1.6 - TCA Cycle and Bioenergetics

Question 1

pyruvate dehydrogenase complexes role (2)

Answer 1

1. catalyses committed step (no going back) of conversion of pyruvate to acetyl-CoA
2. regulates irreversible step in the TCA cycle

Question 2

how is the irreversible step in the TCA cycle controlled? (PDC) (2)

Answer 2

1. product inhibition (allosteric control)
2. reversible phosphorylation (covalent modification)

Question 3

how are pyruvate dehydrogenase complexes inhibited?

Answer 3

by the products, acetyl-CoA and NADH

Question 4

what are pyruvate dehydrogenase complexes (PDC)

Answer 4

multi subunit enzyme complex: E1, E2 and E3

Question 5

coenzymes in pyruvate dehydrogenase complexes (5)

Answer 5

1. FAD
2. CoA-SH
3. NAD+
4. TPP
5. Lipoate

Question 6

allosteric control of PDC (2)

Answer 6

1. binding of end products (NADH and Acetyl-CoA) inhibit PDC activity (enough and PDC inactive)
2. binding of substrates (pyruvate and ADP) promotes enzyme catalytic activity of PDC

Question 7

covalent modifications of PDC (2)

Answer 7

1. serine residue on PDC is phosphorylated by PDK kinase (PDK) - leads to inactivation of PDC
2. PDC has phosphate group removed by PDC phosphatase - PDC activated reversibly

Question 8

reactions catalyses by “X” generate electron donors

Answer 8

dehydrogenase

Question 9

electron transport chain (ETC) purpose

Answer 9

transport protons (H+) from matrix to inter-membrane space across inner membrane of mitochondria via complexes I,III and IV

Question 10

what is required to accept e- at the end of the ETC

Answer 10

molecular oxygen

Question 11

half-channel I (F-ATP synthase)

Answer 11

allows H+ to move from exoplasmic medium to -ve charged side chain of Asp-61 in centre of c subunit near middle of membrane

Question 12

half-channel II (F-ATP synthase)

Answer 12

(arginine residue) permits H+ to move from Asp-61 of adjacent c subunit into the cytosolic medium

Question 13

final common pathway for carbohydrate (and lipid) oxidation

Answer 13

TCA cycle

Question 14

why are aerobic conditions required for the TCA cycle?

Answer 14

oxygen acts as the terminal e- acceptor for respiratory complex IV

Question 15

how does entry to the TCA cycle occur?

Answer 15

via acetyl CoA, occurs in mitochondria

Question 16

ATP per NADH

Answer 16

2.5

Question 17

ATP per FADH2

Answer 17

1.5

Question 18

SLP step

Answer 18

substrate level phosphorylation giving GTP -> ATP

Question 19

total ATP yield (1mol glucose)

Answer 19

reduced NADH/FADH2 re-oxidised via cytochrome (respiratory) chain to yield ~30-36 mols ATP (oxidative phosphorylation) from oxidation of 1mol glucose

Question 20

three irreversible reactions of TCA cycle (enzymes) (3)

Answer 20

1. citrate synthase
2. isocitrate dehydrogenase
3. a-ketoglutarate dehydrogenase

Question 21

what activates the three irreversible enzymes of the TCA cycle?

Answer 21

Ca^2+, levels rise during muscle contraction (increasing TCA cycle and ATP production)

Question 22

what allosterically regulate the three irreversible enzymes of the TCA cycle? (3)

Answer 22

1. ATP
2. Cycle intermediates
3. NADH

• high levels inhibit cycle

Question 23

what do the three irreversible reactions of the TCA cycle ensure?

Answer 23

cycle operates in clockwise direction

Question 24

why is it crucial the TCA cycle occurs within the mitochondrial matrix? (2)

Answer 24

1. intermediates all di-/tri- carboxylic acids (-ve charged at neutral pH)
2. traps them in matrix because of permeability characteristics of inner mitochondrial membrane

Question 25

why are the permeability characteristics of the inner mitochondrial membrane important for the TCA cycle? (2)

Answer 25

1. many intermediates can cross, only in exchange for dicarboxylic acids from the cytosol 2. exchangers help maintain high matrix concentrations of intermediate keeping cycle operating

Question 26

respiratory control

Answer 26

overriding control of TCA cycle

Question 27

effect of ADP concentration on electron transport chain (respiratory control) (2)

Answer 27

1. high ADP concentration - ETC proceeds and ATP made 2. low ADP concentration - ETC slows down - availability of ADP controls respiration rate in mitochondrion

Question 28

purpose of respiratory control

Answer 28

ensures e- flow occurs only when ATP synthesis is required