SURVEY OF BIOCHEMISTRY
Citric Acid Cycle
1
PRS
• Which substrates in the TCA cycle are
coupled to the production of CO2?
1.
2.
3.
4.
Isocitrate and alpha-ketoglutarate
Citrate and alpha-ketoglutarate
Oxaloacetate and citrate
Succinate and malate
2
PRS Answer
Isocitrate
and
Alpha-ketoglutarate
3
PRS
• In the net TCA cycle rxn, what are
the products?
1.
2.
3.
4.
3
3
3
3
NADH + FAD + GDP + 2CO2 + CoA
NADH + FAD + GTP + 2CO2 + CoA
NAD+ + FADH2 + GTP + 2CO2 + Acetyl-CoA
NADH + FADH2 + GTP + 2CO2 + CoA
4
PRS - Answer
• In the net TCA cycle rxn, what are
the products?
1.
2.
3.
4.
3 NADH + FAD + GDP + 2CO2 + CoA
3 NADH + FAD + GTP + 2CO2 + CoA
3 NAD+ + FADH2 + GTP + 2CO2 + Acetyl-CoA
3 NADH + FADH2 + GTP + 2CO2 + CoA
5
PRS
In what form does carbon enter the TCA
cycle?
1.Pyruvate
2.Glucose
3.CO2
4.Acetyl CoA
6
PRS
• Which enzyme catalyzes the
reversible isomerization of citrate
and isocitrate?
1.
2.
3.
4.
Citrate synthase
Aconitase
Triose Phosphate Isomerase
Isocitrate Dehydrogenase
7
TCA Cycle Overview
CO2
Pyruvate
Acetyl CoA
NAD+
NADH
Oxaloacetate
L-Malate
Citrate
3 NAD+ + FAD + GDP + Pi + Acetyl-CoA
3NADH + FADH2 + GTP + 2 CO2
Fumarate
Succinate
Isocitrate
AlphaKetoglutarate
Succinyl-CoA
8
Pyruvate Dehydrogenase
CO2
Pyruvate
NAD+
Acetyl CoA
NADH
• Multienzyme complex: E1, E2, E3
• Oxidative decarboxylation
• Requires 5 cofactors
– CoA
– NAD+
– FAD
– Lipoic Acid
– Thiamine Pyrophosphate
9
Pyruvate Dehydrogenase
E1 Cleaves off CO2
+
10
Pyruvate Dehydrogenase
E2 Catalyzes Acetyl-CoA Formation
11
Pyruvate Dehydrogenase
E2 Catalyzes Acetyl-CoA Formation
Acetyl-CoA is formed, but the oxidized form of the lipoic acid cofactor must be
regenerated.
12
Pyruvate Dehydrogenase
E3 Regenerates the Disulfide
Notice the redox chemistry here:
The lipoic acid group of E2 has been regenerated.
Now the oxidized form of E3 needs to be regenerated. How does this happen?
13
Pyruvate Dehydrogenase
E3 Produces NADH
Now the oxidized form of E3 needs to be regenerated. How does this happen?
14
Citrate Synthase
15
Aconitase
16
Isocitrate Dehydrogenase
17
Alpha-Ketoglutarate
Dehydrogenase
Notice the similarities between this reaction and that of
pyruvate dehydrogenase.
18
Alpha-Ketoglutarate
Dehydrogenase
Notice the similarities between this reaction and that of
pyruvate dehydrogenase.
Both Pyruvate and alpha-ketoglutarate are alpha-keto
acids.
19
Alpha-Ketoglutarate
Dehydrogenase
Notice the similarities between this reaction and that of
the pyruvate dehydrogenase complex.
Both rxns produce CO2 and NADH and H+.
Both rxns use CoA as a cofactor.
20
Succinyl-CoA Synthetase
21
Succinate Dehydrogenase
Rxn is strongly
inhibited by malonate
By what mode of inhibition
would you expect malonate
to work?
22
Fumarase
23
Malate Dehydrogenase
+
24
Malate Dehydrogenase
25
Regulation of the TCA Cycle
Excess Products
1.
Prohibit entry by building
excess Acetyl-CoA and
NADH
2.
Deactivate pyruvate
dehydrogenase
26
Regulation of the TCA Cycle
Inactivate Pyruvate Dehydrogenase
27
Controlling Enzymes
in the TCA Cycle
Citrate Synthase
Isocitrate
Dehydrogenase
Alpha-Ketoglutarate
Dehydrogenase
28
Coming Up
• Wed: Cover Ch. 18
• Thur: Exam #3
– Chapter 15: Glycolysis
– Chapter 16: Glycogen Metabolism
– Chapter 17: TCA Cycle
– Chapter 18: Electron Transport and
Oxidative Phosphorylation
29