Metabolism: TCA Cycle

Question 1

List 3 sources of acetyl-CoA

Answer 1

1. glycogenolysis/glycolysis (glucose => pyruvate oxidation)
2. triglyceride lipolysis and fatty acid beta oxidation
3. proteolysis and deamination/oxidation of amino acids

Question 2

What do deficiencies in PDH coenzymes lead to?

Answer 2

• deficiencies in B1, B2, B3, B5 cause inhibition of TCA cycle due to decreased levels of aCoA
• reduced energy production

Question 3

How many ATP molecules are generated by 1 glucose molecule?

Answer 3

1 glu = 2 pyr = 2 aCoA
1 aCoA = 10 ATP
1 glu = 20 ATP

Question 4

What is different about acetyl-CoA supplied by glycolysis vs fatty acid oxidation?

Answer 4

FA oxidation provides MANY more acetyl-CoA molecules => MUCH MORE ATP made

Question 5

List fates of acetyl-CoA.

Answer 5

1. TCA => ETC => ATP
2. ketone body synthesis during starvation
3. fatty acid and sterol synthesis

Question 6

What is Coenzyme-A? (structure, function)

Answer 6

Structure: ATP + pantothenate + mercaptoethylamine + acetyl group (of various carbons)
Function: acyl carrier; transfers acetyl groups for degradation and synthesis

Question 7

List net production of TCA.

Answer 7

3 NADH (2.5ATP/1 NADH) = 7.5 ATP
1 FADH2 (1.5ATP/1 NADH) = 1.5 ATP
1 GTP = 1 ATP 
(total 10 ATP)
2 CO2

Question 8

Where does the TCA cycle occur?

Answer 8

mostly mitochondrial matrix, except succinate dehydrogenase on the inner membrane

Question 9

What are the 3 stages of ATP production? Where do they occur?

Answer 9

1. aCoA production (glycolysis occurs in cytosol; pyruvate transported into mitochondria => acetyl-CoA)
2. TCA (matrix)
3. ETC and oxidative phosphorylation (inner membrane)

Question 10

What general reactions lead to ATP production?

Answer 10

1. NADH/FADH2 oxidation to NAD+/FAD
2. ADP + Pi => ATP
3. 1/2 O2 => H2O
   occurs simultaneously

Question 11

What are the 2 ways to regulate TCA?

Answer 11

• fine control via allosteric regulation of enzymes

- coarse control via cellular environment

Question 12

What 2 enzymes are involved in fine control of TCA?

Answer 12

• isocitrate dehydrogenase

- alpha-ketoglutarate dehydrogenase

Question 13

Describe allosteric regulation of isocitrate dehydrogenase.

Answer 13

reaction: isocitrate => alpha-ketoglutarate
- activators: high ADP (need more energy)
- inhibitors: high ATP, NADH (sufficient energy; NADH inhibits PDH)

Question 14

Describe allosteric regulation of alpha-ketoglutarate dehydrogenase.

Answer 14

reaction: alpha-ketoglutarate => succinyl-CoA
- activators: high Ca2+ (indicator of low energy)
- inhibitors: high ATP, NADH, GTP, succinyl-CoA (feedback inhibition, energy indicators)

Question 15

What factors are involved in coarse control of TCA?

Answer 15

ATP, ADP, NADH, NAD+
aCoA
OAA

Question 16

How does ATP/ADP/NADH/NAD+ affect coarse control?

Answer 16

Since reoxidation of NADH and ATP synthesis are joint, supply of ADP and ATP synthesis are rate-limiting steps.

Question 17

How does cellular activity affect coarse control?

Answer 17

increased activity would deplete ATP stores and increase ADP supply => activate TCA

Question 18

How do oxygen levels affect coarse control?

Answer 18

anaerobic conditions would prevent reoxidation of NADH => inhibition of TCA enzymes, PDH
=> can’t continue TCA without enough NAD+

Question 19

How does OAA affect coarse control? Give 2 examples of things that affect its supply.

Answer 19

OAA is the substrate for the first step. OAA supply is affected by the amphibolic intertwining of TCA with other biosynthetic pathways.

ex: OAA used for AA synthesis
ex: OAA generated by AA breakdown

Question 20

What are the 2 forms of PDH deficiency and their etiology?

Answer 20

1. X-linked dominant mutation in E1 alpha

2. autosomal recessive various mutations

Question 21

What can you expect in the serum of children with PDH deficiency?

Answer 21

elevated lactate, pyruvate, alanine

lactic acidosis

Question 22

How do you treat PDH Deficiency?

Answer 22

thiamine
carnitine
lipoate

Question 23

Describe etiology, symptoms, and tx of fumarase deficiency.

Answer 23

reaction: fumarate => malate
- autosomal recessive mutation
- symptoms = neurological deficits, encephalomyopathy, dystonia, elevated TCA intermediates in urine
- tx = nutritional supplementation, PT, wheelchair

Question 24

What is fluoroacetate?

Answer 24

rat poison
blocks aconitase => blocks TCA
LETHAL