Biochemistry: Citric Acid Cycle

Where does the citric acid cycle take place?

Mitochondria

How does pyruvate gain access to the mitochondrial matrix?

• Via the pyruvate-H+ symport

• Uses the electrochemical gradient created by the ETC

What are the advantages of using a multienzyme complex (pyruvate dehydrogenase) to convert pyruvate to acetyl-CoA?

• Diffusion distance

• Minimize side reactions

• Coordinated control of reactions

What are the 5 coenzymes of the pyruvate dehydrogenase complex?

• Thiamine pyrophosphate (TPP)

• Lipoic Acid

• Coenzyme A (CoA)

• Flavin adenine dinucleotide (FAD)

• Nicotinamide adenine dinucleotide (NAD+)

What is the function of TPP?

Decarboxylates pyruvate, yielding a hydroxyethyl-TPP carbanion

What is the function of lipoic acid?

Accepts the hydroxyethyl carbanion from TPP as an acetyl group

What is the function of CoA?

Accepts the acetyl group from lipoamide

What is the function of FAD?

• An electron carrier

• Reduced by lipoamide

What is the function of NAD+?

• An electron carrier

• Reduced by FADH2

Pyruvate => Acetyl-CoA

Step 1

Catalyzing Acetyl-CoA

Step 1: Substrate

• 2 Acetyl-CoA

• 2 Oxaloacetate

• 2 H2O

Step 1: Product

• 2 Citrate

• 2 CoASH

Step 1: Enzyme

Citrate Synthase

For step 1 of the reaction, why is there a high amount of energy being used? (-32.2kJ/mol)

Allows for the condensation to take place even when the concentration of oxaloacetate is low

Step 2

Isomerization of Citrate

Step 2: Substrate

2 Citrate

Step 2: Product

2 Isocitrate

Step 2: Enzyme

Aconitase

Step 2: Purpose of chemical reaction

Isomerization facilitates future oxidation

Step 3

Oxidative Decarboxylation

Step 3: Substrate

• 2 Isocitrate

• 2 NAD+

• 2 H+

Step 3: Product

• 2 a-Ketoglutarate

• 2 H+

• 2 NADH

• 2 CO2

Step 3: Cofactors

• Mg2+ or Mn2+

• NAD+

Step 3: Purpose for chemical reaction

Serves as an important site for regulation

Step 4

Oxidative Decarboxylation

Step 4: Substrate

• 2 a-Ketoglutarae

• 2 CoASH

• 2 NAD+

Step 4: Product

• 2 Succinyl-CoA

• 2 CO2

• 2 NADH

• 2 H+

Step 5

Generation of GTP

Step 5: Substrate

• 2 Succinyl-CoA

• 2 Pi

• 2 ADP or GDP (primarily GDP)

Step 5: Product

• 2 Succinate

• 2 GTP

• 2 CoASH

Step 5: Enzyme

Succinyl-CoA synthase

Step 6

Catalyzation of Succinate

Step 6: Substrate

• 2 Succinate

• 2 E-FAD

Step 6: Product

• 2 Fumarate

• 2 E-FADH2

Step 6: Enzyme

Succinate Dehydrogenase

Step 7

Hydration of Fumarate

Step 7: Substrate

• 2 Fumarate

• 2 H2O

Step 7: Product

2 Malate

Step 7: Enzyme

Fumarase

Step 8

Catalyzing Malate

Step 8: Substrate

• 2 Malate

• 2 NAD+

Step 8: Product

• 2 Oxaloacetate

• 2 NADH

• 2 H+

Step 8: Enzyme

Malate Dehydrogenase

What are the regulatory steps for the citric acid cycle? How are they controlled?

• Step 1 (substrate concentration)

• Step 3 (product inhibition)

• Step 4 (feedback inhibition)

Step 1: Importance of the reaction

To facilitate the oxidation of the carbons coming from Acetyl-CoA

Step 2: Importance of the reaction

It takes the tertiary alcohol and rearranges it to a secondary alcohol, which is more reactive, to prepare for the oxidation of the secondary alcohol in step 3

Step 3: Importance of the reaction

Oxidation of a carbon to make NADH

Step 4: Importance of the reaction

Generate NADH for the ETC

Step 5: Importance of the reaction

To generate the only 2 GTP to do work in other pathways

Step 6: Importance of the reaction

Prepares the molecule for the nucleophilic addition of water

Step 7: Importance of the reaction

The -OH will become the ketone in oxaloacetate

Step 8: Importance of the reaction

Regenerate oxaloacetate