KREB’S CYCLE (TRICARBOXYLIC ACID

(TCA)CYCLE/CITRIC ACID CYCLE)

BIOCHEMISTRY LEC

WEEK 3

METABOLIC PATHWAYS

• A series of consecutive biochemical reactions catalyzed by enzymes that produce a specific end product.

• Catabolism- the breakdown of food stuffs to simple organic chemicals.

• Anabolism- the synthesis of biomolecules from simple organic chemicals.

METABOLIC PATHWAYS

• Very Few metabolites are used to synthesize a large variety of biomolecules:

• Acetyl-Coenzyme A (acetyl-CoA)

• Pyruvate

• Citrate acid cycle intermediates

METABOLIC PATHWAYS

• Three main pathways for energy production:

• Glycolysis

• Citric acid cycle

• Oxidative-Phosphorylation

METABOLIC PATHWAYS

• Certain pathways are involved in both breakdown and buildup of molecules these pathways are called amphibolic. The citric acid cycle is an example of this.

OVERALL GOAL/IMPORTANCE

• Makes ATP

• Makes NADH

• Makes FADH2

• A cyclical series of biochemical reactions that is fundamental to the

metabolism of aerobic organisms, i.e. animals, plants, and many

microorganisms

Geography

• Krebs in mitochondrial matrix• Mitochondrion

– Outer membrane very permeable• Space between membranes called intermembrane space

– Inner membrane (cristae)• Permeable to pyruvate,• Impermeable to fatty acids, NAD, etc

– Matrix is inside inner membrane

The citric acid cycle enzymes are found in the matrix of the mitochondria

OVERVIEW• Described by Hans Adolf Krebs in 1937

• A feature of cell chemistry shared by all types of life.

• A complex series of reactions beginning and ending with the compound oxaloacetate.

• The cycle produces carbon dioxide and the energy-rich compound ATP.

OVERVIEW

• Eight successive reaction steps.

• The six carbon citrate is formed from two carbon acetyl-CoA and four carbon oxaloacetate.

• Oxidation of citrate yields CO2 and regenerates oxaloacetate.

• The energy released is captured in the reduced coenzymes NADH and FADH2.

Conversion of pyruvate to Acetyl CoA

CH3

O

O

O

pyruvate

CO2HSCoA

CH3 SCoA

O

acetyl CoA

NADHNAD+

pyruvate dehydrogenase complex

• 2 per glucose (all of Kreb’s)• Oxidative decarboxylation• Makes NADH

Fates of Acetyl CoA

CH3 SCoA

O

acetyl CoA

Kreb's

CO2, ATP, NADH...energy

ketone bodies

no CHO present

TAG's

• In the presence of CHO– Metabolized to CO2, NADH, FADH2,GTP and, ultimately, ATP

• If energy not being used (Lots of ATP present)– Made into fat

• If energy being used, but no CHO present– Starvation– Forms ketone bodies (see fat metabolism slides)– Danger!

Kreb’s CycleCH3 C

O

SCoAacetyl CoA

C O

CH2C

O

COO

O

oxaloacetate

CoASH

citrate synthase

COO

CH2CCH2

C

OH C O

O

O Ocitrate

aconitase

COO

CHCHCH2C

C O

O

OO

OH

isocitrate

NADNADH

CO2

COO

CCH2CH2

COO

O

isocitrate dehydrogenase

alpha ketoglutarate

NADNADH

CoASHCO2

CCH2

CH2C

OO

OSCoA

succinyl CoA

alpha ketoglutaratedehydrogenase

GDP

GTPCoASH

CC

CC

OO

O O

H

H

succinate

succinyl CoAsynthetase

FAD

FADH2succinatedehydrogenase

CCH2

CH2

COO

OO

fumarate

OH2

CCH

CH2

COO

O O

OH

malatefumarase

NADNADH

malatedehydrogenase

Kreb's Cycle

OH2+

1. Citrate Synthase Reaction

acetyl CoAoxaloacetate

CoASH

citrate synthase

citrate

OH2

CH3 C

O

SCoA

C O

CH2C

O

COO

O

COO

CH2CCH2

C

OH C O

O

O O

+

2. Aconitase Reaction

citrate

aconitase

isocitrate

COO

CH2CCH2

C

OH C O

O

O O

COO

CHCHCH2C

C O

O

OO

OH

• Forms isocitrate

3. Isocitrate Dehydrogenase

isocitrate

NAD NADH CO2

isocitrate dehydrogenase

alpha ketoglutarate

COO

CHCHCH2C

C O

O

OO

OH

COO

CCH2CH2

COO

O

• All dehydrogenase reactions make NADH or FADH2

4. α-ketoglutarate dehydrogenase

alpha ketoglutarate

NAD NADHCoASH

CO2

succinyl CoA

alpha ketoglutaratedehydrogenase

COO

CCH2CH2

COO

OCCH2

CH2C

OO

OSCoA

• Same as pyruvate dehydrogenase reaction

5. Succinyl CoA synthetase

succinyl CoA

GDP GTP CoASH

succinate

succinyl CoAsynthetase

CCH2

CH2C

OO

OSCoA

CCH2

CH2

COO

OO

• Coupled to synthesis of GTP– GTP very similar to ATP and interconverted later

6. Succinate dehydrogenase

succinate

FAD FADH2

succinyl CoAdehydrogenase

fumarate

CCH2

CH2

COO

OO

CC

CC

OO

O O

H

H

• Dehydrogenation• Uses FAD

7. Fumarase

fumarate

OH2

malate

fumarase

CC

CC

OO

O O

H

H

CCH

CH2

COO

O

OH

O

• Addition of water to a double bond

8. Malate Dehydrogenase

oxaloacetatemalate

NAD NADH

malatedehydrogenase

CCH

CH2

COO

O

OH

O

C O

CH2C

O

COO

O

• Makes NADH• Regenerates oxaloacetate for another round

Net From Kreb’s

• Oxidative process– 3 NADH– FADH2

– GTP

• X 2 per glucose– 6 NADH– 2 FADH2

– 2 GTP

• All ultimately turned into ATP (oxidative phosphorylation)