Lecture 34

Question 1

what does the krebs cycle serve as?

Answer 1

serves as the focal end point for oxidation of carbohydrate, fats, and amino acids via acetyl coenzyme A.

Question 2

what is pyruvate converted to and by what?

Answer 2

converted to acetyle coenxyme A by pyruvate dehydrogenase complex

Question 3

what do the reactions of the TCA cycle generate?

Answer 3

carbon dioxide, reduced NAD, reduced FAD, and GTP

Question 4

What are the three paths leading to acetyl CoA?

Answer 4

1) Proteins to amino acids to acetyl CoA
2) Polysaccharides to monosaccharides to Acetyl CoA
3) Lipids to Glycerol/Fatty Acids to Acetyl CoA

Question 5

What are the enzymes associated with the pyruvate dehydrogenase complex?

Answer 5

1) Pyruvate decarboxylase
2) Dihydrolipoyl transacetylase
3) Dihydrolipoyl dehydrogenase

Question 6

which is the first coenzyme to act in the pyruvate dehydrogenase complex?

Answer 6

thiamine pyrophosphate or thiamine diphosphate (TPP)who is an acceptor for a decarboxylated pyruvate and passes it along to the second coenzyme.
- works with pyruvate decarboxylase

Question 7

what is the second coenzyme to act in the pyruvate dehydrogenase complex?

Answer 7

Lipoic acid who has two roles:

1) Disulfide oxidized role - accepts and oxidizes decarboxylated pyruvate (hydroxyethyl) then passes it along to Coenzyme A.
2) Sulfhydryl reduced form - passes its electrons to FAD who then gets reduced to FADH2.
- works with dihydrolipoyl transacetylase.

Question 8

what is the final electron acceptor in the pyruvate dehydrogenase complex?

Answer 8

NAD

Question 9

how is the pyruvate dehydrogenase complex directly regulated?

Answer 9

it can be phosphorylated by Protein Kinase (DEACTIVATED); it can be dephosphorylated by Phosphoprotein phosphatase (ACTIVATED)

Question 10

how is the pyruvate dehydrogenase complex (PDC) indirectly regulated?

Answer 10

Via action on Protein Kinase (which normally deactivates PDC)

• Excitatory action via ATP, Acetyl CoA, or NADH
• Inhibitory action via Pyruvate, NAD+, or CoA alone.

Question 11

What is the first step of the Kreb Cycle?

Answer 11

the 2 carbon Acetyl-CoA reacts with the 4 carbon Oxaloacetate to form the 6 carbon Citrate.

Question 12

Why does Aconitase have its name?

Answer 12

Because there is an intermediate called Aconitate during the step that it catalyzes however it is usually not mentioned.

Question 13

how are alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase similar?

Answer 13

• both use the same 3 enzymes (Pyruvate decarboxylase, Dihydrolipoyl transacetylase, and Dihydrolipoyl dehydrogenase)
• both use the same 5 coenzymes (TPP, lipoic acid [reduced and oxidized forms], NAD, and FAD)

Question 14

what is significant about the reaction Succinate to Fumarate via enzyme Succinate dehydrogenase?

Answer 14

the coenzyme FAD is reduced to FADH2 during this reaction which is the only NET reduction in the Krebs cycle.

Question 15

what is the enzyme fumarase sometimes referred to as?

Answer 15

Fumarate hydrotase

Question 16

what is the enzyme that catalyzes the last step in the TCA cycle and what is interesting about it?

Answer 16

Malate Dehydrogenase is the enzyme and it functions in both the mitochondria and the cytosol whereas most of the other enzymes only function in the mitochondria.

Question 17

What is an inhibitor of Pyruvate Dehydrogenase?

Answer 17

ATP; NADH; its product Acetyl CoA

Question 18

what is an inhibitor of Citrate Synthase?

Answer 18

ATP

Question 19

what are the regulators of Isocitrate Dehydrogenase?

Answer 19

inhibitors: ATP and NADH
activators: ADP

Question 20

what is an inhibitor of alpha-ketoglutarate dehydrogenase?

Answer 20

NADH and its product Succinyl CoA

Question 21

What catalyzes one of the most important reactions for the production of oxaloacetate?

Answer 21

pyruvate carboxylase

Question 22

which mutations in the TCA cycle can sometimes result in cancerous cells?

Answer 22

1) mutation in isocitrate dehydrogenase results in formation of 2-hydroxyglutarate (2HG) INSTEAD of alpha-ketoglutarate. 2HG inhibits alpha-ktoglutarate dependent dioxygenases which are responsible for demethylation of histones (so causes hypermethylation.
2) lowered expression of fumarase which results in reduced TCA cycle activity and since glycolysis and TCA usually compete, this would mean Glycolysis now has the edge and can increase in its activity. Cancer cells tend to have increased glycolysis (Warburg effect)

Question 23

what is the warburg effect?

Answer 23

aerobic glycolysis tends to be increased in cancer cells.

Question 24

what is the difficulty associated with cytosolic NADH and how is this solved?

Answer 24

Problem: the inner mitochondrial membrane is impermeable to NADH

Solution: two electron shuttles Glycerol phosphate shuttle and Malate-aspartate shuttle.

Question 25

How does the Glycerol phosphate shuttle work?

Answer 25

In the cytosol, DHAP gets reduced to Glycerol 3-Phophate by NADH (takes its valuable electron), then goes through the membrane into the mitochondrial matrix where it loses that electron to FAD (reducing it to FADH2) and reforms into its oxidized state DHAP.

Question 26

How does the Malate-Aspartate shuttle work?

Answer 26

In the cytosol, Oxaloacetate gets reduced to Malate by NADH (takes its valuable electron), then gets transported into the mitochondrial matrix where it loses that electron to NAD+ (reducing it to NADH) and is then transaminated into Aspartate (with help from glutamine) then transported back into the cytosol and re-transaminated to oxaloacetate.

Question 27

what aminoacid does the malate-aspartate shuttle need in order to function?

Answer 27

glutamine is necessary for transamination of oxaloacetate into aspartate which is then transported back out of the mitochondrial matrix and into the cytosol.

Question 28

Under anaerobic conditions what is the limiting factor for the TCA cycle?

Answer 28

NAD concentration