2. The Tricarboxylic Acid Cycle

Question 1

What is the role of pyruvate dehydrogenase in glucose metabolism? Why is this a key regulatory step?

Answer 1

• Converts pyruvate (3C) to acetyl CoA (2C):
  Pyruvate + CoA + NAD+ –> acetyl CoA + CO2 + NADH + H+
• Involves loss of 1C as CO2 - irreversible reaction

Question 2

Where is pyruvate dehydrogenase located?

Answer 2

Mitochondrial matrix

Question 3

Deficiency in what would affect pyruvate dehydrogenase activity? Why?

Answer 3

PDH activities requires various cofactors: FAD, thiamine pyrophosphate and lipoid acid.

Cofactors provided by B vitamins… so reaction is sensitive to vitamin B1 deficiency.

Question 4

How is pyruvate dehydrogenase regulated?

Answer 4

1. Activated by low energy signals (inc. own substrates) - dephosphorylation:

• pyruvate
• CoA
• NAD+
• ADP
• insulin (indicates increased glucose so need metabolism to produce energy for biosynthesis)

2. Inhibited by high energy signals (inc. own products) - phosphorylation:

• acetyl CoA
• NADH
• ATP
• citrate

Question 5

What is the physiological result of pyruvate dehydrogenase deficiency?

Answer 5

Lactic acidosis (as pyruvate diverted to lactate production)

Question 6

Which substrates can be used to produce acetyl CoA?

Answer 6

1. Pyruvate (from carbohydrates)
2. Fatty acids (from lipids)
3. Amino acids (from proteins)
4. Alcohol

Question 7

What is the function of the TCA cycle?

Answer 7

1. Break down acetyl CoA (2C) to produce reducing equivalents and some direct energy (with subsequent release of CO2):
   
   • 6 NADH
   • 2 FADH2
   • 2 GTP
     (For every 2 TCA cycles from 1 glucose)
     I.e. Transfer of energy from glucose to reducing agents for use in ETC.
2. Produces precursors for biosynthesis

Question 8

What does acetyl CoA combine with at the start of the TCA cycle? How many carbons does the resulting molecule have?

Answer 8

4C oxaloacetate to produce 6C molecule

Question 9

How is the TCA cycle regulated?

Answer 9

2 key enzymes (isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase) regulated by:

• inhibited by high energy signals/own products = NADH, ATP, succinyl-CoA
• activated by low energy signals = ADP

Question 10

Why can the TCA cycle be considered anabolic as well as catabolic?

Answer 10

Intermediates can be used in biosynthetic processes, e.g:

• fatty acids (from citrate)
• amino acids (from alpha-ketoglutarate, succinate, malate, oxaloacetate)
• GLUCOSE (from oxaloacetate)