Metabolism 5 - Mitochondria & Oxidative Phosphorylation

Question 1

Where do the reactions of oxidative phosphorylation take place?

Answer 1

The inner mitochondrial inner membrane.

Question 2

Discuss the possible origins of mitochondria.

Answer 2

Mitochondria are believed to be the evolutionary descendants of prokaryotes following an endosymbiotic relationship with the ancestors of the eukaryotic cells.

Question 3

What is the supporting evidence for the endosymbiotic theory?

Answer 3

• Mitochondria only arise from preexisting mitochondria
• Mitochondria have their own genome in the same form as prokaryotes (a single circular molecule of DNA)
• Mitochondria can synthesise proteins
• The start amino acid in mitochondria is formylated methionine residue (fMet) like with bacteria
• Antibiotics that block protein synthesis of bacteria also block protein synthesis of mitochondria

Question 4

What are the two steps of oxidative phosphorylation?

Answer 4

• The translocation of proteins from the mitochondrial matrix to the intermembrane space. This is controlled by the electron transport chain
• Protons diffuse down their gradient through ATP synthase, making ATP.

Question 5

What are the enzymes and carriers in the electron transport chain?

Answer 5

Enzymes- NADH dehydrogenase complex, cytochrome b-c1 complex, cytochrome oxidase complex
Cofactors- ubiquinone and cytochrome C

Question 6

Why are electrons important in the ETC?

Answer 6

The electron acceptors accept electrons from NADH and FADH2 alongside H+. As the electrons lose energy travelling between the electron acceptors, protons are pumped across the membrane.

Question 7

Why are the carriers important in oxidative phosphorylation?

Answer 7

Ubiquinone transports electrons between NADH dehydrogenase and cytochrome b c1 complex. Cytochrome C then takes the electrons to cytochrome oxidase complex. Cytochrome C has a higher affinity for electrons than ubiquinone, so the reactions are directional

Question 8

Describe the structure of ATP synthase.

Answer 8

ATP synthase is multimeric (made of many subunits). F0 is membrane bound, while F1 is the part that projects into the matrix space.

Question 9

How does ATP synthase work?

Answer 9

• The enzyme rotates as protons move down rheir concentration gradient. If protons move into the matrix ATP synthesis occurs, if they move from the matrix ATP hydrolysis occurs.
• The conformational energy of the ATP synthase changes, which promotes the formation or hydrolysis of ATP.

Question 10

How does succinate dehydrogenase affect ATP production and why?

Answer 10

1 less molecule of ATP is made, as succinate dehydrogenase communicates directly with ubiquinone, so one less proton is pumped into the intermembrane space.

Question 11

How does donation of electrons by FADH2 affect ATP production? When does this occur?

Answer 11

FADH2 species result in one less ATP molecule being made, as seen in the glycerol phosphate shuttle. This is because FADH2 enters the chain at a later stage than NADH.

Question 12

How do cyanide and azide affect metabolic processes?

Answer 12

• They bind with high affinity to the ferric form of the haem group in cytochrome oxidase
• This blocks electron flow and consequently ATP production

Question 13

How does malonate inhibit metabolism?

Answer 13

• Malonate resembles succinate and acts as a competitive inhibitor of succinate dehydrogenase.
• This slows the flow of electrons from succinate to ubiquinone ny inhibiting oxidation of succinate to fumarate

Question 14

How does rotenone affect the rate of metabolic reactions?

Answer 14

• It is an isoflavone

- Inhibits transfer of electrons from complex 1 to ubiquinone

Question 15

How does ogliomycin affect metabolic processes?

Answer 15

• An antibiotic that inhibits oxidative phosphorylation by binding to ATP synthase
• Blocks flow of protons through the enzyme

Question 16

How does dinitrophenol affect metabolic reactions?

Answer 16

It is a proton ionophore which shuttles protons across rhe inner mitochondrial membranes.

Question 17

How does non-shivering thermogenesis occur?

Answer 17

In hinernating animals and newborn humans.

• Regulated uncoupling of oxidative phosphorylation occurs in the brown adipose tissue
• This occurs by UCP-1, activated by liberation of free fatty acids and triacylglycerols.
• Heat is released from the dissipation of the proton gradient

Question 18

What are the uses of DNP, in the past and present?

Answer 18

• DNP induces weight loss by transporting protons across the membrane, increasing the metabolic rate and body temperature.
• DNP was used to treat obesity, though it can also result in death.
• DNP is now used in pesticides