The TCA cycle

Question 1

Oxaloacetate

Answer 1

Condensation reaction, to citrate by addition of acetyl coA. Enzyme citrate synthase.

Question 2

Citrate

Answer 2

Isomersim to isocitrate by aconase

Question 3

Isocitrate

Answer 3

Oxidative decarboxylation to alpha ketoglutarate, by isocitrate dehydrogenase. Release CO2 reduce NAD+.

Question 4

Alpha-ketoglutarate

Answer 4

Oxidative decarboxylation to succinyl coA, by alpha ketoglutarate dehydrogenase. Release CO2 and reduce NAD+.

Question 5

Succinyl CoA

Answer 5

Thioesterase deacetylation to Succinate by Succinate thiokinase. Produces one molecule of GTP which can be coupled to produce one ATP. Substrate level phosphorylation.

Question 6

Succinate

Answer 6

Oxidation (not large enough to produce NADH so produces FADH2). To fumarate catalysed by succinate dehydrogenase.

Question 7

Fumarate

Answer 7

Hydration to L-malate by fumarase

Question 8

L-malate

Answer 8

Oxidation to oxaloacetate by malate dehydrogenase

Question 9

Where does the TCA cycle occur?

Answer 9

Mitochondrial matrix

Question 10

Describe course of glucose from outside cell to acetyl CoA in mitochondria

Answer 10

Enters cell and in cytoplasm becomes G6P and pyruvate (glycolysis).

Pyruvate enters through inner mitochondrial transporter.

Pyruvate converted to acetyl CoA in the link reaction.

Question 11

Describe the passage of fatty acids, amino acids and ketone bodies into mitochondria

Answer 11

Pass through plasma membrane and mitochondrial membrane without being converted into anything (then into Acetyl CoA)

Question 12

How is pyruvate converted to acetyl CoA in link reaction?

Answer 12

Pyruvate dehydrogenase enzyme, leads to decarboxylation, addition of CoA and oxidation (produce NADH)

Question 13

What does the TCA cycle produce?

Answer 13

3NADH, and FADH2 for oxidation and energy yield

GTP (converted to ATP)

Question 14

How is GTP formed?

Answer 14

Substrate level phosphorylation

Question 15

How many molecules of ATP from the 3 byproducts of TCA?

Answer 15

10

Question 16

How is FAD different from NADH?

Answer 16

Must be physically associated with inner mitochondrial membrane as bound to dehydrogenase

Question 17

Why is it better for cyclical pathway?

Answer 17

Intermediates are regenerated so that net amount of each one remains same after each turn.
Intermediates present in small amounts and play catalytic role

Question 18

Which steps of TCA cycle is regulated in response to ETC?

Answer 18

Dehydrogenase reactions

Question 19

What governs the rate of TCA cycle?

Answer 19

ATP demand, not substrate availability

Question 20

What inhibits dehydrogenase steps?

Answer 20

NADH, ATP

Question 21

PDH is what sort of complex?

Answer 21

Multi enzyme

Question 22

What inhibits PDH (allosteric)?

Answer 22

ATP, NADH, Acetyl CoA

Question 23

What stimulates PDH?

Answer 23

ADP , NAD+, Pyruvate, CoA-SH (i.e. substrate, or products indicating low energy)

Question 24

Why does high ATP inhibit PDH and high ADP stimulate PDH?

Answer 24

ATP Indicates high energy so need to reduce glucose usage

Lots of ADP suggests low energy situation and so upregulate PDH and glucose breakdown

Question 25

What can inactivate PDH (covalent)?

Answer 25

PDH kinase phosphorylates it (activated by allosteric factors)

Question 26

What can reactivate PDH covalently?

Answer 26

PDH phosphatase

Question 27

What can form amino acids out of TCA intermediates, which amino acids?

Answer 27

Alpha ketoglutarate (glutamate) and oxaloacetate (aspartate)

Question 28

What can form glucose out of TCA intermediates?

Answer 28

Oxaloacetate

Question 29

What can form fatty acids out of the intermediates?

Answer 29

Citrate

Question 30

What is an anaplerotic reaction? Give an example.

Answer 30

Anaplerosis is the act of replenishing TCA cycle intermediates that have been extracted for biosynthesis.

Pyruvate carboxylated to oxaloacetate
Aspartate forms oxaloacetate