Oxidative Phosphorylation and the Electron Transport Chain

what three types of electron transfers do electron carriers facilitate

1. DIrect transfer as in the reduction of Fe3+ to Fe2+

2. Transfer of a hydrogen atom

3. Transfer of a hydride ion, bearing two electrons

NADH

transfers a hydride ion which bears two electrons

FMN and FAD

can accept 2 electrons and 2H+

FMN (oxidized) —> semiquinone intermediate —> FMNH2 (reduced)

flavin nucleotides can accept a hydride from NADH

iron clusters

cysteine sulfur + inorganic sulfur: carry one electron

cytochromes

contain tightly bound heme that carries one electron

type A heme

hydrophobic tail

type B heme

bare bone structure

type C heme

protein link via Cysteine sulfides

ubiquinone

lipid soluble, carries two electrons, one at a time through the membrane

Q (oxidized) - ubiquinone—> semiquinone intermediate —> QH2 (reduced - ubiquinol

reduction potential

the tendency of an oxidized molecule to be reduced by gaining an electron

• O2 readily accepts electrons from all other conjugate redox pairs

• More positive E = better electron acceptor = stronger oxidant

calculating change in reduction potential: delta E = (E electron acceptor)-(E electron donor)

relationship between E and Gibbs

a more positive change in reduction potential means a more negative change in free energy

delta G = -nF(deltaE)

complex I

FMN accepts 2 e from NADH and Fe-S centers pass 1 electron at a time toward Q binding site

• Complex I is a proton pump: 4H+ are pumped per NADH through proton transfer using amino acids on opposite sides of the membrane

• The series of amino acids undergo protonation and deprotonation to the intermembrane space

Overall reaction for complex I: NADH + Q + 5H+N —> NAD+ + QH2 + 4H+P

• N = negative side, the matrix

• P = positive side, the intermembrane space

how do electrons from succinate enter the ETC?

at complex II through FADH2

Complex II

FAD accepts 2 electrons from succinate, passed one at a time from Fe-S to Q

• FADH2 + Q —> FAD + QH2

• this is not a proton pump and doesnt contribute to the proton gradient

• It introduces electrons from the formation of fumarate to the QH2 pool

Complex III

uses 2 electrons from QH2 to reduce two cytochrome C molecules

the Q cycle

accommodates the swtich between the two electron carrier Q and the one electron carrier cytochrome C

N = negative side, the matrix

P = positive side, the intermembrane space

Complex IV

accepts electrons from cytochrome C and passes them to oxygen via copper

• CuA: ions accept 2 electrons from cytochrome C

• CuB: ions transfer 2 electrons to oxygen to form H2O

• 2 H+ picked up to make H2O

• 2 additional H+ pumped into the intermembrane space

what sets the energetics between electron carriers

differences in reduction potential

ATP synthase components

1. rotor: rotates as protons flow through, translates energy from PMF into conformational changes in headpiece (PMF = proton motive force)

2. headpiece: synthesizes ATP with 3 catalytic sites

3. stator: holds the headpiece in place so it doesnt turn with rotor

protons drive rotation of the c ring in response to the proton gradient

effect of rotation

rotation of gamma induces conformational changes in each of the catalytic sites

• each AB dimer exists in a different conformational state because of asymmetrical interactions with gamma

• B subunits: L binds ATP loosely, T binds ATP tightly, O doesnt bind ATP

ATP Synthesis is the reason that there are 2.5

ATP from NADH and 1.5 ATP from FADH2

• Hydrogen carriers donate high energy electrons to the electron transport chain (located on the cristae)

• As the electrons move through the chain they lose energy, which is transferred to the electron carriers

within the chain (complexes I, III, and IV)

• The electron carriers use this energy to pump hydrogen ions from the matrix and into the

intermembrane space

• The accumulation of H+ ions in the intermembrane space creates an electrochemical gradient (or a

proton motive force). 1 proton is needed for the transport of inorganic phosphate to the mitochondria

• H+ ions return to the matrix via the transmembrane enzyme ATP synthase (this diffusion of ions is called

chemiosmosis)

• As the ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP

(from ADP + Pi). 3 protons are needed for ATP synthesis plus 1 for Pi so 10 protons from NADH divided

by 4 equals 2.5 ATP for every 1 NADH. 6 protons for FADH2 yields 1.5 ATP for every FADH2

• The de-energised electrons are removed from the chain by oxygen, allowing new high energy electrons

to enter the chain

• Oxygen also binds matrix protons to form water – this maintains the hydrogen gradient by removing H+

ions from the matrix

Biomolecules needed for oxidative phosphorylation

must be shuttled across the membrane

malate-aspartate shuttle, G3P shuttle

• the malate aspartate shuttle moves electrons from cytosolic NADH to mitochondrial NAD+

• the G3P shuttle transfers electrons from cytosolic NADH to ubiquinone

phosphate and ATP/ADP translocases

provide substrates for ATP synthase and export ATP