43. Citric acid metabolism

Question 1

final common pathway for aerobic oxidation of carbohydrates, lipids, proteins

Answer 1

citric acid cycle /
Krebs cycle/
tricarboxylic acid cycle

Question 2

major pathway for formation of ATP

Answer 2

citric acid cycle

Question 3

provides substrates for gluconeogenesis , amino acid synthesis and fatty acid synthesis

Answer 3

citric acid cycle

Question 4

where does citric acid cycle occur

Answer 4

mitochondria

mitochondrial matrix - except succinate dehydrogenase - inner mitochondrial membrane

Question 5

what is the substrate of citric acid cycle

Answer 5

acetyl CoA

Question 6

products of citric acid cycle

Answer 6

• 2 CO2
• 1 GTP
• 3 NADH
• 1 FADH2

Question 7

rate limiting step of citric acid cyc;e

Answer 7

Isocitrate –> alpha ketoglutarate

Enz: Isocitrate dehydrogenase

Question 8

steps of citric acid cycle

Answer 8

1. acetyl coa + oxaloacetate –> citrate
   enz. citrate synthase
2. citrate –> isocitrate
   enz. aconitase
3. isocitrate -> alpha ketoglutarate
   enz. isocitrate dehydrogenase
   produces: CO2 + NADH
4. alpha ketoglutarate –> succinyl coa
   enz. Alpha ketoglutarate dehydrogenase
   produces : CO2 and NADH
5. succinyl coa –> succinate
   enz. succinate thiokinase
   produces GTP by substrate level phosphorylation
6. succinate –> fumarate
   enz. Succinate dehydrogenase
   produces:
   produces FADH2
7. fumarate to malate
   enz. fumarase
   (fumarate hydratase)
8. malate –> oxaloacetate
   enz. malate dehydrogenase
   produces NADH

Question 9

delivers acetyl ca to the cytosol for fatty acid synthesis

Answer 9

citrate

Question 10

used for heme synthesis, activation of ketone bodies in extahepatic tissues, TCA intermediate

Answer 10

succinyl CoA

Question 11

TCA intermediate; may be used for gluconeogenesis

Answer 11

malate

Question 12

does the TCA cycle synthesize new oxaloacetate

Answer 12

NO

Question 13

what is the hormonal control of TCA cycle

Answer 13

none

Question 14

where is NADH produced

Answer 14

3. isocitrate –> alpha ketoglutarate
   enz Isocitrate dehydrogenase
4. alpha ketoglutarate –> succinyl coa
   enz. Alpha ketoglutarate dehydrogenase
   produces : CO2 and NADH
5. malate –> oxaloacetate
   enz. malate dehydrogenase
   produces NADH

Question 15

where is CO2 produced

Answer 15

3. isocitrate –> alpha ketoglutarate
   enz Isocitrate dehydrogenase
4. alpha ketoglutarate –> succinyl coa
   enz. Alpha ketoglutarate dehydrogenase
   produces : CO2 and NADH

Question 16

where is GTP produced

Answer 16

succinyl coa –> succinate
Enz. Succinate thiokinase
produces: GTP

Question 17

produces FADH2

Answer 17

succinate –> fumarate
enz. succinate dehydrogenase

produces:FADH2

Question 18

ATP yield for TCA

Answer 18

3 NADH (2.5 ATP): 7.5
1 FADH2 (1.5 ATP): 1.5
ATP from GTP: 1.5
total: 10 ATPs

Question 19

complete oxidation of glucose

ATP yield

Answer 19

total : 30-32 ATP yield

glucose -> 2 pyruvate : 5 or 7 ATP
2 pyruvate -> 2 acetyl coa -> 2 NADH : 5 ATP
2 acetyl coa in TCA (10 atp each): 20 ATP