Unit IV: Mitochondria

Question 1

Mitochondrial Import

Answer 1

Most mitochondrial proteins are synthesized from nuclear DNA in the cytosol; they are imported to the mitochondrion via Translocase of Outer Membrane (TOM), which allows passive diffusion of fully folded proteins, and Translocase of Inner Membrane (TIM), which is a gate opened by interaction with the mitochondrial target sequence on the N-terminus of proteins

HSP70 is an ATPase that unwinds proteins as they are translocated through TIM

Question 2

ATP Synthase structure

Answer 2

ATP synthase consists of two subunits:

F0, which forms a proton channel that spans the inner mitochondrial membrane

F1, a globular heterohexamer (a3B3) containing the active site which harnesses the energy released by movement of 3 protons to generate one ATP

Question 3

Role of mitochondria in apoptosis

Answer 3

Usually, BCL-2 and BCL-XL proteins “guard” the mitochondrial membrane; in response to suicide signals, pro-apoptotic proteins Bim and PUMA activate Bak/Bax on the mitochondrial membrane, which oligomerize to make a channel which releases cytochrome c from the mitochondria to the cytosol where it binds several proteins to form an apoptosome, which activates Caspase

It is possible for cells to oxidize cytochrome c in the cytosol, preventing progression of apoptosis

Question 4

Mitochondrial Fusion & Fission

Answer 4

Non-functional mitochondria undergo fusion with healthy mitochondria (mediated by GTPases Mfn and Opa1) and new mitochondria “bud” off via fission (mediated by GTPases Fis1 and Drp);

Mutations in Mfn and OPA cause Autosomal Dominant Optic Atrophy and Charcot-Marie Tooth Type 2A

Question 5

NADH Structure

Answer 5

NAD/H contains a benzene ring that can exist in an oxidized state (NAD+) or in a reduced state (NADH)

Question 6

Glycolysis

Answer 6

Glucose metabolism occurring in the cytosol to generate 2 pyruvate (a 3 carbon molecule) + 2 ATP + 2 NADH

Question 7

Citric Acid Cycle

Answer 7

Occurs in the mitochondrial matrix; transforms pyruvate, through an Acetyl CoA intermediate, to carbon dioxide; generates 4 NADH & 1 FADH

Question 8

Electron Transport Chain

Answer 8

Electrons from NADH are passed through a series of 4 enzyme complexes within the inner mitochondrial membrane, pumping 3 protons into the intermembrane space as O2 is sequentially reduced to H20

Question 9

ATP Synthase mechanism

Answer 9

As protons flow through F0, F1 undergoes 3 conformational changes to catalyze the production of ATP:

Conformation 1: Binds ADP + Pi
Conformation 2: Brings ADP + Pi into a conformation that is favorable to promote reaction
Conformation 3: ATP is released and ADP and Pi can re-bind for the next cycle

Question 10

Mitochondrial Quality Control - 4 Mechanisms

Answer 10

Damaged mitochondria are not effective in producing ATP; they also produce Reactive Oxygen Species (ROS) which may damage other cellular components; therefore, 4 mitochondrial quality control mechanisms are employed:

1. Outer membrane proteins are targeted for ubiquitin-mediated proteolytic degradation in the proteasome; mAAA recognizes and degrades misfolded proteins in the inner membrane; Lon recognizes and degrades misfolded proteins in the matrix

Mutation in mAAA causes hereditary spastic paraplegia

2. Mitochondrial dynamics - mitochondrial fusion can help “dilute” defective proteins
3. Mitophagy - Transport of defective mitochondria to lysosomes for degradation
4. If mitochondrial damage is extensive the cell may induce apoptosis