Essential Cell Biology 5th edition

442
PANEL 13–2 THE COMPLETE CITRIC ACID CYCLE
Overview of the complete citric acid cycle.
pyruvate
NAD + NADH + H +
acetyl CoA 2C
The two carbons from acetyl CoA that
enter this turn of the cycle (shadowed in
O
O
red ) will be converted to CO 2
in
CH 3 C
CH 3 S CoA
COO – C
subsequent turns of the cycle: the two
carbons in the starting oxaloacetate
CO 2 HS CoA
(shadowed in blue ) will be converted to
coenzyme A
HS CoA
COO –
CO 2
in this cycle.
H 2 O
next cycle C O
COO –
CH CH
H + 2
2
NADH +
HO C COO –
COO –
COO – STEP 1
NAD + STEP 2
oxaloacetate 4C
CH 2
C O
COO –
COO –
CH 2
COO – STEP 8 CH 2
citrate 6C
COO –
HC COO –
H C OH
oxaloacetate 4C
isocitrate 6C HO CH
CH
COO –
2 malate 4C
COO –
CITRIC ACID CYCLE
NAD +
STEP 3
H 2 O
COO –
NADH + H +
fumarate
STEP 7 COO –
4C
α-ketoglutarate 5C
CH 2
CO 2
succinyl CoA 4C
CH
CH
2
succinate
C O
CH
COO –
4C
COO – STEP 4
STEP 6
H 2 O
COO –
COO –
CH 2
CH 2
STEP 5
CH 2
CH 2
NAD +
COO –
C O
FADH 2
HS CoA
FAD
GDP S CoA
+
GTP
NADH H +
+
HS CoA P
CO 2
Details of these eight steps are shown below. In this part of the panel, for each step, the part of the molecule that undergoes
a change is shadowed in blue, and the name of the enzyme that catalyzes the reaction is in a yellow box.
To watch a video of the reactions of the citric acid cycle, see Movie 13.2.
STEP 1
After the enzyme removes
a proton from the CH 3
group on acetyl CoA, the
negatively charged CH 2
–
forms a bond to a carbonyl
carbon of oxaloacetate.
The subsequent loss of the
coenzyme A (HS–CoA) by
hydrolysis drives the
reaction strongly forward.
O
COO –
O C S CoA H 2 O COO –
citrate
synthase CH
C S CoA C O
2
CH 2
+
HO C COO –
COO – + HS CoA +
HO C
CH 3 CH 2
COO –
CH 2
CH 2
COO – COO –
acetyl CoA
oxaloacetate
S-citryl-CoA
intermediate
citrate
H +
STEP 2
An isomerization reaction,
in which water is first
removed and then added
back, moves the hydroxyl
group from one carbon
atom to its neighbor.
H
HO
H
COO –
C H
C COO –
C H
COO –
aconitase
H 2 O
H 2 O
H
COO –
C H
C COO –
C H
COO –
H 2 O
H 2 O
H
H
HO
COO –
C H
C COO –
C H
COO –
citrate
cis-aconitate
intermediate
isocitrate