01:694:301 Lecture Notes - Lecture 18: Oxidative Phosphorylation, Electromotive Force, Intermembrane Space
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14 Nov 2018
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Chapter 18 – ‘Oxidative Phosphorylation’
• Understand the overview (25-8). This chapter is about energetics in the mitochondrion, but
mitochondria re symbiotic bacteria, and parallel processes are found everywhere in
prokaryotic cells
o Oxidative phosphorylation – the recycling of ADP to ATP for energy
! Redox reactions allows flow of e- from NADH and FADH2 to O2 (reduced to water)
! Electron flow takes place in four large protein complexes embedded in mito
membrane and make up ETC
! Three complexes use energy by e- flow to pump protons from mito matrix to cyto
• Results in unequal distribution of H+ " pH gradient /electric mem. potent formed
• ATP synthesized when protons flow back to matrix through enzyme complex
! Overall rxn is exergonic
o Collectively, CAC + ETC + synthesis of ATP " cellular respiration
• Mitochondria is where oxy-phos takes place
o Bounded by double membranes
! Outer; Inner (highly folded ridges " cristae (more SA))
• Oxy-phos takes place in inner membrane which has two sides:
o Membrane potential (cyto side + and matrix –)
! Two compartments:
• Intermembrane space
• Matrix – site most CAC and fatty acid oxy rxns
o In prokaryotes, ETC pumps and complexes found in cyto membrane (inner of 2)
o Possibly created by
endosymbiotic event
(large cell engulfs small cell); have their own DNA
!
Rickettsia
– most mitochondrial-like bacterial genome
• You should be able to calculate the STD free energy given half-reaction potentials (Table 18.1)
o
Electron-transfer potential of NADH and FADH2
is converted to
phosphoryl-transfer
potential
of ATP
o Electron-transfer potential represented " E’0
o Redox couple X:X— (oxy:reduc)
! Reduction potential can be determined by
measuring electromotive force in
sample half-cell
o Strong reducing agent (such as NADH) " donate e-
"
negative reduction potential
! Oxidized form has lower affinity e- than H2
o Strong oxidizing agent (such as O2) is " accept
electrons "
positive reduction potential
! Oxidized form has higher affinity e- than H2
• Electron transport chain (ETC) directly produces a proton gradient
o The protons flow back in through the mitochondrial inner
membrane, driving protein-based machinery, which causes ATP to
be synthesized
o The electrons come from NADH and FADH2 in the matrix, and the
ATP is also formed in the matrix
• Electrons move from higher energy (top of the table) to lower energy in
the ETC and divide them into four complexes.
o LEARN this linear version:
o KNOW the components of each complex
! Complex I, II, III assoc. to form supramolecular complex --
respirasome
o Understand that Q and c are ‘mobile’ electron carries that pick up electrons from
complex and deliver them to the next
! Q (ubiquinone) – hydrophobic, diffuses rapidly inner mito membrane
• Coenzyme Q – quinone derivative with long tail consisting of 5 isoprene units
(hence why it’s hydrophobic); most (mammals) common coenzyme Q10 (10 units)
! Cytochrome c – small, soluble protein that shuttles e- from Complex III to IV, where is
what catalyzes reduc of O2
• Complex I (NADH-Q oxidoreductase)
o Flow of 2e- from NADH to coenzyme Q through complex I "
pumping 4 H+ out of matrix
o In accepting 2 e- Q takes up 2 protons from matrix " QH2 (reduced)
• Complex II (Succinate-Q reductase)
o FADH2 enters through this complex; succinate DH is present as
integral protein of inner mito membrane
o FADH2 does not leave complex
! Instead e- are transfer to Fe-S clusters " Q " QH2
o Note that Complex II does not pump protons from one side of
membrane to other (unlike other complexes); hence less ATP
formed by oxidation FADH2
• Complex III (Cytochrome c oxidoreductase) (Q CYCLE)*
o This complex catalyzes the transfer of e- from QH2 " to oxidized Cytochrome c and pump
H+ out of matrix
o Flow of e- leads to net transport " 2 H+ to cyto
o Contains two types of cytochromes:
b
and
c1
! Cytochromes are electron transferring proteins that contain heme prosthetic groups,
(these are newer and lower in energy) (KNOW) (same heme group Hb and Mb)
! Fe ion of cytochromes alternates b/w Fe2+ (reduced) and Fe3+ (oxidized) states
! Together they have 3 heme groups: two in (
b
) and one (
c1
)
•
b
- has bL (low affinity e-) and bH (high affinity e-) (due to different env.)
o *Also contains (2)Fe-(2)S cluster known as
Rieske center
; has two H residues instead 4 C
! This stabilizes center in reduced form raising reduc potential " can accept QH2 e-