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Citric Acid Cycle
Reactions
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Introduction:
Under aerobic conditions the end product of glycolysis is
pyruvic acid. The next step is the formation of acetyl coenzyme
A(acetyl CoA) - this step is technically not a part of the
citric acid cycle, but is shown on the diagram on the top left.
Acetyl CoA, whether from glycolysis or the fatty acid spiral,
is the initiator of the citric acid cycle. In carbohydrate metabolism,
acetyl CoA is the link between glycolysis and the citric acid
cycle.
The initiating step of the citric acid cycle occurs when a
four carbon compound (oxaloacetic acid) condenses with acetyl
CoA (2 carbons) to form citric acid (6 carbons).
The whole purpose of a "turn" of the citric acid
cycle is to produce two carbon dioxide molecules. This general
oxidation reaction is accompanied by the loss of hydrogen and
electrons at four specific places. These oxidations are connected
to the electron transport chain where many ATP are produced.
The reactions for the citric acid cycle are shown in the graphic
on the left. These reactions are more familiar than those from
glycolysis. Unless the instructor states otherwise, you should
study these reactions so that you can: tabulate the ATP and CO2
generated; name the type of reaction at each step; and write
the structure of any compound which has been blanked out. You
should not memorize these structures but derive them from a knowledge
of reaction principles.
Citric Acid Cycle
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Overview
of Metabolism
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Reaction 1: Synthesis of Citric Acid
Acetyl CoA and oxaloacetic acid condense to form citric
acid. The acetyl group CH3COO is transferred from CoA to
oxaloacetic acid at the ketone carbon, which is then changed
to an alcohol. The net effect is to join a 2 carbon piece with
a 4 carbon piece to make citric acid which is 6 carbons. This
is just called the synthesis of citric acid.
This reaction is catalyzed by citric acid synthetase.
Reaction 1 - Chime
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Link to Chime: Citric
Acid Synthetase - Department of Biochemistry,The University
of Arizona
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Reaction 2: Dehydration of an alcohol
Two steps (Rx. 2 and 3) are required to isomerize the position
of the -OH group on citric acid. This first step is a dehydration
of an alcohol to make an alkene. The cis-aconitic acid remains
bound to the enzyme aconitase in readiness for the next step.
This reaction is catalyzed by aconitase.
Reaction 2 - Chime
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Reaction 3: Hydration to make alcohol
This reaction is a hydration reaction of an alkene to make
an alcohol. This hydration does not follow Markovnikov's Rule.
The net effect of reactions 2 and 3 has been to move the -OH
group from C-3 to C-2, which is isocitric acid.
This reaction is catalyzed by aconitase.
Reaction 3 - Chime
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Link to Chime: Aconitase
- Department of Biochemistry,The University of Arizona
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Reaction 4: Oxidation
This is the first oxidation reaction in which an alcohol is
converted to a ketone. Two hydrogens and 2 electrons are transferred
to NAD+ to NADH + H+. This is the entry
point into the electron transport chain.
The product of this reaction, oxalosuccinic acid, remains
attached to the isocitrate dehydrogenase for the next step.
This reaction is catalyzed by isocitrate dehydrogenase.
Reaction 4 Chime
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Reaction 5: Decarboxylation
This is the first step where a carbon group is lost as carbon
dioxide in a decarboxylation reaction. The remaining compound
now has 5 carbons and is called alpha-ketoglutaric acid.
This reaction is also catalyzed by isocitrate dehydrogenase.
Reaction 5 Chime
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Reaction 6: Oxidation, Decarboxylation, Thiol Ester
Synthesis
This complex oxidative decarboxylation is guided by three
enzymes in much the same fashion as the formation of acetyl CoA
from pyruvic acid. This is actually the only non-reversible step
in the entire cycle and prevents the cycle from operating in
the reverse direction.
This is the second oxidation reaction in which an alcohol
is converted to a ketone. Two hydrogens and 2 electrons are transferred
to NAD+ to NADH + H+. This another the
entry point into the electron transport chain.
This is the second step where a carbon group is lost as carbon
dioxide in a decarboxylation reaction. Essentially, although
not the exact same carbons, the two carbons from the acetyl CoA
have been converted to carbon dioxide at the end this step/.
The remaining 4 carbon group is attached to the CoA through
a thiol ester high energy bond. Notice that the final product,
succinyl CoA, has 4 carbons in the succinate group at
one end of the CoA molecule.
This reaction is catalyzed by alpha-ketoglutarate dehydrogenase
complex.
Reaction 6 Chime
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Reaction 7: Hydrolysis of Succinyl CoA;
Synthesis of ATP
The hydrolysis of the thioester bond (exothermic) is coupled
with the formation of ATP (Actually guanosine triphosphate is
formed first but is further coupled with the ADP to make ATP).
This is the only "visible" ATP formed in the entire
cycle.
Succinic acid, a 4 carbon acid, is the product of this
reaction. This is the start of the return to the beginning of
the cycle.
This reaction is catalyzed by succinyl CoA.
Reaction 7 Chime
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Reaction 8: Oxidation
This slightly unusual oxidation reaction results in the removal
of the hydrogens from saturated alkyl carbons to form an alkene,
fumaric acid. The hydrogen acceptor is the coenzyme FAD
instead of the more usual NAD+. This will be significant when
the ATP is tabulated from the electron transport chain, since
this coenzyme is in the enzyme complex 2. Only 2 ATP result from
this reaction in the electron transport chain.
This reaction is catalyzed by succinate dehydrogenase.
Reaction 8 Chime
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Link to Chime: Succinate
dehydrogenase - Department of Biochemistry,The University
of Arizona
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Reaction 9: Hydration to form an alcohol
This is a simple hydration reaction of an alkene to form an
alcohol. Take your pick where you place the -OH group since it
must be adjacent to a carboxylic acid group in either case and
forms malic acid.
This reaction is catalyzed by fumarase.
Reaction 9 - phosphoenol pyruvic acid Chime
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Reaction 10: Oxidation
This is the final reaction in the citric acid cycle. The reaction
is the oxidation of an alcohol to a ketone to make oxaloacetic
acid. The coenzyme NAD+ causes the transfer of two hydrogens
and 2 electrons to NADH + H+. This is a final entry
point into the electron transport chain.
This reaction is catalyzed by malate dehydrogenase.
Reaction 10 Chime
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Conclusion:
Starting with acetyl Co A with 2 carbons, the citric acid
cycle spins these 2 carbons off as two carbon dioxide molecules.
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