1. The Krebs Tricarboxylic Acid Cycle
The Final Common Pathway of Oxidative Metabolism
9/24/07

2. ⑥ ⑤
 Liver ① ④
Gluconeogenesis; 1 Liver, Kidney ② e-
Ox phos ③

3. Citric Acid Cycle (CAC) Tricarboxylic Acid Cycle
“Kreb Cycle”
Tricarboxylic Acid Cycle
2/3 of O2 consumption needed for
oxidation of Acetyl CoA  CO2
Occurs exclusively in the
mitochondrion (matrix)
OAA acts as carrier or acceptor
of acetyl CoA units – is regenerated
“Burns” acetyl CoA to CO2 – during
this oxidation eˉs from acetyl CoA
are trapped in the form of:
Pyruvate
NADH
Pyruvate Dehydrogenase
Complex “links”
glycolysis to CAC
FADH
+ 2eˉ
+ 2eˉ
+ 2eˉ
+ 2eˉ
GTP  ATP (substrate level
phosphorylation)

4. The Three Stages of Metabolism

5. Citric Acid Cycle; The TCA Cycle
The Krebs Cycle
Citric Acid Cycle; The TCA Cycle
Pyruvate (actually the acetyl group) from glycolysis is degraded to CO2
The acetyl group is formed in stage II of metabolism from carbohydrate and amino acid metabolism
1GTP (ATP in bacteria) and 1 FADH2 is produced during one turn of the cycle
3 NADH are produced during one turn of the cycle
NADH and FADH2 energize electron transport and oxidative phosphorylation
Eight reactions make up the Krebs cycle

6. The Chemical Logic of the Krebs Cycle
After condensing acetate with oxaloacetate to form citrate – oxidation yields CO2, oxaloacetate is regenerated, and the energy is captured as NADH, FADH2, and GTP (ATP)
Acetyl-CoA is called the stoichiometric substrate; it is consumed in large amounts
Oxaloacetate is called the regenerating substrate; it is continuously regenerated (it is not consumed)
The cycle is catalytic; oxaloacetate is consumed and then regenerated.

7. Overview of the Krebs Cycle: A Mitochondrial Process

8. Anatomy of the Mitochondrion
Which membrane is impermeable to protons and other ions?
Which membrane will allow for the transport of molecules up to a molecular weight of about 1000?

9. Pyruvate Dehydrogenase Complex Multimolecular aggregate
Cytoplasm Pyruvate
* Pyruvate transporter
Multimolecular aggregate
3 Enzymes
5 Coenzymes
* Pyruvate mito matrix
5 Reactions
Irreversible
CoA contains the
vitamin Pantothenic acid
Links glycolysis to CAC
Product Inhibition
Mitochondrial matrix
Coenzymes
Thiamine Pyrophosphate (TTP) B1
NAD+
FAD+
CoA
Lipoic acid

10. CAC √ Possible treatment is ketogenic diet:
PDH Deficiency – results in Congenital Lactic Acidosis
Pyruvate cannot enter the CAC and results in ↑ Lactic Acid
Primarily affects the brain – neonatal death
3 Forms – psychomotor retardation
√ Possible treatment is ketogenic diet:
Low in CHO
CAC
High in fats
Produces ketone bodies as an alternate form of energy for the brain
Arsenic Poisoning – Pyruvate Dehydrogenase
– a-Ketoglutarate Dehydrogenase
Both require lipoic acid as a cofactor
Arsenite – Trivalent form of arsenic
I° – Forms a stable complex with the thiol (-SH) group of Lipoic Acid
II° – Glyceraldehyde 3-PO4 step forms complex with inorganic Pi thus prevents ATP
formation in glycolysis
Affects the brain – Death, neurologic problems

11. √ Allosteric Regulation
Skeletal muscle
Contraction
cAMP
independent
√ Allosteric Regulation
√ Allosteric
Regulation

12. Carrier
Aldo condensation
The entrance of acetyl CoA does
not ↑ or ↓ intermediates in the CAC
Fluorocitrate
OAA
Fluoroacetate
Fluroacetyl CoA
Rat poison
( ¯ )
Isomerization
Oxidative decarboxylation
ADP (+) e¯
One of the rate limiting Rxs
of the CAC
Irreversible (1)
ATP
NADH ( - )

13. Nucleoside Biphosphate
Oxidative decarboxylation
Very similar to the
Pyruvate Dehydrogenase complex
Irreversible (2)
ATP. GTP
Succinyl CoA
NADH
( ¯ ) e¯
From oxidation of
odd number FAs
ADP
GDP
ATP
“Substrate Level
Phosphorylation”
Nucleoside Biphosphate
Kinase e¯
Oxidation reaction
Hydration reaction

14. Reversible oxidation reaction
( 4 )

16. Main Points of the Krebs Cycle
Occurs in mitochondrion
All enzymes are hydrophilic and occur in the matrix except for succinate dehydrogenase, which occurs in the inner mitochondrial membrane
Citrate synthase, Isocitrate dehydrogenase and a-ketoglutarate dehydrogenase are the three irreversible reactions
ICD is the main regulatory enzyme, and it is activated by ADP
Succinate dehydrogenase is inhibited by malonate and oxaloacetate

17. Dependent on the energy state of the cell which is reflected by
Summary
Regulation of the CAC
Dependent on the energy state of
the cell which is reflected by
[ADP] [Pi]
[ATP]
ratio
This ratio determines the rate of
oxidative phosphorylation
Named “Respiratory Control”
of energy production because oxidation
and phosphorylation of ADP must
occur simultaneously

18. Electrochemical gradient
Oxidized
Reduced

19. 1 2 3 6 4 7 5
2 Shuttle systems to bring cytosolic NADH into
mitochondria for oxidative phosphorylation
1) Glycerophosphate shuttle = 36 ATP 8
2) Malate-aspartate shuttle = 38 ATP
Count ATPs: Anerobic glycolysis = Glycolysis + CAC + oxidative phosphorylation = 38
NADH
FADH2
ATP 1
Glycolysis 2
Glycolysis (G-3-P  1,3,BisP) 6 3
Pyruvate  Acetyl CoA
4, 5, 6
CAC 18 7
CAC-FADH2 4 8
CAC – substrate level ATP
Total 38