Chapter 12: Oxidative Phosphorylation

What is oxidative phosphorylation?

Process in which ATP is formed as electrons are transferred from NADH or FADH₂ to O₂ by series of electron carriers. 3 reactions are linked together

What model is the diagram of oxidative phosphorylation?

Chemiosmotic model

What does the chemiosmotic model reveal of oxidative phosphorylation?

• Movement of electrons from NADH (or FADH₂) through electron transport chain produces an electro-motive force that involves the movement of protons across mitochondrial membrane

• Movement creates a gradient & resultant movement of protons back into mitochondrial matrix through a specific channel creates enough energy for formation of ATP from ADP

• F₁ is catalytic site, F_o is proton channel

Where does oxidative phosphorylation occur in?

Inner mitochondrial membrane

What occurs in the mitochondrial matrix?

CAC, conversion of pyruvate to Acetyl CoA, fatty acid oxidation, & amino acid degradation

Where does glycolysis occur in?

Cytosol

What is the outer membrane of the mitochondria?

Quite permeable to most small molecules & ions due to precense of porins, trans membrane proteins & large pores

What is the inner membrane of the mitochondria?

Impermeable to nearly all ions & polar molecules

What do the contrasting membranes result in?

Access is only gained via a variety of transporters w/in the mitochondrial membrane

What does oxidative phosphorylation begin w/?

Entry of electrons into respiratory chain

• Reduced substrate + NAD+/FAD → oxidized substrate + FAD/NADH + H+

What type of enzymes catalyze the previous reactions?

Dehydrogenase enzymes

Since electrons from NADH & FADH₂ pass through series of membrane-bound carriers, what are the 3 possible processes the electrons are transferred through?

• Direct transfer (Fe3+ → Fe2+)

• Transfer as a hydrogen atom H(H+ + e-)

• Transfer as hydride ion H-

What are the other electron carriers?

• Ubiquinone (coenzyme Q or Q)

  • Semiquinone radical between ox/red forms

  • Can carry 2 electrons

• Cytochromes

  • Proteins that contain porphyrin rings bound to central ion atom that can change oxidation states

• Iron-sulfur proteins

  • Similar to cytochromes, contain bound iron atom that can change oxidation states

  • Iron is co-ordinated by multiple sulfhydryl groups

What is the negative side of the mitochondria?

Matrix

What is the positive side of the mitochondria?

Intermembrane space

What is complex I?

NADH:ubiquinone oxidoreductase (NADH dehydrogenase)

• Hydride is moved from NADH to ubiquinone (Q)

• NADH + Q + 5H+_N → NAD+ + QH₂ + 4H+_P

• 4 protons are “pumped” across inner membrane

Can both complex I & II be used?

No, just one

What is complex II?

Succinate to Ubiquinone

• Succinate dehydrogenase, only membrane bound enzyme w/in CAC, is part of this complex. Complex allows electrons from FADH₂ to enter respiratory chain

• FADH₂ + Q → FAD + QH₂

• No protons are pumped across the inner membrane

• The 2 other enzymes/processes that enter there electrons here are glycerol 3-phosphate (cytosolic) & fatty acyl-CoA

What happens when starting w/ FADH₂ instead of NADH?

Pump 4 less protons across membrane

What is complex III?

Ubiquinone:cytochrome c oxidoreductase

• In this step, electrons from ubiquinone are transferred to cytochrome c, reducing iron atom of cytochrome c

• QH₂ + 2cytc (ox) + 2H+_N → Q + 2cytc (red) + 4H+_P

What is regenerated in complex III & IV?

III: Q

IV: 2cytc (ox)

What is pumped across in complex III & IV?

III: 4H+

IV: 2H+

What is complex IV?

Cytochrome oxidase

• In this step, electrons from cytochrome c are transferred to molecular oxygen

• 2cytc (red) + 4H+_N + ½O₂ → 2cytc (ox) + H₂O + 2H+_P

What is the overall transfer of electrons from NADH to O₂ produced?

NADH + 11H+_N + ½O₂ → NAD+ + H₂O + 10H+_P

What is the major difference when electrons come from FADH₂?

Skip complex 1… only 6 protons are pumped across the membrane

What is the synthesis of ATP process driven by?

Proto-motive (proton motive) force, inward flow of 3H+

What is the enzyme in synthesis of ATP?

ATP synthase & has 2 distinct domains

What is the F₀ subunit?

Hydrophobic segment that spans the inner mitochondrial membrane

• contains proton channel for complex

What is F₁ subunit?

Protrudes into the mitochondrial matrix & contains catalytic activity of enzyme

What does the F₁ subunit have?

3 non-identical alphabeta binding sites

What does each alphabeta binding site have either?

1. Newly released ATP. Binding site is empty & waiting for substrates

2. Tightly bound ADP + P_i are waiting to be coupled

3. Newly synthesized ATP waiting for release

What triggers the next step at all 3 subunits?

Inward movement of 3H+ back into the matrix

What does exit of ATP from mitochondria require?

2 translocase enzymes, phosphate source, & movement of an additional proton

Why is an extra proton needed to exit ATP from mitochondria?

• Relatively low proton concentration in matrix favors the inward movement of an addition H+ (leaks in)

What is the adenine nucleotide translocase (antiporter)?

2 compounds moving in opposite directions

What is the phosphate trasnlocase (symporter)?

2 compounds moving in the same direction

• Also phosphate source

In oxidative phosphorylation, what does the formation of 1 ATP molecule require?

Inward movement of 3H+

In oxidative phosphorylation, what does movement of ATP to cytosol require?

Inward movement of 1 additional H+

In oxidative phosphorylation, what is the total H+ moving inward to allow ATP to be synthesized & released to the cell?

4

In oxidative phosphorylation, what does each electron pair carried by NADH produce?

2.5 ATP

• NADH “pumps out” 10H+ in electromotive phase

• 4H+/ATP moving inward in protomotive phase

In oxidative phosphorylation, what does each electron pair carried by FADH₂ produce?

1.5 ATP

• FADH₂ skips complex I

• “Pumps out” 3 less protons

What do the skeletal muscle & brain use?

Glycerol 4-phosphate shuttle

• Delivers electrons of NADH to ubiquinone through FAD

• Bad currency exchange

When can 32 ATP molecules be formed from a molecule of glucose?

When 2 NADH molecules from glycolysis are able to enter complex I

What happens if the 2 NADH molecules initially use the glycerol 3-phosphate shuttle?

Converted to FADH₂ & enter at complex II

What happens when the shuttle is used instead?

• 8 H+ are not pumped across membrane or

• Since 4H+ are required to synthesize & release ATP, cell loses ability to make 2 ATP → only get 30 ATP

What is another NADH shuttle that functions in the liver, kidney, & heart?

Malate-aspartate shuttle

Why is the Malate-aspartate shuttle better?

NADH is moved into mitochondria as NADH

What is the Malate-asparate shuttle based on?

2 reversible reactions

1. CAC reaction (step 8) that also occurs in gluconeogenesis

• Oxaloacetate → (dehydrogenase) → Malate

2. Aminotransferase or transamination

• Oxaloacetate + Glutamate → Asp + alpha-ketoglutarate

How is oxidative phosphorylation regulated?

• Regulated of ATP production abased on concentration & ratios of ATP/ADP (or AMP) & NADH/NAD+

  • AMP is key indicator of cell’s energy supply

• High concentrations of intermediates (citrate, acetyl CoA, Succinyl CoA) also affect pathways

  • Ratios are important: provide a signal to cell if eerngy supplies are sufficient (or abundant) or deficient

• Only new control

  • inc [substrate] → inc. oxidative phosphorylation