The TCA Cycle

Question 1

Where does the TCA cycle happen?

Answer 1

• Everywhere and in all tissues requiring energy in the fed state
• Except RBC because there are no mitochondria

Question 2

What is the common end product of carbohydrate, fatty acid and amino acid metabolism?

Answer 2

Acetyl CoA

Question 3

What is acetyl CoA used for the biosynthesis of in the cytoplasm?

Answer 3

• Fatty acids
• Ketone bodies (only in the liver)
• Cholesterol

Question 4

Where can it be stored for use in the TCA cycle and why?

Answer 4

• Mitochondrial matrix

- Too big to cross inner mitochondrial matrix

Question 5

What is acetyl CoA converted to in the TCA cycle (step 1)?

Answer 5

• Citrate (6 carbons)
• Combines with oxaloacetate
• Condensation reaction
• Citrate synthase

Question 6

What is released in step 1 of TCA cycle?

Answer 6

• Water

- CoA

Question 7

What is citrate converted to in TCA cycle (step 2)?

Answer 7

• Isocitrate (6 carbons)

- Slight modification of the structure

Question 8

What is isocitrate converted to in TCA cycle (step 3)?

Answer 8

• α-ketoglutarate
• 5 carbon molecule
• Enzyme: isocitrate dehydrogenase

Question 9

What is released in step 3 of TCA cycle (isocitrate–> α-ketoglutarate)?

Answer 9

• NADH (reduced from NAD+)

- One carbon dioxide molecule

Question 10

What inhibits isocitrate dehydrogenase?

Answer 10

• High levels of NADH or ATP in the cell
• Slows down turning of TCA cycle
• When active it donates electrons from isocitrate to NAD

Question 11

What is α-ketoglutarate converted to in TCA cycle (step 4)?

Answer 11

• Succinyl CoA
• 4 carbon molecule
• CoA temporarily re-introduced
• Enzyme: α-ketoglutarate dehydrogenase

Question 12

What is released in step 4 of the TCA cycle (α-ketoglutarate –> succinyl CoA)?

Answer 12

• NADH (reduced from NAD)

- Carbon dioxide molecule

Question 13

What is succinyl CoA converted to in TCA cycle (step 5)?

Answer 13

• Succinate
• CoA quickly lost
• No enzyme

Question 14

What is released in step 5 of the TCA cycle (succinyl CoA–> succinate)?

Answer 14

• CoA

- GTP (GDP + Pi)

Question 15

What is succinate converted to in TCA cycle (step 6)?

Answer 15

• Fumarate
• 4 carbon molecule
• Enzyme: succinate dehydrogenase

Question 16

What is released in step 6 of the TCA cycle (succinate to fumarate)?

Answer 16

• FADH (reduced from FAD)

Question 17

What is fumarate converted to in TCA cycle (step 7)?

Answer 17

• Malate- 4 carbons
• Enzyme: Fumarase
• Fumarate + water

Question 18

What is malate converted to in the TCA cycle (step 8)?

Answer 18

• Oxaloacetate
• 4 carbon
• Enzyme: malate dehydrogenase

Question 19

What is released in step 8 of TCA cycle (malate to oxaloacetae)?

Answer 19

• NADH (Reduced from NAD+)

Question 20

What happens to all the NADH/FADH molecules released from TCA cycle?

Answer 20

• Become electron donors in the electron transport chain
• One molecule of FADH2
• 3 molecules of NADH (each accepting 2 electrons)

Question 21

What are the two major flavin redox coenzymes?

Answer 21

• Flavin adenine dinucleotide (FAD)

- Flavin mononucleotide (FMN)

Question 22

How many electrons and protons do FAD and FMN accept?

Answer 22

• 2 electrons
• 2 protons
• For molecules undergoing metabolism

Question 23

What vitamin do FAD and FMN both contain

Answer 23

• Riboflavin-5’-phosphate

- Produced from vitamin B2 (riboflavin)

Question 24

What is meant by ‘anaplerotic reactions’?

Answer 24

• Filling up reactions

- Intermediates of the TCA cycle solely for use of the TCA cycle

Question 25

What other reactions is oxaloacetate used for?

Answer 25

- Amino acids synthesis (non-essential ones as they can be formed in the body)

Question 26

What other reactions is α-ketoglutarate used for?

Answer 26

- Amino acid synthesis | - Neurotransmitter synthesis in the brain

Question 27

What other reaction is malate used for?

Answer 27

- Gluconeogenesis

Question 28

What other reaction is succinyl CoA used for?

Answer 28

- Haem synthesis

Question 29

What other reaction is citrate used for?

Answer 29

- Fatty acid synthesis

Question 30

What are glucogenic and ketogenic type amino acids?

Answer 30

- Able to join the TCA cycle at different points | - Amino acids can go into TCA cycle and help replenish intermediates- they can be converted to convenient molecules

Question 31

What processes are glycogenic and ketogenic amino acids used for when removed from TCA cycle?

Answer 31

- Gluconeogenesis | - Ketogenesis (by forming acetyl CoA)

Question 32

What does the 'link reaction' convert pyruvate into?

Answer 32

Acetyl CoA

Question 33

How can pyruvate be directly inserted into the TCA cycle?

Answer 33

- Converted directly into oxaloacetate - Requires some energy - Pyruvate carboxylase is used for this reaction

Question 34

Describe the structure of ATP?

Answer 34

- Adenine - Ribose sugar - 3 phosphate groups

Question 35

Describe stage 1 of fuel oxidation

Answer 35

- Production of reduced nucleotide coenzymes during oxidation of fuels and the release of carbon dioxide - Reduced enzymes include NADH and FADH2

Question 36

Describe stage 2 of fuel oxidation

Answer 36

- When electron carriers are passing electrons into electron transport chain - Oxygen required - ATP generated from free energy provided by oxidation of reduced coenzymes