Mitochondrial Shuttles and ETC

Question 1

give a summary of the electron transport

Answer 1

• Complex I (NADH dehydrogenase) and Complex II (succinate dehydrogenase) can donate electrons to the lipid-mobile electron carrier Coenzyme Q (found within the inner-mitochondrial membrane)
• CoQ can donate electrons to Complex III (cytochrome reductase)
• Complex III can donate electrons to the mobile protein cytochrome C (foud within the inner-membrane space)
• cytochrome C will donate electrons to complex IV (cytochrome oxidase)
• complex IV will donate electrons to oxygen

Question 2

name the prosthetic groups of each complex

Answer 2

• Compex I: FMN (riboflavin derivative)
• Complex II: FAD (riboflavin derivative)
• Complex III: heme groups (Fe3+)
• Complex IV: Cu2+ and heme groups (Fe3+)

Question 3

where did the NADH electrons come from?

Answer 3

• NADH oxidized by CoQ at complex I
• pyruvate dehydrogenase
• isocitrate dehydrogenase
• a-KG dehydrogenase
• malate dehydrogenase

Question 4

where did the FADH electrons come from?

Answer 4

• FADH electrons came from succinate dehydrogenase activity
• FADH oxidized by CoQ at Complex II

Question 5

describe the oxidative phosphorylation and the chemiosmotic gradient

Answer 5

• as electrons flow down the electrochemical potential, protons are pumped into the intramembrane space
• protons are pumped into intramembrane space at complexes I, III and IV
• this creates a pH gradient that is relieved by pumping protons back through complex V
  
  • the energy released in this process is coupled to ATP synthesis from ADP + P

Question 6

describe the function of rotenone, piericidin A (antibiotic) and the barbituate amytal

Answer 6

• inhibit NADH dehydrogenase in complex I

Question 7

describe the function of antimycin A (antibiotic)

Answer 7

• inhibits cytochrome b of cytochrome reductase (complex III)

Question 8

describe the function of CO, azide, hydrogen sulfide (H₂S) and cyanide (CN-)

Answer 8

• inhibit cytochrome oxidase (complex IV)

Question 9

describe the function of oligomycin (antibiotic)

Answer 9

• inhibits ATP synthase

Question 10

describe the transport of ATP and ADP

Answer 10

• adenine nucleotide translocase (ANT): unidirectional exchange of ATP for ADP (antiport)
• symport of Pi and H+ is electroneutral

Question 11

name inhibitors of ANT

Answer 11

• atractyloside: toxic glycoside from plant
  
  • binds the outward facing (inter-membrane space) portion of the adenosine nucleotide transporter
• bongkrekic acid: respiratory toxin produced in coconuts
  
  • binds the inward facing (matrix) portion of the ANT

Question 12

describe uncouplers of oxidative phosphorylation

Answer 12

• certain uncouplers (dinitrophenol, DNP), ASA (aspirin), thermogenin, ionophores act by destroying the proton gradient
• uncouplers decrease ATP synthesis and increase ETC and O2 consumption

Question 13

describe thermogenin (aka UCP1)

Answer 13

• uncoupling protein
• found exclusively in brown adipocytes in mammals
  
  • found in newborns in neck and upper back
• the H+ gradient generated from electron transport is uncoupled from ATP synthesis and generates heat
• this is the physiological function of brown adipose tissue

Question 14

describe ionophores

Answer 14

• compounds that make the inner-membrane permeable to compounds that cannot usually cross
• both uncouplers and ionophores allow for dissipation of the proton gradient

Question 15

name examples of ionophores

Answer 15

• gramicidin is a channel forming ionophore
• valinomycin is a mobile carrier
  
  • associated with carrying K+ across bilayers

Question 16

describe the 2 shuttle mechanisms

Answer 16

• cells needs a mechanism to shuttle electrons from NADH (from glycolysis in the cytosol) across inner mt membrane
• 2 shuttle mechanisms:
  
  • glycerophosphate shuttle results in the synthesis of 2 ATPs per cytosolic NADH oxidized (lower because NADH -> FADH)
  • malate-aspartate shuttle results in the synthesis of 3 ATPs per cytosolic NADH oxidized (higher bc NADH stays as NADH)

Question 17

describe the malate-aspartate shuttle

Answer 17

Question 18

describe the glycerophosphate shuttle

Answer 18