Oxidative phosphorylation

Lecture 7

What happens when no ADP is present when mitochondria are placed in a buffer solution?

respire slowly to compensate for leakage of protons across the inner mitochondrial membrane

What happens when ADP is present when mitochondria are placed in a buffer solution?

respiration increases

mitochondria allow protons to flow across the mitochondrial membrane

What substances promote proton re-entry and what do they do?

ionophores & 2,4-dinitrophenol

short-circuit the proton motive force

oxygen can be consumed when no ATP is produced.

What are the parts of the electron transport chain?

Complex I, complex II, complex III, complex IV, complex V

Describe the movement of electrons through the electron transport chain.

QH₂ passes electrons to complex III

complex III passes to cytochrome c

cytochrome c transfers to complex IV

complex IV transfers to oxygen

What oxidising prosthetic groups are used by complex 1 & complex 2 respectively?

complex 1 - FMN

complex 2 - FAD

*NB - immobile prosthetic groups that must be recycle

What complexes have iron-sulfur clusters?

1, 2 & 3

*NB - they transfer single electrons despite variable numbers of Fe atoms.

How is Q reduced to QH₂?

Transports electrons from complex I & complex II to complex III

Also enables complex III to pump protons via the ‘Q cycle’

How & as what are cytochromes categorised?

a,b or c depending on the wavelength of light they absorb

What cytochromes are found in complex III?

b & c1

What cytochromes are found in complex IV?

a & a3

Where is cytochrome c found?

ferries electrons from complex III to complex IV

What is the structure & function of complex I?

uses NADH to reduce ubiquinone

NADH reduces FMN, electrons pass through FeS centres & reduce Q to QH₂

What is the structure & function of complex II?

FAD reduced by succinate to FADH₂

FeS centres pass electrons to Q to make QH₂

What is the structure & function of complex III?

FeS centre passes electrons to cytochrome c1

cytochromes c1 transfers electrons to cytochrome c

What is the structure & function of complex IV?

cytochrome C reduces O₂ to water

What are the 2 subunits of ATP synthase?

F_o & F₁

F₁ function

hydrolyses ATP (using water if detached from F_o)

Describe the function of the F_o complex.

forms a channel so the protons can return to the mitochondrial matrix

torque generated by flow of protons rotates the gamma subunit (stalk)

*NB - channel blocked by oligomycin.

What is the function of the gamma subunit (stalk)?

drives the binding change mechanism

3 protons translocated for every ATP produced.

Describe the structure of the F₀ complex.

8c subunits

What are the 3 types of site in ATP synthase?

O (open) - low affinity for ADP & P

L (loose) - binds ADP & P loosely

T (tight) - tight binding required to squeeze out water

What is step 1 in the use of ATP synthase?

ADP & P binds to L site

What is step 2 in the use of ATP synthase?

energy in to convert L to T

ADP + P_i —→ ATP + H₂O

What is step 3 in the use of ATP synthase?

energy in to convert T to O

ATP is released

What is the difference in how ATP & ADP and phosphate transport into and out of the mitochondria?

ATP & ADP exchange due to the charge difference

Phosphate enters due to the pH difference

How are partially reduced ubiquinone radicals produced?

rate of electron entry into chain is greater than the rate of electron transfer through the chain

What do ubiquinone radicals do?

donate an electron to oxygen to form superoxide

What does superoxide do?

Acts on citric acid enzyme aconitase to release Fe²⁺

Leads to the formation of OH free radical

Donor & acceptor of complex I

donor - NADH

acceptor - ubiquinone

Donor & acceptor of complex II

donor - succinate

acceptor - ubiquinone

Donor & acceptor of complex III

donor - reduced Q

acceptor - cytochrome C

Donor & acceptor of complex IV

donor - reduced cytochrome C

acceptor - O₂