S i l Topic:
Special
T i Carbohydrate/Lipid
C b h d t /Li id D
Dysregulation-6252
l ti 6252
醣類與脂肪代謝特論-6252
Regulation
g
of TCA Cycle
y
授課教師: 陳洵一,
授課教師
陳洵一 動物科學系
Instructor: Chen Shuen-Ei, Dept. Animal Sci.
2 CO2
released
No CO2
N
release
(TCA)
Summary of Reactions
Substrate or Cofactor
Energy
Citrate synthase
Acetyl CoA, OAA
Aconitase
(± H2O)
Isocitrate dehydrogenase
releases CO2
NAD+
3 ATP
α-Ketoglutarate dehydrogenase
NAD+
3 ATP
Succinyl-CoA synthetase
GDP, Pi
1 ATP
S i t dehydrogenase
Succinate
d h d
FAD
2 ATP
Fumarase
(H2O)
Malate dehydrogenase
NAD+
Feature
releases CO2
GTP
3 ATP
TOTAL
12 ATP
GTP
Table 1. Summary of Enzymes and Specific cofactor or products in the Krebs Cycle
Cit t + 3NAD+ + FAD + GDP + Pi + H2O
Citrate
[1]
Oxaloacetate + 3NADH + 3H+ + FADH2 + 2CO2 + GTP
6
4
2
9 ATPs
2 ATPs
1 ATP
Where do we get all that energy?
Citrate + 3NAD+ + FAD + GDP + Pi + H2O
[1]
+
Oxaloacetate + 3NADH + 3H + FADH2 + 2CO2 + GTP
1. How energetic is citrate?
6C
Carbons
b
C l Turns
T
= 3 Cycle
= 3 x 12 ATP per cycle
= 36 ATPs
A
2. How energetic is oxaloacetate
4 Carbons = 2 Cycle turns
to Citrate
= 24 ATPs
3. How energetic
g
is malate
4 Carbons = 2 Cycle turns + 1 NADH= 27 ATPs
Regulation of the Kreb’s Cycle
Pyruvate Dehydrogenase complex
Pyruvate + TPP → Acetal-TPP + CO2
Acetal-TPP + S-S → Ac-S ^ SH + TPP
Ac-S ^ SH + HS-CoA → AcS-CoA + HS ^ SH
HS ^ SH + FAD → S-S + FADH2
FADH2 + NAD+ → FAD + NADH + H+
Pyruvate
y uvate + HS-CoA
S Co + N
NAD+ → Acetyl-CoA
cety Co + N
NADH + H+
Regulators-Activators
Regulators- Inhibitors
and AMP
Fatty acids and ATP
Key Regulatory Points:
Pyruvate dehydrogenase Complex
Inhibited by NADH and Acetyl-CoA
NADH
[NAD+]
Acetyl-CoA
HS CoA
HS-CoA
High NADH means that the cell is experiencing a
surplus
l off oxidative
id i substrates
b
and
d should
h ld not produce
d
more. Carbon flow should be redirected towards synthesis.
High Acetyl-CoA means that carbon flow into the Krebs
cycle
l is
i abundant
b d t and
d should
h ld b
be shut
h td
down and
d rechanneled
h
l d
towards biosynthesis
(TCA)
Mechanism:
Text p621
1. Competitive Inhibition
NADH and acetyl-CoA reverse the pyruvate dehydrogenase
reaction by competing with NAD+ and HS-CoA
2. Covalent Modification (second level regulation)
E-11 subunits of PDH complex is subject to phosphorylation
E
TPP
Active
FAD
HPO4=
1
2
Insulin
3
E1-OH
PDH
phosphatase
H2O
ATP
PDH
kinase
E1-OPO
OPO3
Inactive
ADP
Epinephrine
Glucagon
Cyclic-AMP
Cyclic
AMP
protein kinase
ATP
Regulation of Cycle Enzymes
Enzyme
Citrate Synthase
Aconitase
Isocitrate dehydrogenase
α-Ketoglutarate dehydrogenase
Succinyl-CoA Synthase
Succinate dehydrogenase
Fumarase
Malate dehydrogenase
y g
ΔGo’
(kJ/mol)
-31.5
~5
-21
-33
-2.1
+6
-3.4
+29.7
All regulatory enzymes occur in the first half of the cycle
Regulation of the Citric Acid Cycle
Primary modes:
1. Substrate availability (key enzymes are subsaturated)
Allostery is not a primary mode
2. Product inhibition
3. Feedback inhibition (competitive)
Key regulators:
1 Acetyl-CoA (controls citrate synthase)
1.
2. OAA (controls citrate synthase, regulated by NADH)
3. NADH (controls citrate synthase, isocitrate dehydrogenase
4 Calcium
4.
C l i
(stimulates
( ti l t NADH production)
d ti )
Equilibria to Consider
O2 consumption
NADH oxidation
id ti
ATP production
Tightly coupled: affect one is to affect all
Malate + NAD+
Controls NADH and is
controlled by
NADH
OAA + NADH
A
A working muscle will increase
respiration and oxidize NADH. This
stimulates
i l
OAA synthesis
h i which
hi h
stimulates citrate synthase and
isocitrate dehydrogenase reactions
reactions.
Malate + NAD+
OAA + NADH
Substrate limited
[OAA][NADH]
K =
[Malate][NAD+]
Citrate
K [Malate][NAD+]
[OAA][NADH]
Isocitrate dehydrogenase
α-Kg dehydrogenase
Respiration (O2)
Respiration Increases
P 621
Pyruvate Dehydrogenase
Citrate Synthase
No
Regulation
Isocitrate
Dehydrogenase
α-Ketoglutarate
Dehydrogenase