BIOL2020 - Chapter 19 - Oxidative Phosphorylation

What is Oxidative Phosphorylation?

The process of passing high energy electrons in carriers along the mitochondrial electron transport chain, generating ATP.

What is the final electron acceptor of Oxidative Phosphorylation?

Oxygen (O₂).

What happens to electrons as they are passed through the Electron Transport Chain?

They loose energy.

Why is it possible for electrons to be passed from carrier to carrier in the Electron Transport Chain?

Each carrier has more affinity for electrons than the one before it.

Where to the NADH and FADH₂ entrance branches converge at?

Coenzyme Q.

Why is the energy released from NADH to O₂ in multiple steps?

Because a lot would be lost if it was released in 1 step.

How is the energetics of reduction reactions calculated?

With the standard free energy.

What is the standard free energy equation used to calculate reduction reactions in the Electron Transport Chain?

ΔG°’ = -nFΔE°’

n= number of electrons

F= Faraday constant

ΔE°= E°’ _acceptor - E°’ _donor

Do molecules with more affinity for electrons have a more positive or negative standard reduction potential?

More positive.

Where do NADH and FADH₂ enter the ETC?

NADH: Complex I

FADH₂: Complex II

What reaction of the Citric Acid Cycle happens inside Complex II of the ETC?

Oxidation of Succinate to Fumarate (reduction of FAD).

How are electrons moved through Complex I of the ETC?

Flavin Mononucleotide moves electrons to a Iron-Sulphur cluster containing protein, and then to Coenzyme Q.

How does Complex I Contribute to the Proton gradient?

Pumps 4 H+ per 2 electrons to the intermembrane space against the gradient.

What is the overall equation of the movement of electrons in Complex I?

NADH + 5H⁺ + Q → NAD⁺ +QH₂ +4H⁺

How does Coenzyme Q take electrons?

It is reduced to a semiquinone radical (*QH) to ubiquinol (QH₂)

What are the 2 key features of Iron-Sulphur cluster containing proteins?

• Contains Fe and Cystine

• Has many Fe-S links

How are electrons transported through Complex II in the ETC?

Electrons are transported from succinate via FADH₂ to Iron-Sulphur cluster containing proteins to Coenzyme Q.

How many Protons are pumped across the membrane by Complex II in the ETC?

None.

What is the overall reaction of electron movement in Complex II

Succinate + Q → Fumarate + QH₂

How are electrons moved across Complex III in the ETC

Iron-Sulphur cluster containing cytochromes accept electrons from Coenzyme Q, which are transferred to Cytochrome c1, and then to Cytochrome c. Cytochrome c also accepts electrons from cytochrome b.

What happens in the Q cycle?

1. QH₂ gives 1 electron to Cyt c, and the other to a Q (forms *Q^-)

2. A second QH₂  gives 1 electron to Cyt c, and 1 to the *Q^- (forms QH₂)

Why must the Q cycle happen?

Because Cyt c1 can only take 1 electron, but 2 electrons need to be taken from QH_2, meaning QH₂ needs to run through twice.

How do electrons transfer through Complex IV?

Electrons are transferred to O₂ from Cyt c with cytochromes a and a3, producing H₂O

How many H⁺ are pumped across the membrane in Complex IV?

4H⁺ per 4 electrons and 1 O₂.

What is the overall reaction for the transfer of electrons in Complex IV?

4 cyt c_red (Fe⁺²) + O₂ + 8H⁺←→ 4 cyt c_ox(Fe⁺³) + 4H⁺ +2H₂O

How can toxins be used to determine the order of the ETC?

Different toxin inhibits electron flow in different parts of the ETC, so using them can determine the order of the ETC by analyzing the products produced.

Is the ETC endergonic or exergonic?

exergonic.

Phosphorylation of ADP to ATP is endergonic; where does the energy for this come from?

From chemiosmotic coupling with the Electron Transport Chain.

How many protons are pumped across the membrane per NADH in the ETC?

10.

What powers ATP synthase?

The proton gradient.

What is ATP synthase composed of?

A proton pore coupled to an ATP synthesizing enzyme.

How does protons flowing through ATP synthase drive ATP synthesis?

It causes rotation of the y-subunit, causing conformational changes in the β-subunit synthesizing ATP.

What is the purpose of the 𝛼 subunits of ATP synthase.

Structural.

what orientations can the β-subunits be in and what purposes do they have?

• Tight; just formed ATP

• Loose; About to make an ATP

• Open; Just released an ATP

How does turning of the y-subunit in ATP synthase drive ATP synthesis?

Overcomes the activation energy of ATP synthesis.

What modifications to ATP synthase were made to aid in its discovery?

An actin filament on the c₁₀ (rotational) unit, histone tags on the 𝛼 and β-subunits to attach it to a Nickle complex, and a high ATP concentration to make it run backwards.

What is the purpose of the Malate-aspartate shuttle?

To get NADH from glycolysis across the mitochondrial membrane.

How does the Malate-aspartate shuttle work?

• Oxaloacetate is reduced by NADH to Malate, which is let though the membrane via a Malate-𝛼-ketoglutarate transporter

• Oxidized by NAD⁺ to Oxaloacetate

• Transformed to Aspartate by aspartate aminotransferase by adding an amine group from Glutamate (Citric Acid Cycle)

• Aspartate then leaves the membrane through a Glutamate-aspartate transporter, and can undergo the reverse reaction and form Oxaloacetate, continuing the cycle.

What is the final ATP yield of Glycolysis?

5-7 ATP.

What is the final ATP yield of Pyruvate Oxidation (per glucose)?

5 ATP.

What is the final ATP yield of Acetyl-CoA oxidation and ETC?

20 ATP

What is the total ATP yield per Glucose molecule broken down?

30-32 (31 average).