Lecture 13Electron Transport

ReducedHigh energy

OxidizedLow energy

Glucose + 02--- CO2 + H20

Mitochondria

The pumps

Overview electron transport

Working of respiratory chain is different from enzymatic reactions where energy is passed down from one energy rich bond to another. Here chemical reaction is coupled to physical force.

1 separation of protons and electrons of hydrogen2 Passage of electrons down cascade-four membrane protein complexes (proton pumps expel

protons)3 Electrons travel down chain and are picked up by oxygen and combine with proton forming

H2O4 Proton accumulate outside mito and are pumped back through inner membrane-mol motor

rotates by flow of protons driving synthesis of ATP from ADP+Pi

ADP ATP

Summary

1 Complex1 accepts H+ from NADH2 converting it back to NAD. Electrons are transferred to carrier-ubiquinone. 2 Hydrogen protons + 2 additional protons are expelled/electron (Free energy of electron transfer is used)

2 Complex2 accepts hydrogen from succinate and transfers electrons to ubiquinone (CoenzyQ)

3 Complex3 re-oxidizes CoenzyQ and expels four protons/electron. Electrons are transferred to carrier CytochromeC (Free energy of electron transfer is used to expel proton).

4 Complex4 re-oxidizes CytC and electrons are transferred to oxygen. Free energy of electron transfer is used to expel 4 protons

Electrons do not occur in free form. Proteins provide grps that bind and release electrons. These are in close physical proximity.

Successive transfer of electrons must be exergonic- free energy must be negativeCofactors that bind/release electrons1 Flavins2 Iron-sulfur cluster (Fe held by 4 sulfur)3 Porphyrin (heme- Fe2+ to Fe3+)4 Copper ions

Two electron carriers are mobileA Ubiquinone (CoenzymeQ)- carries electrons as hydrogen and moves within inner

membrane. Transports electrons from complex1 and 2 to complex3B CytochromeC (contains Heme) located between inner and outer membrane. Transports

electrons from complex3 tocomplex4

Std reduction potential

Free energy of electron movement from one cofactor to next must be negative.Cofactors down the chain have higher affinity for electrons- they have higher redox potential.

Redox Carrier E0’

NAD+ -0.32VFMN (complex1) -0.30VFe-S (complex1) -0.03VFAD (complex2) -0.04VUbiquinone (carrier) 0.04VHeme (complex3) 0.235VCytochromeC (carrier) 0.25VHeme (complex4) 0.38VOxygen 0.82V

Complex 1-4

The gradient

Path of electrons

Electrons are stripped of their protons, the protons are released at cytosolic side. This accounts for some of the proton translocation.The mechanism by which the additional protons are expelled from the mitochondria is not known.Possible that binding and release of electrons causes conformational changes in protein complexes (analogous to allosteric change) and these may help expel protons.

Path of electrons 1-2

Path of electrons 3

Path of electrons 4

Protonmotive force

Cytosol Mitochondrial matrix

ATP is synthesized AFTER substrate is vanished in form of CO2 and H2O. The energy is stored in PROTON MOTIVE force. Proton conc in cytosol is 10 times higher than in mitochondrial matrix. Proton moving down the gradient generates free energy (6kJ/mol)Each proton ejected leaves one negative charge inside mitochondria- generates electrostatic membrane potential- 150mV negative inside=15kJ/molTotal protonmotive force is 21kJ/mol

H+ pump

ATP synthase

Mito matrix

Cytoplasm

1 Protons flow between a and F0 subunit (with 10 c chains).

2 This causes F0 subunit to rotate (aspartate in c chain in F0 interacts with proton and undergo conformational change resulting in rotation). The proton is translocated and released into mitochondrial matrix

3 subunit rubs against hexamer

4 hexamer that rub undergo conformational change

5 subunit contains active site that binds ADP+Pi

6 subunit utilizes energy transmitted from conformational change to synthesize ATP

7 subunit gets everted and ATP is released.

ADP to ATP

ATP transport