Electron Transport Chain

inputs into the ETC

NADH, FADH2, O2

main goal of ETC

generate a proton gradient to power ATP synthase

movement of electrons down the ETC

they move from lower to higher reduction potential (thermodynamically favorable), which releases energy that pushes protons across membrane to create gradient, until reaching O2 (final electron acceptor)

reduction potential

measures a molecule’s affinity for electrons

• more negative = more likely to donate electrons

• more positive = more likely to accept electrons

molecule with low reduction potential (strong donor)

NADH

molecule with high reduction potential (strong acceptor)

O2

complex I name

NADH-Q dehydrogenase (or NADH-Q oxidoreductase)

complex II name

succinate dehydrogenase (or succinate-Q reductase)

complex III name

cytochrome bc1 complex (or Q-cytochrome c oxidoreductase)

complex IV name

cytochrome c oxidase

mobile electron carriers

ubiquinone (Q) and cytochrome c (c)

NADH electron entry point

complex I

FADH2 electron entry point

complex II

paths electrons can take through the ETC

1. I → Q → III → c → IV

2. II → Q → III → c → IV

ubiquinone role

move electrons from complex I/II → III

cytochrome c role

move electrons from III → IV

final step of the ETC

electrons are transferred to O2, reducing it to H2O

how complex I generates proton gradient

passing of electrons causes conformational changes which drive proton translocation (piece that sticks into matrix folds up toward inner membrane, pushing its protons across the membrane)

effect of rotenone

it inhibits complex I, physically blocking the electron transfer to ubiquinone

how complex II generates proton gradient

it doesn’t pump protons, it just transfers electrons to complexes that do

ubiquinone carries __ electron(s)

2

cytochrome c carries __ electron(s)

1

Q cycle

process that maintains continuous electron flow and transfer between unequal carriers

how the Q cycle maintains continuous electron flow

by using the Q pool:

• electron 1 → cytochrome c

• electron 2 → reduces ubiquinone to regenerate ubiquinol

how is the proton gradient formed

complexes I, III, IV use energy released from oxidation of carriers to pump H+

where is the proton gradient formed

in the intermembrane space by protons pumped from the matrix (matrix → intermembrane)

what type of gradient is the proton gradient

electrochemical (concentration and electrical)

outcome of the proton gradient

free energy is stored as the proton motive force, which drives ATP synthesis

structural domains of ATP synthase

F₀ (membrane embedded) and F₁ (matrix-facing catalytic domain)

role of the F₀ domain of ATP synthase

forms a proton pathway with the a subunit and c-ring, which rotates as protons pass through ( a subunit does not rotate, only c-ring)

role of the F₁ domain of ATP synthase

have conformational changes induced by γ subunit rotation, which allows ATP phosphorylation to occur

γ subunit of ATP synthase

shaft that rotates as the c-ring rotates, causing conformational changes in the α3β3 hexamer

α3β3 hexamer structure

alternating 3 α and 3 β subunits, where the β subunits act as catalytic sites as the conformation of the hexamer changes due to γ rotation

direction of γ subunit rotation

counterclockwise

role of b₂ subunit

restrict α3β3 hexamer movement (should stay still as γ rotates)

functional states of ATP synthase hexamer

O (open) → L (loose) → T (tight)

each of the 3 β subunits is always in one of these states

open state function

β subunit binds ADP and P_i

loose state function

β subunit holds substrates

tight state function

catalyzes ATP phosphorylation

energy from the proton motive force drives _____ _____, not _____ _____

conformational changes; bond formation