Electron transport chain

Question 1

what is the electron transport chain

Answer 1

aka respiratory chain
Comprises 4 large multi-unit proteins intrinsic to inner mitochondrial membrane
Catalyse a series of reactions
NADH+H++1/2O2>NAD++H2O
Energy released from reaction by tightly coupled production of ATP

Question 2

What are the components of ETC

Answer 2

Four components

Complex I, II, III and IV

Question 3

What proteins are linked to the components of ETC

Answer 3

2 soluble proteins
Ubiquinone (coenzyme Q) – a lipid soluble benzoquinone with a long isoprenoid tail
Cytochrome C
Free to move in membrane by diffusion, not part of complexes

Question 4

What enzymes are involved in the protein complexes of the ETC

Answer 4

NADH dehydrogenase
Succinate dehydrogenase
ubiquinone: cytochrome C oxidoreductase
cytochrome C (moves between complex III and IV)
Cytochrome oxidase

Question 5

what is complex I

Answer 5

NADH dehydrogenase

Question 6

what does complex I do

Answer 6

Initially electrons are passed to FMN to produce FMNH2
Subsequently transfer to series of iron-sulphur clusters then transfer to ubiquinone (coenzyme Q)
So enzyme catalyses NADH+H++Q=NAD++QH2
Proton pump, moving protons from matrix to intramitochondrial space
QH2 released outside and NAD+ stays in
4H pass

Question 7

what is complex II

Answer 7

succinate dehydrogenase

Question 8

What does complex II do

Answer 8

FAD within complex II is reduced to FADH2 by electrons gained from conversion of succinate to fumarate in TCA cycle
Complex II passes electrons to ubiquinone
Other substrates for mitochondrial dehydrogenases also pass on their electrons to ubiquinone but not through complex II (eg from G3P shuttle)
QH2 released

Question 9

Sources of electrons entering the ETC

Answer 9

NADH +H+
Succinate to fumarate
FAD
ETF:Q oxidoreductase

Question 10

what is complex III

Answer 10

Ubiquinone: cytochrome c oxidoreductase

Question 11

what does complex III do

Answer 11

Accepts electrons from reduced ubiquinone and pass to complex IV via cyt c
2nd of 3 proton pumps in respiratory chain
4H pass

Question 12

what is complex IV

Answer 12

Cytochrome oxidase

Question 13

What does complex IV do

Answer 13

3rd and final proton pump
Carries electrons from cytochrome c to molecule oxygen, produces water
2h pass

Question 14

how is ATP synthesised

Answer 14

Inner mitochondrial leaflet is generally impermeable to charged species but 3 specific systems in the membrane
Transport ADP and Pi into matrix
Synthesis ATP
Transport ATP into cytosol 
ADP3-+P2-+H+=ATP4-+H2O

Question 15

How is the adenine nucleotide translocated

Answer 15

Integral protein of inner mitochondrial membrane
transport ADP from intramitochondrial membrane to matrix in exchange for ATP (favoured by electrochemical gradient generated by proton pump)
Known as anti porter

Question 16

how can adenine nucleotide translocation be inhibited

Answer 16

Atractyloside, a glycoside isolated from a thistle is a specific inhibitor of adenine nucleotide translocase

Question 17

how is phosphate translocated

Answer 17

A second membrane transporter essential for oxidative phosphorylation and synthesis of ATP
Transports both phosphate and hydrogen ions into matrix, symporter
favoured by transmembrane proton gradient

Question 18

what synthesises ATP

Answer 18

An F-type ATPase
Two functional domains
1 Fo an oligomycin-sensitive proton channel
2 F1 ATP synthase

Question 19

What is the structure of ATP synthesis

Answer 19

Fo comprises of three different types of SU a b and c
Forms a complex of 13-15 Sus
Sus c1-10 arranged in a circle

F1 comprises 5 different types of SU a3, B3, y, delta and epsilon
Forms a complex of 9 SU (3B and 3a Sus, have catalytic sites for ATP synthesis)

Question 20

how are the B SUs of F1 arranged

Answer 20

B Sus are arranged alternately with a SUs like segments of an orange
Form a knob-like structure held by a stalk of y and epsilon Sus
Delta SU interacts with 2 b Sus of Fo

Question 21

what is the. theory of rotational catalysis

Answer 21

3 B Sus take it in turns catalysing the synthesis of ATP
Any given B SU starts a conformation for binding ADP and Pi
Then changes conformation so active site now binds the product ATP tightly
Then changes conformation to give active site a low affinity for ATP (B-empty conformation) so ATP released

Question 22

how does the y SU affect the B-catalytic units

Answer 22

The y-SU rotates and the properties of B-catalytic units change

Question 23

what type of reaction is the reduction of NADH

Answer 23

Highly exergonic reaction

Question 24

What results from the reduction of NADH

Answer 24

Energy released is coupled to movement of H+ across inner membrane
Electrochemical energy generated represents temporary conservation of energy of e- transfer
Protons flow spontaneously down their electrochemical gradient releasing energy available to do work

Question 25

what are uncoupling reagents

Answer 25

Normally e- flow and phosphorylation of ADP tightly coupled

Uncouplers dissipate the pH gradient by transporting H+ back into the matrix of mitochondria bypassing the ATP synthase

Question 26

what is an example of uncoupling agents

Answer 26

An uncoupler (eg DNP ) severs link between e- flow and ATP synthesis with energy released as heat
Can occur naturally eg UCPI (thermogenin) is found in brown adipose tissue and has a specific H+ channel through which H+ may be dissipated- energy released as heat

Question 27

how does uncoupling link to brown fat thermogenesis

Answer 27

BAT – high numbers of mitochondria (contain UCP1)
Extra protein in membrane for protons to bypass ATP synthesis
Specialised for heat generation – important in newborns, important in obesity/diabetes

Question 28

how is DNP an uncoupler

Answer 28

exogenous uncoupler
Weak acid that crosses membrane ferrying H+
Each DNP molecule collects a proton from IMS and moves through membrane
Can return to collect more

Question 29

what are the effects of DNP uncoupling

Answer 29

Toxicity arises from liver damage, respiratory acidosis and hyperthermia