Electron Transport and ATP Synthesis

C483 Spring 2013

1. To reduce one molecule of O2, ________ electron(s) must be passed through the electron transport chain and ________ molecule(s) of NADH is(are) oxidized.A) 4; 2B) 2; 1C) 1; 1D) 1; 2E) 4; 4

2. The chemiosmotic theory is a concept that ________. A) the transport of Na+ and K+ across cell membranes is by active transport B) explains how transport by facilitated diffusion reaches a saturation limit C) explains the blood-brain barrier D) a proton gradient drives the formation of ATP

3. Which is the proper ranking of greatest to least reduction potential? A) NAD+, Q, O2B) O2, Q, NAD+C) Q, NAD+, O2D) Q, O2, NAD+

4. According to the binding change mechanism, the (alpha3 beta3 ) oligomer of ATP synthase has 3 catalytic sites which can each have ________ different conformations.A) 2B) 3C) 6D) 9

5. The P/O ratio for passing electrons through complexes I, III and IV is ________.A) 1B) 1.5C) 2D) 2.5E) 3

Goal: ATP Synthesis

Overview• Redox reactions• Electron transport chain• Proton gradient

• ATP synthesis• Shuttles

Analogy: How does burning coal put flour in the grocery store?

Redox reactions: electricity

• 2 e- transfer• Calculate DG by

reduction potential• NADH: Eo’ = -.32• FMN: Eo’=-.30• DGo’ = -nFDEo’ = -2(96485)(0.02)= -3.9 kJ/mol

N

R

H2N

O H H

NR

N

N

NHR

R

O

O

H+

NADH FMN

N

R

H2N

O

NR

HN

NH

NHR

R

O

O

NAD+ FMNH2

Coenzyme Q: Mobile Carrier

• FADH2 is a one e- donator

• Many reactions, including metals

• Ubiquinone is a key intermediate

• Can diffuse through nonpolar regions easily

Numerous Redox Substrates• O2: high “reduction potential”• Substrates– Organic cofactors– Metals (iron/sulfur clusters)– cytochromes

Oxygen: the final electron acceptor

• Water is produced—has very low reactivity, very stable

• Superoxide, peroxide as toxic intermediates

• Overall reactionNADH + H+ + ½ O2 NAD+ + H2O

Flow Through Complexes

Downhill Flow of Electrons

Compartmentalization

Protonmotive Force• Flow of electrons is

useless if not coupled to a useful process– Battery connected

to wire• Proton gradient

across mitochondrial membrane

Overview of Complexes I-IV• Don’t need to know which cofactors in which

complexes, mechanism of proton pumping• Complex I and II are different entry points into

Q pool, which goes to Complex III

Protonmotive Force

• NADH + H+ + ½ O2 NAD+ + H2O + 10 H+ pumped

• succinate + ½ O2 fumarate + H2O + 6 H+ pumped

Proton Gradient

• Gradient driven by concentration difference + charge difference

• Free energy of ATP hydrolysis = -48 kJ/mole• How many protons needed to fuel ATP

formation? Minimum of 3

Using the Gradient

• Coupled to ATP synthesis• Uncouplers used to show

link of oxygen uptake and ATP synthesis

Complex V: ATP Synthase

• Molecular motor• Rotor: c, g, e– Proton channel

Proton Channel• Protons enters channel

between rotor and stator (unit a--purple)

• Rotor rotates to release strain by allowing proton to enter matrix

• “Stalk” (g) moves inside the “knob”—hexameric ATP synthase

• 9 or 10 protons = full rotation

Binding-Change Mechanism• Stalk causes ATP synthase to have three different

conformations: open, loose, tight• In “tight” conformation, energy has been used to

cause an energy conformation that favors ATP formation

• 9 protons = 3 ATP (or 1 ATP/3 protons)

Remember Analogy

• Fuelelectricitywater pumped uphillflows down to grind flour

• But we don’t have bread until flour is transported to where it needs to go!

• Compartmentalization: ATP is in matrix, but must get to the rest of the cell

Active Transport of ATP

• ATP must go out, ADP and Pi must go in• Together, use about 1 proton of protonmotive

force

Energy Accounting• ATP costs 4 protons– 3 protons in ATP synthase, 1 proton in transport

• NADH pumps 10 protons – 4 protons in Complex I, 4 protons in Complex III, and 2

protons in Complex IV– 2.5 ATP/NADH • called P/O ratio--# of phosphorylation per oxygen atom

• QH2 pumps 6 protons– 4 protons in Complex III and 2 protons in Complex IV– 1.5 ATP/QH2

Net ATP Harvest from Glucose

• Glycolysis = 2 ATP– Plus 3 or 5 ATP from

NADH– What leads to

difference in this case?• Pyruvate DH = 5 ATP• Citric Acid Cycle = 20

ATP• Total: 30-32

ATP/glucose

NADH Shuttles

• Glycerol phosphate shuttle (1.5 ATP/NADH)

• Produces QH2

• Operational in some tissues/circumstances

NADH Shuttles

• Malate-Aspartate shuttle (2.5 ATP/NADH)– Actually slightly less

because one proton is consumed in shuttle

• Separate NAD+/NADH pools indirectly interconverted

• Operational in some tissues/circumstances

Answers

1. A2. D3. B4. B5. D