TCA Cycle

Question 1

What molecules does 1 pyruvate molecule form during the TCA Cycle?

Answer 1

• 3 NADH
• 1 FADH2
• 1 ATP
• 2CO2

Question 2

When is the TCA cycle activated and inhibited?

Answer 2

ACTIVATED - high energy demands and low ATP
INHIBITED - low energy demands and high NADH

Question 3

How does pyruvate form Acetyl CoA?

Answer 3

• Pyruvate transported from cytosol to mitochondrial matrix
• Oxidised and decarboxylated by PDH
• Acetyl group transferred to CoA - form Acetyl CoA. CO2 produced, NAD+ reduced to NADH
• Acetyl CoA enters TCA cycle

Question 4

Outline the main features of the TCA cycle.

Answer 4

• 4 oxidation steps where FAD and NAD+ are reduced
• 2CO2 given off
• 1 ATP molecule synthesised per molecule of pyruvate

Question 4

Describe Step 1

Answer 4

• Oxaloacetate (4C) and Acetyl CoA (2C) join to form citrate
• Catalysed by citrate synthase

Question 5

Describe Step 2

Answer 5

• Isomerisation of citrate to isocitrate
• Reversible
• Catalysed by aconitase

Question 6

Describe Step 3

Answer 6

• Isocitrate oxidised to oxalosuccinate, forming NADH
• Undergoes decarboxylation to α-ketoglutarate
• Irreversible - catalysed by isocitrate dehydrogenase
• RATE LIMITING STEP - IDH allosterically regulated (Activated by ADP, Inhibited by ATP and NADH)

Question 7

Describe Step 4

Answer 7

• α-ketoglutarate decarboxylated and binds to CoA to form succinyl CoA
• NAD+ reduced to NADH
• Irreversible - catalysed by α-ketoglutarate dehydrogenase

Question 8

Describe Step 5

Answer 8

• Cleavage of Succinyl CoA to Succinate
• Energy from hydrolysis used for phosphorylation of GDP to GTP
• Transfer of phosphate group from GTP to ADP
• ATP forms

Question 9

Describe Step 6

Answer 9

• Dehydration of succinate to fumarate
• Removal of hydrogen from 2 carbon atoms
• Double bond forms
• Reduction of FAD to FADH2

Question 10

Describe Step 7

Answer 10

• Hydration of fumarate to malate

Question 11

Describe Step 8

Answer 11

• OH group in malate oxidised to oxaloacetate
• NAD+ reduced to NADH
• Oxaloacetate regenerated and enters next cycle

Question 12

Describe gluconeogenesis.

Answer 12

• Synthesis of glucose from non-carbohydrate precursors
• Occurs when glycogen stores are depleted
• Takes place in liver and cortex of kidneys
• First substrate is pyruvate (REVERSE OF GLYCOLYSIS)

Question 13

What does it mean for a substrate to be glucogenic?

Answer 13

• Can be converted to pyruvate or intermediates for TCA cycle

Question 14

Give examples of glucogenic substrates

Answer 14

• LACTATE - Anaerobic glycolysis via Cori cycle
• GLYCEROL - Triglyceride hydrolysis in adipose tissue
• GLUCOGENIC AMINO ACIDS - Mainly alanine and glutamine (except ketogenic leucine and lysine - convert to fatty acids)

Question 15

Describe the Cori Cycle

Answer 15

• Lactate produced during anaerobic conditions in muscle
• Lactate accumulates and moves to liver via bloodstream
• Oxidised to pyruvate by LDH - converted to glucose which is carried back to muscles.

Question 16

What occurs to glycerol during gluconeogenesis?

Answer 16

• Phosphorylated and oxidised to dihydroxyacetone phosphate (glycolysis intermediate)

Question 17

What occurs to glucogenic amino acids?

Answer 17

• Undergoes transamination
• Generates either pyruvate or oxaloacetate

Question 18

Compare and contrast glycolysis and gluconeogenesis

Answer 18

• GLUCONEOGENESIS - different enzymes for rate-limiting steps
• Steps occur in reverse to glycolysis. Pyruvate converted to phosphoenol pyruvate then bisphosphate then glucose

Question 19

What stimulates and inhibits gluconeogenesis?

Answer 19

• Glucagon stimulates/insulin inhibits gluconeogenesis
• Low ATP indicate low energy - inhibit F16BP - decrease gluconeogenesis (increase glycolysis)
• High ATP and citrate indicate high energy - activate PEPCK and F16BP - increase gluconeogenesis (decrease glycolysis)

Question 20

How is gluconeogenesis involved in Type 2 diabetes?

Answer 20

• Disease due to insulin resistance
• Insulin unable to increase with blood sugar
• Cannot inhibit PEPCK, leading to increased gluconeogenesis and hyperglycemia

Question 21

What is the mechanism of action of metformin?

Answer 21

Inhibits gluconeogenesis

Question 22

What are the rate-limiting enzymes in glycolysis and gluconeogenesis?

Answer 22

• GLYCOLYSIS - Phosphofructokinase
• GLUCONEOGENESIS - Fructose 1,6 - bisphosphatase