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Lecture 30
Pyruvate Oxidation and
the Citric Acid Cycle
Aerobic Fate of Pyruvate: Overview
Proteins
Glycogen
Fats
Glucose
Pyruvate
NADH
Acetyl-CoA
3 NADH
FADH2
Citric acid
(Krebs) cycle
CO2
GTP
CO2
Mitochondria: Site of Oxidations
Pyruvate Oxidation to Acetyl-CoA
Lipoic acid, FAD,
thiamine pyrophosphate
CoASH
CO2
+
NAD
NADH
O
H3C
C
O
C
O
O Pyruvate dehydrogenase
complex
H3C
C
S
CoA
Pyruvate Dehydrogenase Complex
Complex enzyme
– 60 polypeptides of 3 kinds
 In mitocondrion matrix
 Regulated
– Inhibited by NADH and GTP
– Stimulated by insulin

Pyruvate Dehydrogenase Complex

Three enzymes and 5 coenzymes
Enzyme Coenzyme(s) MW
Enz
Enz2
Enz3
TPP
96 kDa
Lipoate, CoA 70 kDa
+
FAD, NAD
56 kDa
Subunits
24
24
12
Total MW: 4.5 x 106!
Also present: regulatory protein kinase, phosphatase
Lipoic Acid: Acyl- and Redox Carrier
S
S
Lipoic
acid
H
N
Lysine
NH
Oxidized form
C
O
CH3
O
SH SH
C
S HS
Reduced form
Acylated form
On enzyme 2
O
Thiamine Pyrophosphate (TPP)
H
NH2
C
CH2 N
N
S
H3C
H3C
N
R1
H3C
O
CH 2 CH 2 O
P
H+
O
C
N
TPP anion
(stable)
S
R2
O
O
P
O
O
Pyruvate Dehydrogenase Mechanism
O
H3C
C
O
C
O
OH
+
H
H 3C
C
O
C
O
C
R1
C
R1 N
N
S
H 3C
H3C
S
R2
R2
Pyruvate Dehydrogenase Mechanism
OH
H 3C
C
O
C
O
C
R1
N
H 3C
S
R2
H3C
CO2
OH
C
C
R1
N
H3C
S
R2
Pyruvate Dehydrogenase Mechanism
H3C
OH
OH
C
H3C
C
R1
N
H3C
C
C
S
Resonance
R2
R1
N
H3C
S
R2
Pyruvate Dehydrogenase Mechanism
OH
OH
H3C
H 3C
C
H+
C
R1
N
H3C
S
R2
C
H
C
R1
N
H 3C
S
R2
Hydroxyethyl-TPP
Pyruvate Dehydrogenase Mechanism
OH
H3C
C
O
H
TPP
E1
S
H3C
S
FAD
E2
E3
C
TPP
E1
HS
S
FAD
E2
E3
Pyruvate Dehydrogenase Mechanism
CoA
SH
Acetyl-CoA
O
O
H3C
C
TPP
E1
HS
H 3C
C
HS
S
CoA
TPP
S
HS
FAD
FAD
E2
E3
E1
E2
E3
Pyruvate Dehydrogenase Mechanism
TPP
E1
HS
S
HS
S
FAD
E2
E3
TPP
E1
FADH2
E2
E3
Pyruvate Dehydrogenase Mechanism
S
TPP
E1
S
NAD+ NADH + H+
S
FADH2
E2
E3
TPP
E1
S
FAD
E2
E3
PDH: The Overall Reaction
CoASH NAD+
O
H3C
C
NADH CO2
O
C
O
Pyruvate dehydrogenase
complex
Acetyl-CoA enter Krebs cycle
 NADH passes e– to O2

H3C
O
C
S
CoA
Krebs Cycle Summary
C2
NADH
C4
C6
NADH
CO2
C5
FADH2
C4
GTP
NADH
CO2
Importance of the Krebs Cycle
1. Central energy-yielding path
2. Point of convergence of catabolism of
fats, CHO, protein
3. Source of precursors for biosynthesis
Citrate Synthase: “Condensing Enzyme”
Acetyl-CoA
O
S
CoA
∆Gº= –32.2 kJ/mol
C
O
CH3
O
C
CoASH
CH2
COO
C
O
CH
OOC
C
OH
CH
COO
COO
Oxaloacetate
(OAA)
Citric Acid
(citrate)
Aconitase
Citrate
H2O cis-aconitate
CH 2COO
CH 2COO
HO
C
COO
C
COO
H
C
COO
C
COO
H
H
Isocitrate CH2COO
H
C
COO
HO
C
COO
H
H2 O
∆Gº =
+13.3 kJ/mol
Isocitrate Dehydrogenase
H2C
COO
NAD(P)+ NAD(P)H + H+
H2C
CH COO
HO
C
H
COO
COO
CH2
CO2
O
C
COO
-Ketoglutarate
Isocitrate
∆Gº= –20.9 kJ/mol
-Ketoglutarate Dehydrogenase Complex
-KG
H2C
Succinyl-CoA
COO
CoASH
CH2
C
CO2
H 2C
COO
O
Compare with
pyruvate DH
CH 2
C
NAD
+
NADH
∆Gº= –33.5 kJ/mol
COO
O
S
CoA
-Ketoglutarate Dehydrogenase Complex
 Cofactors:
FAD, NAD+, lipoate, TPP,
CoASH
 Mechanism  PDH complex
Succinic Thiokinase
Succinyl-CoA
H2C
COO
Succinate
GDP + Pi
CH2
C
O
S
CoA
GTP
H2C
COO
H2C
COO
∆Gº= –2.9 kJ/mol
Succinic Thiokinase

Multistep reaction:
Pi attack
GTP
acyl-phosphate
Enz-his-P
GDP
Enz-his
succinate
Nucleoside Diphosphate Kinase
GTP + ADP
GDP + ATP
∆Gº= 0
Also uses
– CTP
– UTP
 So NTPs are in equilibrium with each
other

Succinate Dehydrogenase
COO
H
C
H
H
C
H
FAD
FADH2 COO
C
H
C
COO
COO
Succinate
Fumarate
∆Gº= 0

H
Enzyme is membrane bound
Fumarase (Fumarate Hydratase)
H2O
COO
C
H
H
C
COO
HO
C
H
H
C
H
COO
COO
Fumarate
L-malate
∆Gº= –3.8 kJ/mol
Malate Dehydrogenase
COO
HO
C
H
H
C
H
NAD+
NADH + H+
COO
O
C
H
C
H
COO
COO
L-malate
Oxaloacetate
∆Gº= +29.7 kJ/mol
[OAA] normally < 10-6 M
Overall Reaction of Krebs Cycle
GDP
O
H3C
GTP
2CO2
C
SCoA
FAD FADH 2
3NAD
+
3NADH
Energry conserved
from oxidation reactions
Fate of NADH, FADH2ATP
ADP + Pi
ATP
NADH
NAD+
O2
Chain of electron
carriers
H2O
FADH 2
Each NADH3 ATP
 Each FADH22 ATP

FAD
Energy Yield from Glucose Oxidation
Glucose
Glycolysis
2 ATP
2 NADH
ATP Equivalents
2
6
2 Pyruvate
2 NADH
6
2 Acetyl-CoA + 2 CO 2
Krebs Cycle
6 NADH
2 FADH 2
2 GTP
4 CO2
18
4
2
38 ATP
Citric Acid Cycle: Metabolic Hub
ala, ser, cys, gly, thr
ala, lys, ile, val, leu
Pyruvate
phe, tyr, leu,
lys, trp, ile
asp
thr, met
Acetyl-CoA
OAA
Citrate
Krebs
cycle
phe, tyr
lipids
Fumarate
-KG
glu, pro, arg
glu, arg, his, pro
Succinate
ile, val, met met, lys porphyrins
heme, chlorophyll
Regulation of the
Citric Acid Cycle
Krebs Cycle in Motion
QuickTime™ and a
Microsof t Video 1 decompressor
are needed to see this picture.
Jon Maber
Dept of Biochemistry and Molecular Biology
The University of Leeds, UK
http://bmbwww.leeds.ac.uk/designs/tcasteps