01:694:301 Lecture Notes - Lecture 18: Oxidative Phosphorylation, Electromotive Force, Intermembrane Space

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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
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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-
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