Bio98 Lecture 18 ETC

Oxidative metabolism

Glycolysis + PDH:

• step 1: make pyruvate (3C)

• step 2: make acetyl-CoA (2C)

TCA:

• add 2C from acetyl-CoA to OAA to make citrate

• burn 2C from citrate to regenerate OAA

Oxidative phosphorylation (OX-PHOS)

• use all the e- generated in this process to make a bunch of ATP!

During glycolysis, electrons are stored as…

NADH.

-cannot cross the mitochondrial membrane.

REMINDER: electrons are not floating around in cells

Electron shuttle mechanism I: Glycerol phosphate shuttle

1) NADH (cytoplasmic) reduces DHAP to glycerol-3-phosphate (G3P)

2) G3P reduces FAD (mito) to FADH2 & G3P is oxidized back to DHAP

Electron shuttle mechanism II: Malate-aspartate shuttle (MAS)

• Malate and aspartate are transported by exchange proteins, 1:1 ratio

• Every malate into mitochondria = 1 αKG into cytosol

• Every aspartate into cytosol = 1 glutamate into mitochondria

• αKG and glutamate are used to regenerate OAA and aspartate

How many complexes exist in ETC?

5 (Complex I-IV & ATP Synthase)

Redox Coupling (5 biological e- carriers)

1) Pyridine linked Dehydrogenases

• Free diffusable in cell

• NAD+ ←→ NADH + H+

• NADP+ ←→ NADPH + H+

• ^^ 2e-s^^

2) Flavin linked Dehydrogenases

• tightly bound enzyme prosthetic groups

• FMN ←→ NADH + H+

• NADP ←→ NADPH + H+

• ^^ 2e-s^^

3) Inorganic FeS centers (No H+ No Heme)

• Fe+++ ←→ Fe++ (1e-)

4) Cytochromes (Heme; no H+)

• Fe+++ ←→ Fe++

• Cu++ ←→ Cu+

5) Ubiquinone (Coenzyme Q)

• H+ and e- (one at a time)

• membrane restricted, but shuttles e- between different membrane bound complexes

• 

3 redox states of CoQ/ubiquinone

1) fully oxidized (ubiquinone)

2) semiquinone (semiubiquinone)

3) fully reduced (ubiquinol)

• capacity for both 1e- and 2e- transfer

• necessary cofactor bc part of ETC can only accept 1e- at a time.

• membrane restricted due to its long prenyl tail (~40 carbons in length)

Proton Motive Force

Proton pumps shoots H+ from N sire (matrix) to P side (intermembrane space).

-movement of protons across membranes downhill the electrochemical gradient.

Complexs I, III, and IV: ?
Complex V: ?

Two mobile electron carriers: ?

NADH pumps ?

FADH2 pumps ?

Complexs I, III, and IV: produced proton gradient
Complex V: (ATP Synthase) converts proton gradient into ATP

Two mobile electron carriers: 1) Ubiquinone, Q (Coenzyme Q); carries 2 e-s. 2) Cytochrome c (Cyt c); carries 1 e-

NADH pumps 10 H+ = 2.5 ATP

FADH2 pumps 6 H+ = 1.5 ATP

(4 protons = 1 ATP)

Complex I: NAD Dehydrogenase

-Complex I uses NADH electrons to pump 4 protons into the intermembrane space, AGAINST the gradient, which COSTS energy

-electrons are TRANSFERRED to membrane UBIQUINONE (UQ)

Complex II: Succinate Dehydrogenase

-Complex II is an enzyme in TCA (SDH)

-FADH2 generated never leaves the enzyme

-Electrons are instead TRANSFERRED to ubiquinone (UQ)

Complex III: Ubiquinone: Cytochrome c - oxidoreductase

-Complex III harvest the electrons from ubiquinone made in complexes I & II

-Electrons are transferred to a molecule called Cytochrome c

-Energy generated in this reaction pumps 4 more protons across gradient

Complex IV: Cytochrome Oxidase

-Complex IV harvests the electrons from the Cytochrome c made in complex III

-Electrons are transferred to molecular oxygen, making H2O

-Energy generated in this rxn pumps 2 more protons across gradient

Why is the whole ETC process aerobic?

it’s the final electron sink/acceptor!

Complex V: ATP Synthase

-Purpose: pumps in ATP into gradient

-Composed of:

-F1 Domain (Catalytic; 5 subunits): α (alpha), β (beta), γ (gamma), δ (delta), and ε (epsilon) subunits; the rotating mushroom

-F0 Domain (Membrane-Bound): the anchor; composed of a ton of subunits (a, b, c, etc.)

-120° rotation per ATP cycle intermediate

• ~3-4 ATP per turn of the synthetase

• 4 protons per 1 ATP

• 230 rotations per second (20,000 rpm)

It appears that only 3 protons are necessary to produce 1 ATP. So why do 4 protons are necessary, as stated in the textbook?

-The 4th proton is needed to change conformation of ATP synthase

-The 4th proton is needed to transport Pi, ATP and ADP across the membrane

-The 4th proton is a ghost proton

-The 4th proton needs to balance the surface charge

-The 4th proton is needed to transport Pi, ATP and ADP across the membrane

Mitochondrion

ET; H+ goes from matrix (inside; n side) to intermembrane space (p side) to ATP synthetase to back inside matrix (n side), but as ATP

Chloroplast

Photosynthesis; H+ goes from stroma (oustide; n side) to thylakoid lumen (inside; p side) to ATP synthase to stroma again but as ATP