Biochemistry - Citric Acid Cycle

Question 1

Acetyl CoA

Answer 1

• Can use Carbohydrates, Proteins and fats to produce the 2 carbon acetyl portion
• Pyruvate can also be converted to Acetyl CoA by pyruvate dehydrogenase (+CoA):
  
  • 1 CO2
  • 1 NADH + 1nH+
• This happens in the mitochondria - inhibited by ATP, acetyl CoA and NADH

Question 2

CAC Input

Answer 2

Occurs in the mitochondria matrix

- 2 Pyruvate

Question 3

CAC Output

Answer 3

FOR 1 PYRUVATE (for 1 glucose this is 2x)

• 4 NADH + 4 H+
• 1 FADH2
• 1 GTP (can be converted to ATP readily)
• 3 CO2

Question 4

Role of CAC

Answer 4

Produce NADH and FADH2 for oxidative phosphorylation

Question 5

Regulation of CAC

Answer 5

Can monitor ATP utilisation by:

• the phosphorylation state of ATP as indicated by ADP:ATP
• the reduction state of NAD+ as indicated by NAD+/NADH
• Rate limiting enzyme is isocitrate dehydrogenase which is allosterically activated by ADP and inhibited by ATP or NADH (Can also be increased by increased Ca2+)
• Alpha-Ketogluturate dehydrogenase is also a key enzyme which is inhibited by Succinyl CoA, ATP, NADH

Question 6

Coenzymes of CAC

Answer 6

• CoA, NAD+, FAD
• FAD and NAD+ are oxidation reduction coenzymes
• FAD can accept single electrons
  NAD+ accepts 2 electrons in a single step and has more reducing power

Question 7

CAC Step 1

Answer 7

Splitting

• Citrate Synthase
• Acetyl CoA adds 2C acetyl group to oxaloacetate (Step 8) producing citrate

Question 8

CAC Step 2

Answer 8

Rearrangement

• Aconitase (Fe-S, sensitive to super oxides)
• Produce isocitrate by removal of one H2O and addition of another

Question 9

CAC Step 3

Answer 9

Oxidation

• Isocitrate Dehydrogenase
• oxidation to produce Alpha-Ketoglutarate
• 1 NADH + H+
• 1 CO2

Question 10

CAC Step 4

Answer 10

Oxidation

• Alpha-Ketoglutarate dehydrogenase
• oxidation and addition of CoA to produce Succinyl CoA
• 1 NADH + H+
• 1 CO2

Question 11

CAC Step 5

Answer 11

Condensation/Splitting

• Succinyl CoA Synthetase
• CoA replaced by phosphate producing Succinate
• 1 GTP (made into ATP)`

Question 12

CAC Step 6

Answer 12

Oxidation

• Succinate Dehydrogenase (in ETC)
• oxidation to produce Fumarate
• 1 FADH2

Question 13

CAC Step 7

Answer 13

Hydration

• Fumarase
• Addition of H2O producing Malate

Question 14

CAC Step 8

Answer 14

Oxidation

• Malate Dehydrogenase
• Oxidation to reform oxaloacetate
• 1 NADH + H+