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Lecture 13 Electron Transport. Reduced High energy. Oxidized Low energy. Glucose + 02--- CO2 + H20 . Mitochondria. The pumps. Overview electron transport. ADP. ATP.
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Reduced High energy Oxidized Low energy Glucose + 02--- CO2 + H20
Overview electron transport ADP ATP 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. separation of protons and electrons of hydrogen Passage of electrons down cascade-four membrane protein complexes (proton pumps expel protons) Electrons travel down chain and are picked up by oxygen and combine with proton forming H2O 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
Summary 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) Complex2 accepts hydrogen from succinate and transfers electrons to ubiquinone (CoenzyQ) 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). 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 negative Cofactors that bind/release electrons Flavins Iron-sulfur cluster (Fe held by 4 sulfur) Porphyrin (heme- Fe2+ to Fe3+) Copper ions Two electron carriers are mobile A Ubiquinone (CoenzymeQ)- carries electrons as hydrogen and moves within inner membrane. Transports electrons from complex1 and 2 to complex3 B 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 DE0’ NAD+ -0.32V FMN (complex1) -0.30V Fe-S (complex1) -0.03V FAD (complex2) -0.04V Ubiquinone (carrier) 0.04V Heme (complex3) 0.235V CytochromeC (carrier) 0.25V Heme (complex4) 0.38V Oxygen 0.82V
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.
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/mol Total protonmotive force is 21kJ/mol
ATP synthase Mito matrix Cytoplasm
Protons flow between a and F0 subunit (with 10 c chains). 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 g subunit rubs against ab hexamer ab hexamer that rub undergo conformational change b subunit contains active site that binds ADP+Pi b subunit utilizes energy transmitted from conformational change to synthesize ATP b subunit gets everted and ATP is released.
