Carbohydrates 5: Citric Acid Cycle

Question 1

where does citric acid cycle occur?

Answer 1

mitochondrial matrix

Question 2

what are 3 common “fuel” molecules?

Answer 2

• carbohydrates
• amino acids
• fatty acids

Question 3

does the cycle produce ATP directly?

Answer 3

No

Question 4

does the cycle include O2 as the reactant directly?

Answer 4

No

Question 5

As electrons are formed, what two reduced electron carriers are produced?

Answer 5

NADH and FADH2

Question 6

In which glycolysis step is the NAD converted to NADH ( reduction)

Answer 6

step 6

Question 7

in which step in glycolysis is NADH re-oxidised? (to form NAD again)

Answer 7

Pyruvate to lactate step

Question 8

what is citric acid cycle in simple words?

Answer 8

Stripping molecules from their electrons which are carried to terminal respiration step. Electrons are harvested to completely oxidise food molecules to CO2, H2O and make lots of energy

Question 9

what 2 substances can be oxidised to form acetyl CoA?

Answer 9

1. pyruvate (from glycolysis)

2. fatty acids

Question 10

what enzyme is used to break down pyruvate into acetyl CoA?

Answer 10

pyruvate dehydrogenase

Question 11

what 3 steps/ series of reactions occur when acetyl CoA is formed from pyruvate?

Answer 11

1. decarboxylation of pyruvate
2. oxidation
3. transfer/attachment of CoA complex

Question 12

what does the carboxylation step in formation of acetyl CoA from pyruvate release?

Answer 12

2 electrons (2H ions)

Question 13

what electron carrier goes through reduction in the formation of acetyl CoA from pyruvate?

Answer 13

NAD turns to NADH (carries the 2 electrons released through pyruvate dehydrogenase)

Question 14

what is pyruvate dehydrogenase made up of?

Answer 14

Many copies of each enzyme subunit

Question 15

what is the advantage of having several enzyme subunits making up pyruvate dehydrogenase?

Answer 15

each subunit catalyses a different part of the reaction

Question 16

what does E1 subunit in pyruvate dehydrogenase catalyse?

Answer 16

first decarboxylation of pyruvate

Question 17

what does E2 subunit in pyruvate dehydrogenase catalyse?

Answer 17

transfers the acetyl group to coenzyme A

Question 18

what does the E3 subunit in pyruvate dehydrogenase catalyse?

Answer 18

recycles lipoyllysine through reduction of FAD which is recycled by passing electrons to NAD+

Question 19

why is CAC called the “black box”?

Answer 19

because intermediate molecules that make up the cycle remain CONSTANT at all times

Question 20

what molecule is involved in energy transfer which is then readily converted to ATP?

Answer 20

GTP

Question 21

what is the order of intermediates from CAC starting from citrate? (8)

Answer 21

1. citrate
2. isocitrate
3. alpha-ketoglutarate
4. succinyl-CoA
5. Succinate
6. fumarate
7. malate
8. oxaloacetate

Question 22

what 2 molecules are released between isocitrate and alpha-ketoglutarate?

Answer 22

• NADH

- CO2

Question 23

what 2 molecules are released between alpha-ketoglutarate and succinyl-CoA?

Answer 23

• NADH

- CO2

Question 24

what is released at succinate intermediate

Answer 24

GTP

Question 25

what is released between succinate and fumarate intermediates?

Answer 25

FADH2

Question 26

what is release at oxaloacetate intermediate?

Answer 26

NADH

Question 27

between which two steps in the CAC is CoA group removed completely?

Answer 27

between succinyl CoA and succinate steps

Question 28

If the cell has enough energy, what will happen to the production of pyruvate dehydrogenase?

Answer 28

it will be stopped/ inhibited

Question 29

what 3 things can inhibit pyruvate dehydrogenease that makes acetyl CoA

Answer 29

- ATP - NADH - acetyl CoA

Question 30

what 2 things can stimulate production of pyruvate dehydrogenase to make acetyl CoA?

Answer 30

- ADP | - pyruvate

Question 31

at what 2 intermediates in CAC, are the other control points?

Answer 31

1. isocitrate | 2. alpha-ketoglutarate

Question 32

What sort of reactions are the control points in CAC?

Answer 32

non-reversible reactions (exergonic steps, deltaG -ve)

Question 33

what 2 things inhibit isocitrate dehydrogenase?

Answer 33

1. ATP | 2. NADH

Question 34

what stimulates isocitrate dehydrogenase?

Answer 34

ADP

Question 35

what 3 things inhibit alpa-ketoglutarate dehydrogenase?

Answer 35

- ATP - NADH - succinyl CoA

Question 36

are isocitrate and citrate interconvertible?

Answer 36

yes

Question 37

if there is citrate build up when isocitrate dehydrogenase stops working, what happens?

Answer 37

citrate is shuttled into cytoplasm causing phosphofructokinase to stop glycolysis

Question 38

what happens if there is a build up of alpha-ketoglutarate if alpha-ketoglutarate dehydrogenase is inactive?

Answer 38

alpha-ketoglutarate switches its use to production of amino acids

Question 39

what sort of pathway is CAC?

Answer 39

amphibolic pathway (both catabolic and anabolic processes occur)

Question 40

what does build up of citrate produce? (2)

Answer 40

fatty acids, sterols

Question 41

what does build up of alpha-ketoglutarate produce?

Answer 41

- glutamate (leading to AAs like glutamine, proline, arginine etc) and purines

Question 42

what does build up of succinyl CoA produce?

Answer 42

porphyrins, heme

Question 43

what enzyme is used to convert pyruvate to oxaloacetate?

Answer 43

pyruvate carboxylase

Question 44

what sort of a reaction is conversion of pyruvate to oxaloacetate?

Answer 44

anabolic process (formation)

Question 45

what are the 3 main products produced as a result of intermediates from CAC building up?

Answer 45

1. nucleotide bases 2. heme groups 3. proteins

Question 46

if there is a build up of Acetyl CoA (excessive production) during exercise, what needs to be produced more of to keep CAC going?

Answer 46

oxaloacetate

Question 47

what pathway can produce oxaloacetate directly?

Answer 47

Pyruvate can be converted to oxaloacetate through pyruvate carboxylase

Question 48

what is the term given to conversion of pyruvate to oxaloacetate by pyruvate carboxylase?

Answer 48

anaplerotic