Electron transport chain

Final stage of aerobic oxidation! _________________________________

• Also known as:

-oxidative phosphorylation (when coupled to ATP synthase)

-respiration (when coupled to ATP synthase)

• Purpose:

-Recycle reduced molecules

-Convert the energy gained from ________________ into ATP

• This occurs by:

-Oxidizing NADH and FADH2

-The e- gained from the above reactions are transported through a membrane bound electron

transport system

-This generates a membrane gradient (potential)

-This potential energy allows for the phosphorylation of ADP (to generate ATP)

Recycling

The amount of cellular adenine is constant.

- It exists as either ATP, ADP, or AMP (the concentration of these vary)

- But [Adenine] remains constant.- But [Adenine] remains constant.

- So we recycle the adenine to whatever energy we need.

- Table 10.2

Same is true for NAD and FAD; constant amount

-It exist as either NAD or NADH (FAD or FADH2)

-But [FAD] or [NAD] remains constant

- So we recycle the NAD and FAD to the desired form.

glycolysis:

glucose + 2NAD+ + 2 Pi + 2 ADP

2 pyruvate + 2 ATP + 2 NADH + H+

Pyruvate dehydrogenase & TCA:

pyruvate + 4 NAD+ + FAD + GDP + 2 H2O

3 CO2 + 4 NADH + 4 H+ + Pi + GTP + FADH2

NAD+ and FAD are in short supply, so we need to recycle. A process has developed to generate

energy from this recycling!

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• We have covered much of the background

14.2: ETC occurs in mitochondria

Know details about mitochondria

ETC proteins are present in the inner mitochondrial membrane

Transport occurs from the matrix to the intermembrane space

14.3: Membrane gradients

Just remember to switch equation to [from]/[to]

May use ‘ln’ as we have done,

or as book does, ‘log’, by putting constant of 2.303

into our equation

Use of log allows easier movement to pH, which is a

measure of H+

We may discuss decouplers later

The ETC is:

• A complex of integral membrane proteins, located in the inner mitochondrial membrane

• These proteins oxidize NADH and FADH2 by passing electrons on to O2, leading to the

production of H2O

• The energy from oxidation creates a proton gradient (protons pumped from the matrix to the • The energy from oxidation creates a proton gradient (protons pumped from the matrix to the

intermembrane space)

• This gradient provides energy for production of ATP from ADP (ATP synthase)

NADH + H+ + ½ O2 + ADP + Pi NAD+ + H2O + ATP

FADH2 + ½ O2 + ADP + Pi FAD + H2O + ATP

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This whole system is a _______________________

Electrons move from a reducing agent to an oxidizing agent

(thus a series of redox reactions)

NADH is the strongest reducing agent in biochemistryNADH is the strongest reducing agent in biochemistry

O2 is the strongest oxidizing agent in biochemistry

Electrons flow from a negative voltage to a positive voltage

(high energy) (low energy)

This flow of e- is spontaneous and thermodynamically favorable!

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ETC components

The ETC is composed of 4 complexes and 2 mobile electron carriers.

Complexes:

Complex I

Complex II

Complex III

Complex IV

Carriers:

Ubiquinone (Q)

Cytochrome c

Chain:

I or II Q III cyto c IV O2 H2O

See why FADH2 produces less energy??

Complex I

• NADH donates its electrons to Complex I

• Complex I has 34-46 subunits!

• NADH-ubiquinone oxidoreductase

• Contains FMN and proteins with Fe-S clusters

This is where redox occurs

• NADH transfers _______________to FMN

• FMN transfers e- to Fe-S clusters, releases H+ to matrix

• Fe-S transfers e- to coenzyme Q

• ~ 4 H+ are transported for every 2 electrons transferred to Q

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Complex II

• FADH2 donates its electrons to Complex II

• Complex II has 3 multisubunit enzymes

• Succinate dehydrogenase

• Contains FAD and proteins with Fe-S clusters

This is where redox occurs

• FADH2 transfers _____________________

• FAD transfers e- to Fe-S clusters, releases H+ to matrix

• Fe-S transfers e- to coenzyme Q

• Complex II does not contribute to H+ transport across membrane!

Ubiquinone (Coenzyme Q)

• Not a protein!

• Membrane soluble, low molecular weight

molecule

• Contains a long hydrophobic tail that keeps

Q in the inner mitochondrial membrane

• Accepts e- one at a time (not as a hydride)

• Shuttles e- from complex I OR complex II to

complex III

Complex III

• QH2 donates its electrons to Complex III

• Complex III has 3 main subunits

• Ubiquinol-cytochrome c oxidoreductase

• Contains several cytochromes and Fe-S clusters

This is where redox occurs

• QH2 transfers _______________________

• Complex III transfers e- to cytochrome c

• ~ 4 H+ are transported for every 1 QH2 oxidized

(2 from matrix & 2 from QH2)

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Cytochrome

Cytochrome c

• Peripheral membrane protein

• Shuttles e- (and H+) from Complex III to Complex IV

• Another heme containing protein

Complex IV

• Cytochrome c donates e to Complex IV

• Cytochrome c oxidase

• 10 subunits

• Contains 2 cytochromes (a & a3) and proteins

with Cu or Fe

This is where redox occurs

• Transfers 4 electrons to O2, to reduce O2 to H2O

• 4 H+ are transported for every O2 reduced (2 e / O

atom)atom)

BUT it takes 2 NADH to reduce 1 O2.

So for every NADH, we only get 2 H+

transported at this step

Summary of ETC

1. NADH is oxidized by Complex

I (Complex I is reduced)

2. Complex I is oxidized by Q (Q

is reduced to QH2)

1. FADH2 is oxidized by Complex

II (Complex II is reduced)

2. Complex II is oxidized by Q (Q

is reduced to QH2)

1. Succinate is oxidized by Complex

II (Complex II is reduced)

2. Complex II is oxidized by Q (Q is

reduced to QH2)

1. QH2 is oxidized by Complex III (complex III is reduced)

2. Complex III is oxidized by cytochrome c (cytochrome c is reduced)

1. Cytochrome c is oxidized by Complex IV (Complex IV is reduced)

2. Complex IV is oxidized by O2 (O2 is reduced to form H2O)

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Generation of a membrane gradient

1. Oxidation of NADH, succinate, and FADH2 produces energy that allows H+ to be pumped

across the membrane

2. This generates an electrical AND a chemical (concentration) gradient

(electrochemical gradient)

3. End result is the matrix is negative, while the intermembrane space is positive

Linkage of gradient to ATP production

• Chemiosmotic hypothesis (Nobel Prize, 1978)

• The proton gradient IS the energy source for ATP generation

• The proton gradient is also referred to as the protonmotice force (∆p)

• H+ flow out of the matrix due to the ETC

• H+ flow back in via ATP synthase

ATP synthesis is driven by the H+ gradient

This is how NADH and FADH2 provide additional ATP

How does cytoplasmic NADH get into the mitochondria?

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