Download The Complete Citric Acid Cycle - Part 2

The Complete Citric Acid Cycle - Part 2
©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter
. http://www.essentialcellbiology.com
Published by Garland Publishing, a member of the Taylor & Francis Group.
COO–
Step 3
In the first of
four oxidation steps in the
cycle, the carbon carrying
the hydroxyl group is
converted to a carbonyl
group. The immediate
product is unstable, losing
CO2 while still bound to
the enzyme.
H
C
H
H
C
COO–
C
H
HO
COO–
isocitrate
dehydrogenase
NAD+ NADH + H
COO–
C
H
H
C
H
H
C
COO–
H
C
H
C
O
C
O
H
COO–
+
a-ketoglutarate
C
H
H
C
H
C
O
COO–
a-ketoglutarate dehydrogenase complex
+ HS CoA
NAD+
COO–
NADH + H
C
H
H
C
H
C
succinyl-CoA synthetase
C
H
H
C
H
COO–
H 2O
GDP
Pi
GTP
succinate
COO–
H
C
H
C
H
COO–
COO–
succinate dehydrogenase
C
H
FAD
COO–
Step 7
C
H
COO–
fumarase
HO
C
H
COO–
C
H
C
H
COO–
H 2O
fumarate
Step 8
C
fumarate
H
COO–
malate
COO–
malate dehydrogenase
HO
C
H
C
H
C
H
CH2
COO–
malate
H
COO–
FADH2
succinate
In the last of four
oxidation steps in the cycle,
the carbon carrying the
hydroxyl group is converted
to a carbonyl group,
regenerating the oxaloacetate
needed for step 1.
H
O
H
H
C
O
H
COO–
S CoA
Step 6
C
succinyl-CoA
COO–
H
C
H
S CoA
CO2
a-ketoglutarate
H
+
succinyl-CoA
The addition of
water to fumarate places a
hydroxyl group next to a
carbonyl carbon.
COO–
oxalosuccinate
intermediate
H
Step 5
In the third
oxidation step in the cycle,
FAD removes two hydrogen
atoms from succinate.
CO2
COO–
Step 4
A phosphate
molecule from solution
displaces the CoA, forming a
high-energy phosphate
linkage to succinate. This
phosphate is then passed to
GDP to form GTP. (In bacteria
and plants, ATP is formed
instead.)
H
+
isocitrate
The aketoglutarate dehydrogenase
complex closely resembles the
large enzyme complex that
converts pyruvate to acetyl CoA
(pyruvate dehydrogenase). It
likewise catalyzes an oxidation
that produces NADH, CO2, and a
high-energy thioester bond to
coenzyme A (CoA).
COO–
NAD+
NADH + H
+
O
COO–
oxaloacetate
+ HS CoA