section 13.3
Tricarboxylic Acid (TCA) Cycle
243
Oxidative Decarboxylation o f Isocitrate
to OL-Ketoglutarate
Isocitrate dehydrogenase catalyzes the first of two
decarboxylations and dehydrogenations in the cycle.
Three different isocitrate dehydrogenases are present: one
specific for NAD+ and found only in mitochondria, the
other two specific for NADP+ and found in mitochondria
and cytoplasm. The NAD+-specific enzyme is the primary
enzyme with regard to TCA cycle operation. All three re-
quire Mg2+ or Mn2+. The reaction yields a-ketoglutarate
(2-oxoglutarate),
NAD(P)H,
and
CO
2
and
involves
enzyme-bound oxalosuccinate as an intermediate.
C H 2— C O C T
j
NAD*
NApH
+ H+
C H — C O O '
------ ------------------------»
I
H O — C H — C O O '
Is o c itra te
C H
2
— C O O '
0
= C — C O O '
a -K e to g lu ta ra te
C H
2
— C O O '
I
+ NADH + H +
C H
2
C O — S C o A
The reaction is of the nonequilibrium type, with a
AG 0'
of
— 8
kcal/mol (—33.5 kJ/mol). Unlike the pyruvate de-
hydrogenase complex, the a-ketoglutarate dehydrogenase
complex does not possess a complex regulatory mech-
anism involving a kinase and a phosphatase. However,
activity is inhibited by high ratios of [ATP]/[ADP],
[succinyl-CoA]/[CoASH],
and
[NADH]/[NAD+]
and
stimulated by Ca2+. a-Ketoglutarate is reversibly trans-
aminated to glutamate and is utilized in the hydroxylation
of prolyl or lysyl residues of collagen (Chapter 25).
C H
2
— C O O '
H
2
C — C O O '
0
= c — CO O '
0 = C — COO '
O x a lo s u c c in a te
o c-K eto g lu tarate
(e n z y m e -b o u n d )
(
2
- o x o g lu ta r a te )
The reaction is nonequilibrium in type and has a AG0'
of —5.0 kcal/mol (—20.9 kJ/mol). Although in mitochon-
dria both NAD+- and NADP+-linked enzymes are in-
volved in the cycle, the NAD+-linked enzyme, which is
also under allosteric regulation, is the more predominant.
Positive effectors are ADP and Ca2+, and negative ef-
fectors are ATP and NADH. Thus, under conditions of
abundance of energy the enzyme is inhibited, and under
conditions of low energy the enzyme is stimulated.
Oxidative Decarboxylation o f a-Ketoglutarate
to Succinyl-CoA
The a-ketoglutarate dehydrogenase complex is analo-
gous to the pyruvate dehydrogenase complex and con-
sists of three enzymes: a-ketoglutarate dehydrogenase,
dihydrolipoyl transsuccinylase, and dihydrolipoyl dehy-
drogenase (the latter dehydrogenase is identical in both
complexes). The cofactor, prosthetic groups, and coen-
zyme requirements are identical to those of pyruvate oxi-
dation.The overall reaction is
Conversion o f Succinyl-CoA to
Succinate Coupled to
Formation o f GTP
In
this
complex
reaction
catalyzed
by
succinyl-
CoA synthase (succinate thiokinase), the energy-rich
thioester linkage of succinyl-CoA is hydrolyzed with
release of free energy that is conserved in the sub-
strate phosphorylation of GDP with phosphate to form
GTP:
C H jC O C T
|
+ G D P 3 ' -t-P ,2'* ±
C H
2
C O -S C o A
S u c c in y l-C o A
C H
2
C O O ~
I
+ G T P 4' + C oA S H
C H 2C O C T
S u c c in a te
This reaction is readily reversible (a near-equilibrium re-
action) and has a
AG 0'
of —0.7 kcal/mol (—2.9 kJ/mol).
It proceeds with the formation of intermediates of the
enzyme with succinyl phosphate and with phosphate
(P;), the latter being linked to histidyl residue of the
enzyme (E):
E + succinyl-CoA + P2+ ^
E-succinyl phosphate + CoASH
E-succinyl phosphate ^ E-phosphate + succinate
E-phosphate + GDP ^ E + GTP
