The Krebs Tricarboxylic Acid Cycle

The Final Common Pathway of Oxidative Metabolism

9/24/07

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⑥

Liver

①

④

②

Gluconeogenesis; 1 Liver, Kidney

e-

Ox phos ③

Citric Acid Cycle (CAC)“Kreb Cycle”

Tricarboxylic Acid Cycle

2/3 of O2 consumption needed foroxidation 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:

NADHFADH

Pyruvate

Pyruvate DehydrogenaseComplex “links”glycolysis to CAC

+ 2eˉ

+ 2eˉ

GTP ATP (substrate levelphosphorylation)

+ 2eˉ

+ 2eˉ

The Three Stages of Metabolism

The Krebs CycleCitric 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

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.

Overview of the Krebs Cycle: A Mitochondrial Process

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?

Pyruvate Dehydrogenase Complex

Multimolecular aggregate3 Enzymes5 Coenzymes

5 Reactions

CoA contains thevitamin Pantothenic acid

Product Inhibition

CoenzymesThiamine Pyrophosphate (TTP) B1NAD+

FAD+

CoA

Lipoic acid

Mitochondrial matrix

Cytoplasm Pyruvate

* Pyruvate transporter

* Pyruvate mito matrix

Irreversible

Links glycolysis to CAC

PDH Deficiency – results in Congenital Lactic AcidosisPyruvate cannot enter the CAC and results in ↑ Lactic AcidPrimarily affects the brain – neonatal death3 Forms – psychomotor retardation√ Possible treatment is ketogenic diet:

Low in CHOHigh in fats

Produces ketone bodies as an alternate form of energy for the brain

CAC

Arsenic Poisoning – Pyruvate Dehydrogenase – -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

√ Allosteric Regulation

√ AllostericRegulation

cAMP independent

Skeletal muscleContraction

Fluorocitrate OAA

( ¯ )

FluoroacetateFluroacetyl CoA

Rat poison

ADP (+)

One of the rate limiting Rxsof the CAC

ATP

Carrier

Aldo condensation

The entrance of acetyl CoA doesnot ↑ or ↓ intermediates in the CAC

Oxidative decarboxylatione¯Irreversible (1)

Isomerization

NADH ( - )

Oxidative decarboxylationVery similar to the Pyruvate Dehydrogenase complex

Irreversible (2)ATP. GTPSuccinyl CoANADH

( ¯ )

e¯

From oxidation ofodd number FAs

ADP GDPATP

Nucleoside BiphosphateKinase

“Substrate LevelPhosphorylation”

e¯ Oxidation reaction

Hydration reaction

e¯( 4 )

Reversible oxidation reaction

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 -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

Summary

Regulation of the CACDependent on the energy state ofthe cell which is reflected by[ADP] [Pi]

[ATP]ratio

This ratio determines the rate ofoxidative phosphorylation

Named “Respiratory Control”of energy production because oxidationand phosphorylation of ADP must occur simultaneously

Electrochemical gradientOxidized Reduced

1

2 3

4

6

75

8

2 Shuttle systems to bring cytosolic NADH intomitochondria for oxidative phosphorylation

1) Glycerophosphate shuttle = 36 ATP

2) Malate-aspartate shuttle = 38 ATP

Count ATPs: Anerobic glycolysis = 2 Glycolysis + CAC + oxidative phosphorylation = 38NADH FADH2 ATP

1 Glycolysis 2

2 Glycolysis (G-3-P 1,3,BisP) 2 6

3 Pyruvate Acetyl CoA 2 6

4, 5, 6 CAC 6 18

7 CAC-FADH2 2 4

8 CAC – substrate level ATP 2

Total 38