section 13.3
Tricarboxylic Acid (TCA) Cycle
241
13.3 Tricarboxylic Acid
(TCA) Cycle
The tricarboxylic acid (TCA) cycle (also called the
citric
acid cycle
or
Krebs cycle
) consists of eight enzymes that
oxidize acetyl-CoA with the formation of carbon dioxide,
reducing equivalents in the form of NADH and FADH
2
,
and guanosine triphiosphate (GTP):
C H 3 C O S C 0 A +
3
N A D + + F A D + G D P 3
+ H P O ^ "
+ 2H20 -* CoASH + 2C0
2
+ 3NADH
+ FADH
2
+ GTP4“ + 2H+
The reducing equivalents are transferred to the electron
transport chain, ultimately to reduce oxygen and generate
ATP through oxidative phosphorylation (Chapter 14).
Thus, oxidation of acetyl-CoA yields a large supply of
ATP.
Since the intermediates in the cycle are not formed or
destroyed in its net operation, they may be considered to
play catalytic roles. However, several intermediates are
biosynthetic precursors of other metabolites (amphibolic
role) and hence may become depleted. They are replen-
ished (anaplerotic process) by other reactions to optimal
concentrations.
The TCA cycle is the major final common path-
way of oxidation of carbohydrates, lipids, and pro-
teins, since their oxidation yields acetyl-CoA. Acetyl-
CoA also serves as precursor in the synthesis of fatty
acids, cholesterol, and ketone bodies. All enzymes of
the cycle are located in the mitochondrial matrix except
for succinate dehydrogenase, which is embedded in
the inner mitochondrial membrane. Thus, the reducing
equivalents generated in the cycle have easy access to
the electron transport chain. TCA cycle enzymes, with
the exception of a-ketoglutarate dehydrogenase complex
and succinate dehydrogenase, are also present outside
the mitochondria. The overall TCA cycle is shown in
Figure 13-12.
Reactions of TCA Cycle
Condensation o f Acetyl-CoA
with Oxaloacetate to
Form Citrate
The first reaction of the TCA cycle is catalyzed by
citrate synthase and involves a carbanion formed at the
methyl group of acetyl-CoA that undergoes aldol con-
densation with the carbonyl carbon atom of the oxalo-
acetate:
SCoA
o
I
II
”2°
*c=o + c—coo-
I
1
*CH
3
CH
2
— C O O "
Acetyl-CoA
O xaloacetate
ch2—(kxr
I
HO—c—COO"
+
CoASH+ H +
I
CH2—COO'
Citrate
The condensation reaction is thought to yield a transient
enzyme-bound intermediate, citroyl-CoA, which under-
goes hydrolysis to citrate and CoASH with loss of free
energy. This reaction is practically irreversible and has a
AG° of —7.7 kcal/mol (—32.2 kJ/mol). Formation of cit-
rate is the committed step of the cycle and is regulated by
allosteric effectors. Depending on the cell type, succinyl-
CoA (a later intermediate of the cycle), NADH, ATP,
or long-chain fatty acyl-CoA functions as the negative
allosteric modulator of citrate synthase. Citrate formation
is also regulated by availability of substrates, and citrate
is an allosteric inhibitor.
Citrate provides the precursors (acetyl-CoA, NADPH)
for fatty acid synthesis and is a positive allosteric
modulator of acetyl-CoA carboxylase,
which is in-
volved in the initiation of long-chain fatty acid synthe-
sis (Chapter 18). It regulates glycolysis by negative mod-
ulation of
6
-phosphofructokinase activity (see above).
All of the above reactions occur in the cytoplasm, and
citrate exits from mitochondria via the tricarboxylate
carrier.
Isomerization o f Citrate to Isocitrate
In this reaction, the tertiary alcoholic group of citrate
is converted to a secondary alcoholic group that is more
readily oxidized. The isomerization catalyzed by aconi-
tate dehydratase (or aconitase) occurs by removal and
addition of water with the formation of an intermediate,
cA-aconitate:
CH
2
— C O O '
I
H
20
H O — C — C O O '
—2—
C H
2
— C O O '
C itrate
C H
2
— C O O '
I
H2o
C — C O O '
— s -
II
C H — C O O " _
c/s-A conitate
C H
2
— C O O "
I
CH — C O O '
I
HO— C H — C O O '
Isocitrate
The cA-aconitate may not be an obligatory inter-
mediate, since the enzyme presumably can isomerize
