Lecture 10 - TCA cycle

Question 1

What does TCA stand for?

Answer 1

tricarboxylic acid

Question 2

What are the other names for the TCA cycle?

Answer 2

Citric acid cycle; Krebs cycle

Question 3

What is the first step? (i.e., what happens to pyruvate?

Answer 3

it’s converted to acetyl coenzyme A by oxidative decarboxylation

Question 4

What is decarboxylation?

Answer 4

a chemical reaction that removes a carboxyl group and releases carbon dioxide

Question 5

What begins the TCA cycle?

Answer 5

the entry of acetate as acetyl CoA

Question 6

What are the products of the TCA cycle?

Answer 6

CO2, ATP, NADH and FADH2

Acetyl CoA -> 3 NADH + FADH2 + ATP + 2 CO2

Question 7

What were the 3 important enzymes mentioned?

Answer 7

CAC-3: isocitrate dehydrogenase
CAC-4: alpha-ketoglutarate dehydrogenase
CAC-6: succinate dehydrogenase

Question 8

What enzyme is inhibited by its products? What are the products? What else inhibits it?

Answer 8

oxoglutarate/ alpha-ketoglutarate dehydrogenase

succinylcholine CoA and NADH

high energy charge in cell is also inhibitive

Question 9

What are the allosteric activators of oxoglutarate dehydrogenase?

Answer 9

ADP and calcium ions

Question 10

What is an allosteric regulator?

Answer 10

a substance that binds to a site on an enzyme distinct from the active site

Question 11

True or false: succinate dehydrogenase is not the only enzyme that participates in both the citric acid cycle and the electron transport chain

Answer 11

false

Question 12

What else does the TCA cycle play an important role in?

Answer 12

the catabolism of proteins and lipids

Question 13

Catabolism of TAG molecules

Answer 13

begins with their hydrolysis to glycerol and free fatty acids

Question 14

Fatty acids and the TCA cycle

Answer 14

fatty acids are linked to coenzyme A to form fatty acyl CoAs, which are then degraded by beta oxidation, a catabolic process that generates acetyl CoA and the reduced coenzymes NADH and FADH2

Question 15

Proteins and the TCA cycle

Answer 15

• free amino acids can be catabolized for energy
• protein catabolism beings with hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain – proteolysis
• the enzymes responsible = proteases
• all of the pathways for amino acid catabolism eventually lead to pyruvate, acetyl CoA, or a few key intermediates in the citric acid cycle

**amino acids can fuel the Krebs cycle

Question 16

Cataplerotic vs anaplerotic pathways

Answer 16

cataplerotic: TCA intermediates lost to this pathway to provide precursors for biosynthesis
anaplerotic: regenerate TCA intermediates