyruvate dehydrogenase complex that synthesises the acetyl-CoA
needed for the first reaction of the TCA cycle.pyruvate
dehydrogenase Also the enzymes citrate synthase, isocitrate
dehydrogenase and alpha-ketoglutarate dehydrogenase, that regulate
the first three steps of the TCA cycle, are inhibited by high
concentrations of ATP. This regulation ensures that the TCA cycle
will not oxidise excessive amounts of pyruvate and acetyl-CoA when
ATP in the cell is plentiful. This type of negative regulation by
ATP is by an allosteric mechanism allosteric
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Several enzymes are also negatively regulated when the level of
reducing equivalents in a cell are high (high ratio of NADH/NAD+).
This mechanism for regulation is due to substrate inhibition by
NADH of the enzymes that use NAD+ as a substrate. substrate
inhibition This includes both the entry point enzymes pyruvate
dehydrogenase and citrate synthase.