The Citric Acid Cycle

The citric acid cycle is a series of reactions that
connects the intermediate acetyl CoA from stage 2 with electron
transport and the synthesis of ATP in stage 3.

The Citric Acid Cycle

The citric acid cycle (stage 3)
?operates under aerobic conditions.
?oxidizes the two-carbon acetyl group in acetyl CoA to CO2.
?is named from citric acid (C6H8O7), formed in the first reaction.
?is also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle.

Citric Acid Cycle Overview

?In the citric acid cycle, eight reactions oxidize acetyl CoA,
producing CO2 and the high-energy compounds FADH2, NADH, and GTP.
?Reactions involved in the citric acid cycle include condensation,
dehydration, hydration, oxidation, reduction, and hydrolysis.

Reaction 1: Formation of Citrate

In the first reaction (condensation)of the citric
acid cycle,
?
citrate synthase catalyzes the condensation of an
acetyl group (2C) from acetyl CoA with oxaloacetate (4C) to yield
citrate (6C) and coenzyme A.
?the energy to form citrate is provided by the hydrolysis of the
high-energy thioester bond in acetyl CoA.

Reaction 2: Isomerization

In reaction 2 (isomerization) of the citric acid cycle,
?citrate rearranges to isocitrate, a secondary alcohol.
?
aconitase catalyzes the dehydration of citrate
(tertiary alcohol) to yield cis-aconitate, followed by a
hydration that forms isocitrate (secondary alcohol).

Reaction 3: Oxidation, Decarboxylation

In reaction 3, isocitrate undergoes oxidation and
decarboxylation by
isocitrate

dehydrogenase.
?One carbon is removed by converting a carboxylate group (COO?) to CO2.
?The dehydrogenase removes hydrogen ions and electrons, used to
reduce NAD+ to NADH and H
+
.

Reaction 4: Oxidation, Decarboxylation

In reaction 4, oxidation and
decarboxylation catalyzed by
?-

ketoglutarate

dehydrogenase,
??-ketoglutarate (5C) undergoes decarboxylation to yield
(4C) succinyl CoA.
?oxidation of the thiol group (� SH) in HS � CoA provides hydrogen
that is transferred to NAD+ to form a second molecule of NADH
and H
+
.

Reaction 5: Hydrolysis

In reaction 5, hydrolysis catalyzed by
succinyl

CoA

synthetase,
?hydrolysis of the thioester bond in succinyl CoA yields succinate
and HS � CoA.
?energy from hydrolysis is transferred to the condensation of
phosphate and GDP forming GTP, a high-energy compound
similar to ATP.

Reaction 6: Hydrolysis

In reaction 6, hydrolysis catalyzed by
succinate dehydrogenase,
?succinate is oxidized to fumarate, a compound with a C = C bond.
?2H lost from succinate are used to reduce the coenzyme FAD to FADH
2.

Reaction 7: Hydration

In reaction 7, hydration catalyzed by
fumarase, water is added to the double bond of
fumarate to yield malate, a secondary alcohol.

Reaction 8: Oxidation

In reaction 8, oxidation catalyzed by
malate dehydrogenase,
?the hydroxyl group in malate is oxidized to a carbonyl group,
yielding oxaloacetate.
?oxidation provides hydrogen ions and electrons for the reduction of
NAD+ to NADH and H
+
.

Summary, Citric Acid Cycle

?Molecular oxygen molecules do not
directly participate in citric acid cycle. However, the cycle operates
only under aerobic conditions.
?Glycolysis has both an aerobic
and an anaerobic mode, whereas the citric
acid cycle is strictly aerobic.

Regulation of the Citric Acid Cycle

?The following enzymes:
1.
pyruvate dehydrogenase
2.
isocitrate

dehydrogenase
3.
?

-

ketoglutarate

dehydrogenase
?are activated by high [ADP] (signs ATP consumption)
?are inhibited by high [ATP], high [NADH] (signs
that ATP is not being used)
?
Citrate synthase is inhibited by
high [ATP], high [NADH]

Electron Transport

The reduced coenzymes NADH and FADH
2 produced from glycolysis, oxidation of pyruvate,
and the citric acid cycle are oxidized to provide the energy for the
synthesis of ATP.
In electron transport or the
respiratory chain,
?hydrogen ions and electrons from NADH and FADH2 are passed from one
electron acceptor or carrier to the next until they combine with
oxygen to form H2O.
?energy released during electron transport is used
to synthesize ATP from ADP and Pi during oxidative phosphorylation.

Electron Transport System

In the electron transport system,
?there are
five protein complexes, which are numbered I, II,
III, IV, and V.
?two electron carriers,
coenzyme Q and
cytochrome c, attached to the inner
membrane of the mitochondrion, carry electrons between
these protein complexes bound to the inner membrane.

NADH to Complex I

In complex I,
?electron transport begins when hydrogen ions and electrons are
transferred from NADH to complex I.
?loss of hydrogen from NADH regenerates NAD+ to
oxidize more substrates in oxidative pathways such as the citric acid cycle.
?hydrogen ions and electrons are transferred to the mobile electron
carrier CoQ, forming CoQH
2.

Complex I, Electron Transfer

During electron transfer, complex I generates energy
from electron transfer,
?H+ ions are pumped through complex I into the intermembrane space,
producing a reservoir of H+ (hydrogen
ion
gradient).
?for every two electrons that pass from NADH to CoQ, 4H+ are pumped
across the mitochondrial membrane, producing a charge separation on
opposite sides of the membrane.

Coenzyme Q

?Coenzyme Q is also known as
ubiquinone because of its widespread occurrence.
?CoQ is a mobile electron carrier that can accept one or two electrons.
?CoQ
is lipid-soluble and can readily diffuse into the
membrane. transports electrons from Complexes I and II to
Complex III.

Complex II

?Complex II is the enzyme
succinate dehydrogenase from the citric acid cycle.
?In complex II, CoQ obtains electrons directly from FADH
2. This produces CoQH
2 and regenerates the oxidized coenzyme
FAD, which becomes available to oxidize more substrates.
No H+ ions are pumped into the intermembrane space

CoQH
2
to Complex III

?The CoQH2 obtained from complex I and II will transfer electrons to
Complex III.
?Two electrons are transferred from the mobile carrier CoQH
2 to a series of iron-containing proteins called
cytochromes inside complex III.
?Complex III generates energy from the electron
transfer and pumps 4H+ from the matrix into the intermembrane space,
increasing the hydrogen ion gradient.

Cytochrome

c

Cytochrome

c
?contains Fe3+/Fe2+, which is reduced to Fe2+ and oxidized to Fe3+.
?is water-soluble and can only transfer one electron at a time.
?moves electrons from complex III to complex IV.
?For every 1 molecule of CoQH
2, 2 molecules of
cytochrome c is required.

Complex IV

At complex IV,
?four electrons from four cytochrome

c are passed to other electron carriers inside
complex IV.
?electrons combine with hydrogen ions and oxygen (O2) to form two
molecules of water.
?energy is used to pump H+ from the mitochondrial matrix into the
intermembrane space, further increasing the hydrogen ion gradient.

Oxidative Phosphorylation

?Energy in electron transfer is coupled with the production of ATP in
a process called oxidative phosphorylation.
?In 1978, Peter Mitchell theorized about a
chemiosmotic

model, which links the energy
from electron transport drives the synthesis of ATP.
?Complexes I, III, and IV act as hydrogen ion pumps, producing a
hydrogen ion gradient.
?The high [H
+
] in the intermembrane space will
move H

+

back to the matrix.
?The
potential energy gained as they were moved against
the gradient
is
now released and drives the oxidative phosphorylation.

Oxidative Phosphorylation, ATP

In the chemiosmotic model,
?H+ cannot move through the inner membrane but returns to the matrix
by passing through a fifth protein complex in the inner membrane
called ATP synthase (also called complex V).
?the flow of H+ from the intermembrane space through the ATP
synthase generates energy that is used to
synthesize ATP from ADP and Pi.
This process of oxidative phosphorylation couples
the energy from electron transport to the synthesis of ATP

Electron Transport and ATP Synthesis

?When NADH enters electron transport at complex I,
the energy transferred can be used to synthesize 2.5 ATP.
?When FADH
2 enters electron transport at complex II, it
provides energy for the synthesis of 1.5 ATP.
?Current research indicates that the oxidation of one NADH yields
2.5 ATP and one FADH2 yields 1.5 ATP.

Regulation of Electron Transport and Oxidative Phosphorylation

?When a cell is active and ATP is consumed
rapidly, the elevated levels of ADP will activate the synthesis
of ATP.
?The activity of electron transport is strongly dependent on the
availability of ADP for ATP synthesis.

Complete Oxidation of Glucose

The complete oxidation of glucose to CO2 and H2O yields a maximum of
32 ATP.

Efficiency of ATP Production

?In a laboratory calorimeter, 1 mole of glucose produces 690 kcal.
C6H12O6 + 6O2 � 6CO2 + 6H2O + 690 kcal
?To calculate the ATP energy produced in the mitochondria from
glucose, we use the energy of the hydrolysis of ATP (7.3 kcal/mole of ATP).
Our cells are about 33% efficient in converting the total available
chemical energy in glucose to ATP.
The remainder of the energy produced from glucose during the
oxidation in our cells is lost as heat.

quality improvement

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quality assessment

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performance improvement (QAPI)

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continuous

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total quality management (TQM)

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What is the purpose of quality improvement? Pg. 497

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