Lecture 6 Electron Transport and Oxidative Phosphorylation

Question 1

Where does oxidative phosphorylation occur?

Answer 1

• oxidative: Electron transport chain (carried out by complexes I, II, III and IV
• phosphorylation: ATP synthase → ADP phosphorylation to ATP

Question 2

What are 4 ways electrons can be transferred?

Answer 2

• Directly as electrons (e-).
• As hydrogen atom (H+ + e-) = proton + 1 electron (transferred together)
• As a hydride ion (H+ + 2e-) = hydrogen anion → reactions with 2 e- transfers
• Through direct combination with oxygen (oxygen is covalently incorporated).
  
  • R-CH3 + 1⁄2O2 → R-CH2-OH

Question 3

Define reducing equivalent

Answer 3

any chemical species which transfer the equivalent of one electron in an oxidation-reduction reaction.

Question 4

Define redox-potential

Answer 4

E → The measure of a compounds affinity for electrons → the magnitude of E is relative to the affinity of hydrogen for electrons

Question 5

How is the magnitude of E measured?

Answer 5

measured using a hydrogen electrode is based on the redox half cell: 2H+(aq) + 2e− → H2(g). The redox potential of the hydrogen electrode is set to 0 volts (by convention).

• A positive E means the compound has a higher affinity for electrons than hydrogen
• a negative E means a lower affinity

Question 6

What is the standard reduction potential?

Answer 6

The standard reduction potential (Eº) is measured at

• 25℃
• with 1M [electron donor] and 1M [electron acceptor]
• the pressure of 1 atmosphere is used for gases.
• Biochemists use a standard condition of pH = 7.0; i.e., [H]=10-7 M and denote this as E’º.
• The E’º of the hydrogen electrode is -0.414V.

Question 7

How does an electrochemical cell work?

Answer 7

Experiment: Two separate cells containing electrolytes (e.g., aqueous solutions of Fe and Cu salts) are connected.
What happens?
Electrons will flow from the lowest standard reduction potential Eº towards the highest standard reduction potential Eº

Question 8

What is the Nernst equation?

Answer 8

Question 9

What half-reactions have the highest and lowest standard reduction potentials?

Answer 9

• Highest reduction potential: ½ O₂ + 2H⁺ + 2e^- ⇌ H₂O
• Lowest reduction potential: Acetate^- + 3H⁺ + 2e^- ⇌ acetaldehyde + H₂O

Question 10

What do dehydrogenase enzymes do?

Answer 10

extract electrons from catabolic substrates and transfer them to ‘universal’ electron acceptors

• catalyze the loss of (usually) 2e- + 2H+ (= 2 hydrogen atoms).
• reduced substrate + NAD+ = oxidized substrate + NADH

Question 11

What do oxidases do?

Answer 11

Catalyze oxidation reactions involving an oxygen atom (= 1/2 O2) as an electron acceptor.

• Oxidases class of enzymes do not incorporate molecular oxygen (dioxygen = O2) into an organic substrate. Rather oxidases accept electrons released from an organic substrate, through intermediate electron carriers (ubiquinone, cytochrome c) to form superoxide (as in NADPH-oxidase), hydrogen peroxide (as in xanthine oxidase) or water (as in cytochrome c oxidase).

Question 12

How do electrons flow in the ETC?

Answer 12

Electrons will flow from lower reduction potential to higher reduction potential sites.