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Chapter 19 Oxidative Phosphorylation and Photophosphorylation Part I) Electron Transport. From Garrett & Grisham. Electrons transferred via NADPH. Electrons transferred via NADPH. {80% proteins}. {30-40% lipids & 60-70% proteins}. Provide inner membrane with large surface area.

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Chapter 19

Oxidative Phosphorylation and

Photophosphorylation

Part I) Electron Transport



Electrons transferred via NADPH

Electrons transferred via NADPH


{80% proteins}

{30-40% lipids & 60-70% proteins}

Provide inner membrane with large surface area

Intermembrane space

  • Outer Membrane

  • Contains porin

  • Allows free diffusion of molecules

  • with molecular weight less than

  • 10,000

Porins are transmembrane channels for small molecules

  • Inner Membrane

  • Impermeable to molecules & ions


  • Matrix

  • contains all of TCA cycle enzymes {except, succinate dehydrogenase which is located in the inner membrane}

  • contains circular DNA, ribosomes and enzymes required to synthesize proteins encoded within the mitochondrial genome

From Lehninger

Principles of Biochemistry


Separation of functional complexes of the respiratory chain

Components of the electron transport chain can be purified from the mitochondrial inner membrane

From Lehninger

Principles of Biochemistry


Electron Transport

Link between glycolysis, TCA cycle, fatty acid oxidation and electron transport chain

Direct link between TCA cycle and electron transport chain

  • e- carried by reduced coenzymes are passed through a chain of proteins and coenzymes to drive the generation of a proton gradient across the inner mitochondrial membrane



From Lehninger

Principles of Biochemistry


Electron transfer from NADH to O2 involves multi-subunit inner membrane complexes I, III, & IV, plus coenzyme Q and cytochrome c.

Within each complex, electrons pass sequentially through a series of electron carriers.


The electron transport chain

NADH

FADH2

Fe.S

FMN

Fe.S

Q

Cyt b

Fe.S

Cyt c1

Free Energy Relative to O2 (kcal / mol)

Cyt c

Cyt a

Cyt a3

O2

NADH (reductant) + H+ + ½ O2 (oxidant) NAD+ + H2O

Electrons generally fall in energy through the chain - from complexes I and II to complex IV


Redox reactions are among a cell's most important enzyme-catalyzed reactions.

Oxidation and reduction refer to the transfer of one or more electrons from a donor to an acceptor, generally of another chemical species.

The donor is oxidized, the acceptor reduced.


Oxidative Phosphorylation enzyme-catalyzed reactions.

The proton gradient runs downhill to drive the synthesis of ATP


Electron Carriers: enzyme-catalyzed reactions.

NAD+/NADH and FAD/FADH2 were introduced earlier

  • FMN (Flavin Mono Nucleotide) is a prosthetic group of some flavoproteins{It is similar in structure to FAD, but lacks the adenine nucleotide}

  • In solution FMN (like FAD) can accept 2 e- + 2 H+ to yield FMNH2

  • When bound at the active site of some enzymes, FMN can accept 1 e-, converting it to the half-reduced semiquinone radical. The semiquinone can accept a second e- to yield FMNH2


Prosthetic groups of cytochromes enzyme-catalyzed reactions.

  • Heme  is a prosthetic group of cytochromes

  • Mitochondria has 3 classes of cytochromes, designated a, b, and c

Found in Hb

The heme iron atom can undergo a 1 electron transition between ferric and ferrous states:       Fe3+ + e- <--> Fe2+

From Lehninger

Principles of Biochemistry


Structure of mitochondrial cytochrome enzyme-catalyzed reactions. c

Heme is covalently linked to the protein via S atoms

From Garrett & Grisham


Iron-sulfur centers (Fe-S) enzyme-catalyzed reactions.

  • - electron transfer proteins may contain multiple iron-sulfur centers.

  • transfer only one electron even if they contain two or more iron atoms, because of the close proximity of the iron atoms. 

  • a 4-Fe center might cycle between the redox states: Fe3+3, Fe2+1 (oxidized) + 1 e-<-->Fe3+2, Fe2+2( reduced)


Iron-sulfur centers enzyme-catalyzed reactions.

From Lehninger

Principles of Biochemistry


Ferredoxin of the cyanobacterium Anabaena 7120 enzyme-catalyzed reactions.

2Fe-2S center

Fe

S

From Lehninger

Principles of Biochemistry



Ubiquinone (Q or coenzyme Q) converts between Cu

From Lehninger

Principles of Biochemistry


A lipid soluble coenzyme (UQ) shuttle between protein complexes

Hydrophobic tail allows it to diffuse freely in the hydrophobic core of the inner mitochiondrial membrane

Mobile electron carrier


Complex I complexes


NADH dehydrogenase aka complexes

NADH-Coenzyme Q reductase

o

NADH 2e- donor

FMN 1 or 2 e- donor

Fe-S clusters 1 e- donor

  • Estimated mass of this complex 850 kD

  • Involves more than 30 polypeptide chains

  • One molecule of FMN

  • As many as 7 Fe-S clusters (2Fe-2S & 4Fe-4S)

  • Precise mechanism of this complex is unknown

From Lehninger

Principles of Biochemistry


Complex I complexes

o

r

o

r

o

r

o


Inhibitors of complex I complexes

  • Rotenone is a common insecticide that inhibits complex I

  • Rotenone is obtained from the roots of several species of plants

  • Rats exposed to rotenone over a period of weeks develop symptoms of Parkinson’s disease

  • Appear to inhibits reduction of Q and the oxidation of Fe-S clusters of complex I

    {Painkiller Demerol also exert inhibitory actions on this complex}



Complex II complexes


Complex II complexes

H+ transport does not occur in this complex

Succinate-CoQ Reductase

aka

Succinate dehydrogenase

(from TCA cycle!)

o

  • Succinate

  • Fumarate

r

r

FAD

Succinate dehydrogenase

o

FADH2

r

o

  • Mass of 100 – 140 kD

  • Composed of 4 subunits, including 2 Fe-S proteins

  • Three types of Fe-S cluster: 4Fe-4S, 3Fe-4S, 2Fe-2S

  • Path: Succinate FADH2 2Fe2+ UQH2


Inhibitors of complex II complexes

2-Thenoyltrifluoroacetone & carboxin block complex II

Inhibitors of complex II


Electron Transport complexes

From Lehninger

Principles of Biochemistry


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