section 13.2
Pyruvate Metabolism
237
4 '- P h o s p h o p a n te th e in e
H
CH
,
O
O
I l
I
I
H S — C H — C H — N H — c — C H — C H — NH— c — c — c — C H — o — p— o —
o
o
O
OH
CH
,________
o
o
ß -M e rc a p to e th y la m in e
P a n to th e n ic a d d
F I G U R E 1 3 -7
Coenzyme A. The terminal sulfhydryl group is the reactive group of the molecule.
acetyl (and acyl groups) by formation of thioesters, and
the acetyl-sulfur bond has a AG° comparable to that of
the phosphoanhydride bond of ATP.
Dehydrogenation and decarboxylation of pyruvate oc-
cur in five reactions requiring three enzymes and six
coenzymes, prosthetic groups, and cofactors (Mg2+, TPP,
lipoic acid, CoASH, flavin adeninedinucleotide [FAD],
and NAD+). The mammalian complex (M.W. 7-8.5 x
106) consists of 20 or 30 units of pyruvate dehydrogenase
(depending on the source), 60 units of dihydrolipoyl
transacetylase (also called dihydrolipoamide acetyltrans-
ferase), and 5 or
6
units of dihydrolipoyl dehydrogenase
(also called dihydrolipoamide reductase). A kinase and a
phosphatase are tightly bound to the pyruvate dehydro-
genase subunit and participate in the regulation of the
activity.
In the first reaction, pyruvate is decarboxylated by re-
action with TPP bound to pyruvate dehydrogenase (Ei),
with formation of a hydroxyethyl group attached to the
thiazole ring of TPP:
O
I
C H
3
— C — C O O " + T P P — E , ------------ »
H
I
s
C H
3
— C — T P P — E, +
I
I
S
OH
H S-
+ E ,— T P P
i
t
C H
3
Ez
A detailed mechanism of the first two reactions is shown in
Figure 13-8. In the third step, catalyzed by dihydrolipoyl
transacetylase, the acetyl group attached to E
2
is trans-
ferred to CoASH, with formation of the dithiol form of
the lipoyl group and acetyl-CoA.
/^S H
V
- s — c — C H
3
+ CoA SH
e
2
O
o
SH
SH
e
2
+ C H
3
C — S — C oA
A cetyl-C oA
H
I
C H
3
— C — T P P — E , + C 0
2
I
OH
In the fourth step, catalyzed by the same enzyme, the hy-
drogen atoms of the dithiol group of E
2
are transferred
to the FAD prosthetic group of dihydrolipoyl dehydro-
genase (E
3
).
In the second step, also catalyzed by pyruvate dehydroge-
nase, the hydroxyethyl group is transferred to the oxidized
form of the lipoyl-lysyl prosthetic group of dihydrolipoyl
transacetylase (E2):
a^
sh
W SH
+ E3— FAD
f
+ E3— FA D H j
E
2
e
2
