Oxidative Phosphorylation - General

Question 1

What is the primary objective of oxidative phosphorylation?

Answer 1

To synthesize ATP.

Question 2

What is the ultimate electron acceptor?

Answer 2

Oxygen.

Question 3

Where does oxidative phosphorylation occur?

Answer 3

The inner mitochondrial membrane.

Question 4

How do molecules like ATP and other molecules get out of the matrix of the mitochondria?

Answer 4

Special transporters or shuttles are used.

Question 5

E₀’ is called the reduction potential. What does a negative one indicate? What about a positive one?

Answer 5

-E₀’: strong reducing agent (donates electrons)

+E₀’: strong oxidizing agent (accepts electrons)

Question 6

When the electrons from NADH flow through the ETC, what is the order of proteins they pass through?

Answer 6

NADH-Q oxidoreductase → Ubiquinone (Q) → Q-cytochrome c oxidoreductase → Cytochrome c → Cytochrome c oxidase → O₂

Question 7

When the electrons from FADH₂ flow through the ETC, what is the order of proteins they pass through?

Answer 7

Succinate-Q oxidoreductase → Ubiquinone (Q) → Q-cytochrome c oxidoreductase → Cytochrome c → Cytochrome c oxidase → O₂

Question 8

What direction are the redox potentials of the ETC proteins? Why is it set up this way?

Answer 8

From lowest redox potential to the most positive redox potential. This ensures that electrons flow down the chain to a protein with higher electron affinity.

Question 9

What metal element is present in all of the complexes? What two forms does it exist in?

Answer 9

Iron.

1. Iron-sulfur clusters
   
   • Formed when protein folding brings multiple cysteine side chains in close proximity allowing sulfur groups to bind one or more iron molecules.
2. Heme prosthetic groups called cytochromes
   
   • Same as present in hemoglobin, cycles between the ferrous and ferric forms as it accepts and donates electrons.

Question 10

What allows the citric acid cycle to be physically linked to the respiratory chain?

Answer 10

The location of the CAC in the mitochondrial matrix produces the NADH and FADH₂ in the compartment where these carriers can easily donate their electrons to the ETC which is situated on the inside of the inner mitochondrial membrane.

Question 11

The ETC transforms the energy in the electrons to this.

Answer 11

A proton gradient.

Question 12

What is the chemiosmotic hypothesis?

Answer 12

The hypothesis that there are high concentrations of protons in the intermembrane space and lower levels in the matrix of the mitochondria, which is used to create ATP using a proton motive force.

Question 13

What is the proton motive force?

Answer 13

An electrochemical gradient between the two sides of the inner mitochondrial membrane ised to create force to drive the synthesis of ATP.

Question 14

What is the enzyme complex that uses the protein gradient to synthesize ATP? Briefly describe its structure.

Answer 14

ATP synthase. It is consists of two components:

1. F₀ portion: embedded in the inner mitochondrial membrane and contains a proton channel
2. F₁ portion: contains the catalytic activity

Question 15

Summarize the binding-change mechanism.

Answer 15

Binding of protons that enter the a subunit cause the c ring to rotate which causes rotation of the g and e subunits. This causes conformational changes in the a/b ring where ATP synthesis occurs.

Question 16

How many alpha and beta subunits does ATP synthase have? What three conformations can the b subunit adopt and what do each do? How does it interchange between forms?

Answer 16

3 of each. The b subunit can adopt:

1. Open (O): binds and releases adenine nucleotides including ATP
2. Loose (L): binds ADP and P_i
3. Tight (T): binds ATP very tightly, to the point where it converts the bound ADP and P_i to ATP

Interchanging occurs via rotation of the gamma subunit which rotates when the c ring rotates.

Question 17

Describe the role of the glycerol-3-P shuttle in helping NADH cross the inner mitochondrial membrane. (3)

Answer 17

1. NADH passes its electrons to DHAP which becomes reduced to glycerol-3-P which able to pass through the outer mitochondrial membrane into the intermembrane space, where it is then oxidized back to DHAP.
2. FAD (prosthetic group of the enzyme that converts glycerol-3-P to DHAP) gets reduced to FADH₂
3. Production of FADH₂ in the inner mitochondrial membrane facilitates entry into the ETC through coenzyme Q reduction, meaning that the electrons have been passed through the chain and NAD⁺ is regenerated.

Question 18

Describe the role of the malate-aspartate shuttle in helping NADH cross the inner mitochondrial membrane (4). Where in the body is this located?

Answer 18

In the heart and liver.

1. A transporter in the inner mitochondrial membrane transports malate into the mitochondrial matrix in exchange for a-KG
2. Malate is oxidized back to oxaloacetate producing NADH
3. Oxaloacetate does not cross the inner membrane so it is converted to aspartate
4. Subsequent reactions regenerate oxaloacetate in the cytosol so the shuttle system can continue

Question 19

What problem does the ATP-ADP translocase solve? How much of the energy generated by electron transfer is used to power these pumps?

Answer 19

ATP in the mitochondrial matrix must be moved to the cytosol and other compartments where it is needed, and ADP must be moved back to the matrix where it can be phosphorylated. ATP-ADP translocase is a specialized carrier that brings ATP to the cytosol and ADP to the matrix. As these pumps are present in very high concentrations in the mitochondrial membrane, it uses about 25% of the energy to drive this exchange.

Question 20

What need regulates cellular respiration?

Answer 20

The need for ATP.