Session 3: TCA cycle

Question 1

what enters the TCA cycle?

Answer 1

acetyl coA

Question 2

how is acetyl CoA formed from pyruvate (the product of glycolysis)?

Answer 2

pyruvate dehydrogenase

Question 3

what is acetly CoA?

Answer 3

• contains a sulfhydryl group that reacts with carbonyl group to form acrylic thioesters
• complex structure derived from vitamins
• binds with enzymes and participates with catalysts —> puts reactive molecule in right chemical environment for reactions to proceed favourably

Question 4

what does pyruvate dehydrogenase do?

Answer 4

pyruvate + CoA + NAD —> acetyl CoA + CO2 + NADH + H+

• first carboxylation reaction (CO2 produced)
• occurs in the mitochondria (pyruvate transported from cytoplasm)

Question 5

what is the structure of pyruvate dehydrogenase?

Answer 5

large multi enzyme complex: 5 enzymes

• 3 enzymes are catalytic
• 2 enzymes are regulatory
• irreversible reaction

Question 6

why is pyruvate dehydrogenase sensitive to vitamin B1 deficiency?

Answer 6

different enzyme activities require various coenzymes (FAD and thiamine pyrophosphate)

• B vitamins provide the coenzymes

Question 7

what is PHD activated by?

Answer 7

• pyruvate
• CoA
• NAD
• ADP
• insulin (signals there are adequate amounts of glucose for metabolism to proceed)

Question 8

what is PDH inhibited by?

Answer 8

• acetyl - CoA
• NADH
• ATP
• citrate

Question 9

what enzyme activates PDH?

Answer 9

phosphatase regulatory enzyme

Question 10

what enzyme inhibits PDH?

Answer 10

kinase regulatory enzyme

Question 11

what does the TCA cycle produce?

Answer 11

• lots of reducing power
• GTP (equivalent energy to ATP)
• produces precursors for biosynthesis

Question 12

why is it called the TCA cycle?

Answer 12

citrate and isocitrate have 3 COOH groups

Question 13

how does the TCA cycle work?

Answer 13

1. acetyl co enzyme A had 8 high energy e-
2. TCA cycle efficiently harvests high energy e-
3. these e- are transferred onto coenzymes (NAD and FAD) —> generates reducing power

Question 14

how does the TCA cycle efficiently harvests high energy e-?

Answer 14

1. acetyl coenzyme A binds to oxoaloacetate to form 6C citrate
2. C-C and C-H bonds broken and e- are released
3. CO2 also released

Question 15

how many NADH and FADH2 and GTP is made?

Answer 15

NADH = 3

FADH2 = 1

GTP = 1

Question 16

how is the TCA cycle regulated?

Answer 16

energy availability:
- the ratio of ATP/ADP
- the ratio of NADPH/NAD

Question 17

what is the rate limiting enzyme of the TCA cycle?

Answer 17

isocitrate dehydrogenase

Question 18

how is isocitrate dehydrogenase regulated?

Answer 18

activated by high ADP levels

inhibited by high levels of NADH

Question 19

how is alpha - ketogluterate dehydrogenase regulated?

Answer 19

inhibited by NADH, ATP and its product succinyl -CoA

Question 20

what are the different intermediated produced from the TCA cycle used in the liver?

Answer 20

citrate = fatty acids

a-ketoglutarate = amino acids

succinate = haem and amino acids

malate = amino acids

oxoaloacetate = glucose

Question 21

where does Stage 4 of carbohydrate metabolism occur?

Answer 21

mitochondria
- e- transport and ARP synthesis
- NADH and FADH2 re oxidised
- requires O2

Question 22

what are the 2 processes that occur in stage 4?

Answer 22

1. electron transport
   - released energy
2. oxidative phosphorylation
   - free energy released used to drive ATP synthesis

Question 23

what structures help transport protons across the inner mitochondrial membrane?

Answer 23

3 proton translocating complexes

• PTC1, PTC2, PTC3

Question 24

what happens when protons move from outside to inside the mitochondrial membrane?

Answer 24

1. accumulation of H+ inside the membrane
2. electrochemical gradient established (more + inside than out)

Question 25

how does the electron transport chain work?

Answer 25

1. e- transferred through series of carrier molecules towards O2 2. release energy as they go down 3. energy is used to transport H+ across inner mitochondrial membrane = proton motive force

Question 26

where is the oxygen found in the ETC (final e- acceptor)?

Answer 26

on PTC3 - PTC3 has a haem group that has O2 bound to it - reacts with e- and H+ to form H2O

Question 27

how much energy released from the ETC is used to transport H+?

Answer 27

30% - rest of energy is released as heat

Question 28

what molecule is used to make ATP?

Answer 28

ATP synthase/ F1F0 - ATPase

Question 29

what is the structure of ATP synthase?

Answer 29

top = F0 which is like a turbine/spins middle = axle bottom = F1 which is the generator

Question 30

how does ATP synthase work?

Answer 30

1. F0 is the only site in IMM where H+ can go down their electrochemical gradient (back across the membrane) 2. H+ go through F0 causing it to turn —> turns the axle 3. energy then used to generate ATP from ADP and Pi in F1 (generator)

Question 31

how is oxidative phosphorylation regulated?

Answer 31

- oxygen (cannot happen without it) - if ATP is high and ADP low, no substrate for ATP synthase - so H+ flow stops - conc. of H+ in intermictochondrial space increases - prevents H+ translocation

Question 32

what is the difference between NADH and FADH2?

Answer 32

NADH - e- in NADH have more energy than FADH2 - uses all 3 PTC —> produced more proton motive force —> more ATP FADH2 - only uses PTC2 and PTC3

Question 33

how many moles of ATP from 2 moles of NADH and FADH2?

Answer 33

2 moles of NADH = 5 moles of ATP 2 moles of FADH2 = 3 moles of ATP

Question 34

how many moles of ATP produced from one glucose?

Answer 34

32

Question 35

what determines the efficiency of oxidative phosphorylation?

Answer 35

the tightness of coupling ( of ETC and ox phosphorylation) 1. inhibitors 2. uncouples

Question 36

what are 2 molecules that inhibit oxidative phosphorylation?

Answer 36

carbon monoxide and cyanide

Question 37

how do CO and CN- inhibit oxidative phosphorylation?

Answer 37

- block e- transport - bind to haem on PTC3 and prevents O2 binding and acting as final e- acceptor - lethal

Question 38

what do uncouplers do?

Answer 38

increase the permeability of IMM to protons - ATP synthase no longer the only place in IMM that H+ can be transported through - gets rid of an electrochemical gradient (proton motive force) - can’t be used in ATP synthesis

Question 39

what are 3 examples of how uncoupling is used?

Answer 39

1. dinitrophenol 2. brown adipose tissue 3. uncoupling proteins (UCPs)

Question 40

how is uncoupling used in dinitrophenol?

Answer 40

drug used in weight loss - uncoupling reduces ATP synthesis - so body metabolises other sources for energy - increases metabolism —> however released a lot of heat that individuals can overheat and die

Question 41

how is uncoupling used in brown adipose tissue?

Answer 41

- H+ enters mitochondria without ATP synthase - electron transport continues without ATP synthesis - excess energy released as heat - to maintain body temp.

Question 42

how doe UCPs work?

Answer 42

UCP - 1 is example found in brown adipose tissue - transports H+ back into mitochndria