Mitochondrial respiratory chain

Question 1

How permeable is the outer membrane of the mitochondria?

Answer 1

Freely permeable to small molecules and ions

Question 2

How permeable is the inner membrane of the mitochondria?

Answer 2

Impermeable to small molecules and ions, including H+

Question 3

Where is the electron transport chain located?

Answer 3

On the inner mitochondrial membrane

Question 4

Describe what happens at complex 1

Answer 4

Initially electrons are passed to FMN to produce FMNH2
Subsequently transfer to a series of iron-sulphur clusters
Then transfer to Coenzyme Q, or ubiquinone
So, the enzyme catalyses the overall reaction:
NADH + H+ + Q = NAD+ + QH2
It is a proton pump, moving protons from the matrix into the intramitochondrial space

Question 5

What is the overall function of complex 1?

Answer 5

Acceptor of electrons from NADH

Moves 4 H+ions from matric to space between the two membranes- proton pump

Question 6

Describe what happens at complex 2

Answer 6

Succinate is converted to fumarate by succinate dehydrogenase
Electrons of FADH2 pass on their electrons to complex II
Complex II passes them to ubiquinone

Question 7

How do other substrates for mitochondrial dehydrogenase pass their electrons to ubiquinone?

Answer 7

Straight to ubiquinone

No need for the complexes 1 or 2

Question 8

Describe the action of complex 3

Answer 8

Ubiquinone: cytochrome c oxidoreductase
Second of three proton pumps in the respiratory chain
Carries 4 H+

Question 9

Describe the action of complex 4

Answer 9

Cytochrome oxidase
Third and final proton pump- carries 2 H+
Carries electrons from cytochrome c to molecular oxygen
Produces water

Question 10

Which complex is not a proton pump?

Answer 10

Complex 2

Question 11

Why is oxygen important for the respiratory electron chain?

Answer 11

Acts as a final electron acceptor

Question 12

How are the protons pumped?

Answer 12

From energy harnessed from the acceptance of electrons at various points
Conservation of energy- holding energy till the right time to make the ATP

Question 13

What are the 3 specific systems in the inner mitochondrial membrane that allow movement across the membrane?

Answer 13

Transport ADP and Pi into the matrix
Synthesise ATP
Transport ATP into the cytosol

Question 14

State the equation for the synthesis of ATP

Give the actual substrates

Answer 14

ADP3- + Pi2- + H+ ——–> ATP4- + H2O

Actual substrates are the Mg2+ complexes of ADP and ATP

Question 15

Give the number of negative charges of ADP and ATP in the physiological pH range

Answer 15

ATP has 4 negative charges and ADP has 3 negative charges

Question 16

Describe the Adenine nucleotide translocase

Answer 16

Integral protein of the inner mitochondrial membrane
Transports ADP3- from the intramitochondrial membrane space into the matrix
In exchange for an ATP4- molecule
transported in the other direction
(Favoured by the proton pump)
Known as an ‘antiporter’

Question 17

Give the name of a specific inhibitor of adenine nucleotide translocase

Answer 17

Atractyloside, a glycoside isolated from a thistle

Question 18

Describe phosphate translocase

Answer 18

A second membrane transport is essential for oxidative phosphorylation and synthesis of ATP
Transports both phosphate and hydrogen ions into the matrix: a ‘symporter’ - favoured by the transmembrane proton gradient

Question 19

What is a symporter?

Answer 19

2 things brought in via the same direction

Question 20

What is an antiporter?

Answer 20

One thing in and one thing out

Question 21

What are the two functional domains of ATP synthase?

Answer 21

Fo, an oligomycin-sensitive proton channel

F1, an ATP synthase

Question 22

Describe the subunit structure of F0

Answer 22

Fo comprises three different types of subunit: a, b, and c
Forms a complex of 13-15 subunits
Subunits c1-10 arranged in a circle

Question 23

Describe the subunit structure of F1

Answer 23

F1 comprises five different types of subunit: alpha3, beta3, gamma, delta, and epsilon

Forms a complex of 9 subunits

Question 24

What is special about the 3 beta subunits of F1?

Answer 24

The 3 beta subunits have catalytic sites for ATP synthesis

Question 25

Describe the structure of ATP synthase

Answer 25

• beta subunits are arranged alternately with alpha subunits like segments of an orange
• Form a knob-like structure held by a stalk of the gamma and epsilon subunits
• delta subunit interacts with the two ‘b’
  subunits of Fo

Question 26

Describe the theory of rotational catalysis

Answer 26

3 beta subunits take it in turns catalysing the synthesis of ATP
Any given beta subunit starts in a conformation for binding ADP and Pi
Then changes conformation so the active site now binds the product ATP tightly
Then changes conformation to give the active site a very low affinity for ATP (‘beta-empty’ conformation) so ATP is released

Question 27

What happens when the gamma unit rotates?

Answer 27

The gamma-unit rotates, and the properties of the beta-catalytic units change

Question 28

Is the reaction in the respiratory electron chain endergonic or exergonic?

Answer 28

Highly exergonic reaction

Question 29

Give the overall equation of the respiratory electron chain in terms of NADH

Answer 29

NADH + H+ + ½ O2 ——-> NAD+ + H2O

Question 30

Describe the energy changes in this reaction

Answer 30

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

Question 31

Describe uncoupling reagents and their purpose

Answer 31

Normally e- flow and phosphorylation are tightly coupled
Uncouplers dissipate the pH gradient by transporting H+ back into the matrix of the mitochondria so bypassing the ATP synthase
Thus an uncoupler (e.g. DNP) severs the link between e- flow and ATP synthesis, with the energy being released as heat
Can occur naturally e.g. UCP1 (thermogenin) is found in brown adipose tissue and has a specific H+ channel through which the [H+] may be dissipated - energy released as heat

Question 32

Describe brown adipose tissue

Answer 32

Brown adipose tissue (BAT): 
Specialized for heat generation
High numbers of mitochondria
Mitochondria contain thermogenin (UCP-1)
Important in new-borns, possible role in obesity/diabetes

Question 33

What is the energy yield from oxidation of 1 molecule of glucose

Answer 33

30 or 32

Depending on the shuttle G-3 P or malate-aspartate used

Question 34

What is DNP?

Answer 34

Weak acid that crosses membranes ‘ferrying’ H+ across
Each DNP molecule collects a proton from the IMS and moves through the membrane with it, depositing it in the matrix
Can then return though the membrane to collect another proton