lec. 18 electron transport chain

electron transfer happens from more ___ reduction potential to ____ reduction potential

lower, higher

electron transfer happens from the ___ to the __

reductant to the oxidant

• reductant = donor of electrons = lower reduction potential

• oxidant = accepter of electrons = higher reduction potential

in redox half-reactions, a more positive standard reduction potential means the oxidized form has a higher affinity for electrons

delta E = E acceptor - E donor

coenzyme Q (ubiquinone)

• hydrophobic tail → can move around inner mitochondrial membrane freely

• forms QH aka semiquinone when only 1 electron reduction

• electron carrier

iron-sulfur clusters

• electron carrier

• 4 cysteine residues

• iron business end

heme and cytochromes

• coordinated by central iron ion and pophryin ring or wtv

• cytochromes are electron transfer proteins that carry hemes

• 1 electron carrier

cytochrome C

• small, water soluble, heme containing

• move along intermembrane surface to inner mitochondrial membrane

• transfers electron from complexes III and IV

Complex IV

copper centers and 1 electron carrier

Complex I couples electron transfer to proton pumping (matrix → intermembrane) (4H)

• every 2 electrons from NADH from matrix side to CoQ → 4 protons pumped out to intermembrane space

Complex III: CoQ → Cytochrome C (oxidoreductase) (4 H)

• 2e electrons from fully reduced QH2 → 2 cytochrome C molecules (1 e per cytC)

  • transfer of the second electron is unfavorable

• net translocation of 4 protons to intermembrane

• inhibited by antibiotic antimycin A

Complex III’s Q Cycle

• 2 Q binding sites on complex III

  • primary site → QH2 (reduced)

  • secondary site → Q (oxidized)

• first turnover : 1 electron reduces Cyt C at Q site to form semiquinone at secondary site

• second turnover: 1 electron reduces second Cyt C at the primary site, and the other reduces semiquinone at secondary site

• NET: one QH2 is oxidized, 2 Cyt C molecules are reduced, 4 protons released to intermembrane space, 2 protons taken up from matrix

Complex IV: Cytochrome C to molecular oxygen (oxidase)

• cyt c oxidized by molecular oxygen

• 2 hemes (a and a3) and 2 copper centers

  • copper a accepts electrons from cyt c

  • coper a3: forms binuclear center where O2 binds and is reduced

• inhibited by cyanide — entire transport chain shut off

electron flow from cytochrome C to molecular oxygen

• complex 4 accepts 4 electrons from 4 reduced Cyt Cs and reduces one molecular oxygen to two h2o molecules

• process:

  • 2 electrons flow from 2 cytochrome C to copper center a and heme a → reduces heme a3 and copper b

  • molecular oxygen bines at reduced heme a3-copper a → form peroxide level intermediate

  • 2 electrons and 2matrix protons promote cleavage of O-O bond

  • uptake of two more protons → release 2 water molecules

proton transport by complex 4

• 4 protons are picked up from the matrix, producing 2 water molecules

• 4 protons go to intermembrane space

• oxidizing NADH is a 2 electron process → 2 NADH to fully reduce O2

  • complex 4 pumps 2 protons per electron pair and consumes 2 additional matrix protons to form H2O

complex II

• succinate → FAD → CoQ

• no protons pass thru membrane

electron transport chain summary

• oxidation of NADH

  • complex I = 4 H

  • complex II = 0

  • complex 3 = 4

  • complex 4 = 2

  • half of molecular oxygen equals 1 h2O molecule

  • 10 protons total to intermembrane space

• oxidation of FADH2

  • complex III: 4 H

  • complex IV: 2H

  • 6 protons and one water pumped out

glycerol 3-phosphate shuttle

• solves the problem when NADH is formed in the cytosol

• the 2 electrons reduce hydroxyacetone phosphate → glycerol 3-phosphate

• glycerol 3-phosphate then reoxidizes back to dihodroxyacetone phosphate via mitochondrial glycerol 3-phosphate dehydrogenase

  • IN inner mitochondrial membrane

    • transfers electrons to enzyme-bound FAD

• bypasses complex I which means there are less protons pumped across the inner mitochondrial membrane

• skeletal muscle and brain

cytoplasmic NADH: the malate-aspartate shuttlle

• solves for the same problem as above

• heart and liver

• NADH reduces oxaloacetate to malate

• malate enteres matric through malata alpha ketoglutarate antiporter

• in matrix, the malate is oxidized, causes NAD to turn to NADH

remainings rxns turn to carbon shuttle of oxaloacetate to the cytosol as aspartat

enters the chain like normal then