Electron Transport & Coupled Oxidative Phosphorylation

Electron Transport & Coupled Oxidative Phosphorylation

1. Mitochondrial

   • Structure
     • Inner membrane
       • 1. 75% protein by mass 2. Freely permeable: O2, CO2, H2O 3. Numerous transport proteins control passage of ATP, ADP, pyruvate, Ca+2 phosphate
       • Divides Mitochondria into 2 spaces
         • 1. Intermembrane Space
         • 2. Matrix
       • ADP-ATP Translocator
         • Exchanger. Transports ATP out of matrix in exchange for ADP produced in cytosol
         • 1 binding site, 2 conformations
           • 1. ATP & ADP compete for binding site 2. 2 comformations. 1 with binding site facing out vs. in 3. Ligand binds, conformation changes
       • Electron Carriers
         • 1. Move NADH & FADH2 associated with inner membrane 2. Exergonic process
         • Electron Transport Chain
           • 1. Oxidation of NADH & FADH2 carried out by electron transport chain
           • Protein Complexes Containing redox centers
             • 1.Progressively greater electron affinities. 2. Electrons move from progressively lower to higher standard reduction potentials
             • Properties
               • 1. Several protein components assoc. with variety of redox prosthetic groups with increasing reduction potentials. 2. Complexes are laterally mobile within inner mitochondrial membrane
           • Mechanism
             • CoQ carries e for Complexes I, II, III
               • These complexes sufficient deltaG to power synthesis of ATP
               • Complex I: NADH-CoQ Oxidoreductase
                 • 1. Largest protein complex
                 • Oxidation of NADH by COQ. CoQ(red)
                   • 1. CoQ has long hydrophobic tail. Soluble in mitochondria's lipid membrane
                   • 2. Transit of e from NADH to CoQ unknown sequence
                   • Redox Centers
                     • 1. E "jumps" between centers 2. Transfer rate depends on distance 3. NADH only participates in 2e transfers
                 • FMN: Redox Prosthetic Group
                   • 1. No AMP and presence of 6-7 Fe-S clusters that participate in e transport.
                   • Fe-S clusters
                     • 1. Most common [2Fe-2S] and [4Fe-4S] clusters. Attached to 4 protein Cys sulfhydryl group. tetrahedral.
                     • 2. Undergo one e oxidation/reduction 3. FMN & CoQ have 3 oxidation states; can accept/donate 1-2 electrons
                 • FMN & CoQ
                   • 1. Can do 1 or 2 electron transfers. Provide e conduit between 2e donor NADH and 1e acceptors, the cytochromes
                   • 2. Electrons transferred between redox centers in Complex I, 4 protons translocated from matrix to intermembrane space.
                   • Proton Pumping
                     • 1. Probably driven by conformational changes. Unknown. 2. "Proton Wire"
               • Complex II: Succinate-coenzyme Q oxidoreductase
                 • 1. DeltaG0 = -16.4 KJ/mol 2. Oxidation of FADH2 releases insufficient delta G to synthesize ATP 3. Only function is to inject electrons from FADH2 into ETC
                 • 4. Important because allows relatively "high energy" electrons to enter electron transport by bypassing Complex I
                 • Oxidation of FADH2 by CoQ
                   • 1. Contains TCA cycle enzyme succinate dehydrogenase which passes e's from succinate to CoQ
                   • Transfer of e's to CoQ from succinate
                 • Redox Groups
                   • FAD, one [4Fe-4S], [3Fe-4S], [2Fe-2S], one cytochrome b560
               • Complex III: Coenzyme Q Cytochrome c oxidoreductase
                 • Note: Requires 2 cytochrome
                 • Oxidation of CoQ(Red) by cytochrome c. Cytochrome c (red)
                   • Contains: 2 b-type cytochromes, 1 cytochrome c1, 1 [2Fe-2S] cluster
                 • CoQH2
                   • 1. Q cylce: Permits CoQH2 (a 2 e carrier) to reduce 2 cytochrome c (a 1 e carrier)
                   • 2. Complex III pumps protons from matrix to Intermembrane space
                   • Result
                     • When CoQH2 oxidized, 2 reduced cytochrome c molecules & 4 protons appear on outer side of membrane
               • CoQ
                 • 1. Diffused in lipid bilayer among resp. complexes 2.Collection point for e's. Ex. 1st step of fatty acid oxidation, FADH2 from glycerophosphate shuttle
             • Cytocchrome c transports e from Complex III to IV
               • Complex 4: Cytochrome c oxidase
                 • 1. Terminal electron transfer of chain. deltaG0= -112KJ/mol
                 • 2. 1e oxidations of 4 consecutive reduced cyctochrome c and concomitant 4e reduction of 1 O2
                 • Oxidation of reduced cytochrome c by O2
                   • Enter text here
                 • 4 redox centers
                   • Cytochrome a, a3, CuB, CuA
               • Cytochrome c
                 • 1. Peripheral membrane protein 2. Shuttles e's between complexes III & IV on outer surface of inner mitochondrial membrane
     • Outer Membrane
       • Porins
         • 1. Equivalent to cytosol in conc. of metabolites & ions
   • Contains
     • Pyruvate dehydrogenase, TCA enzymes, fatty acid oxidation enzymes, Electron transport/oxidative phosphorylation enzymes
   • Controlled Permeabilitiy
     • 1. Ion gradients across membrane 2. Compartmentalization
   • Mitochondrial Transport Systems
     • 1. Malate-Aspartate Shuttle (run in reverse)
       • Purpose
         • 1. Regenerate NADH inside mitochondrial membrane during glycolysis & translocate proteins across inner membrane (impermeable to NADH)
         • Outside inner membrane: NADH --> OAA --> Malate Inner membrane Malate --> OAA --> Aspartate (leaves) NAD+ --> NADH
     • 2. Glycerophosphate Shuttle
       • Purpose
         • 1. Regenerates NAD from NADH
     • 3. Phosphate Carrier
       • 1. ATP synthesized from ADP & Phosphate in mitochondria but utilized in cytosol 2. Phosphate returned to mitochondria. Phosphate/H symport
       • ATP synthesis
         • 1. Transmembrane proton gradient generated by electron transport of inner membrane. 2. Driving force for ATP synthesis & motivates transport of raw material-ADP & P

2. Overview

   • 12 e pairs released during glucose oxidation
     • 1. Transferred to NAD+ & FAD forming NADH & FADH2 in dehydrogenase rxns
     • ETC
       • 1. System of linked electron carriers 2. NADH & FADH2 reoxidized to NAD+ & FAD
       • Electrons participate in sequential redox centers in 4 enzyme complexes
         • 1. Reducing O2 to H2O
   • Oxidative Phosphorylation
     • Redox reactions release energy which is used to form ATP
       • 1. Highly efficient. 2. Mitochondria in cells Use their structure, enzymes, & energy released by the oxidation of nutrients to reform ATP
       • 3. Redox rxns carried out by a series of complexes in inner mitochondrial membrane
       • Electron Transport
         • Energy to transport protons across inner mitochondrial membrane
         • Forms a PH gradient w/potential energy
         • Chemiosmosis
           • Protons flow back through ATP synthase