Uncouplers of oxidative phosphorylation

OXIDATIVE PHOSPHORYLATION

•UNCOUPLERS•SHUTTLES•ENZYMES OF OXIDATION

Presented By;

NOMAN HAFEEZ KHOSA

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• ATP move from the mitochondrial matrix to the cytosol via a specialized membrane transport protein, “ATP-ADP translocase.”

• Translocase is tightly coupled to the exchange of ADP for ATP as ATP exits.

Movement of ATP/ADP Through translocase

These compounds abolished the coupling between oxidation and phosphorylation through increasing the permeability of the IMM

Failure of formation of the electrochemical gradient

ATP formation stops while oxidation proceeds.

Inhibitors of the respiratory chain

A- Uncouplers

What is the mechanism of

uncouplers?

They act by transporting H+ to inside the mitochondria without

passing through F₀F1

And energy is released as heat.

A- Uncouplers1. 2,4 dinitrophenol2. Dinitrocresol3. Snake venoms phospholipases4. Pentachlorophenol5. trifluorocarbonyl-cyanide phenyl

hydrazone

2,4-dinitrophenol (DNP)

۞ A small lipophilic molecule۞ A protein carrier۞ Can easily diffuse through

the IMM۞ Also used as drug to lose

weight۞ Due to many side effects

FOOD & DRUG ADMINISTRATION has banned this drug

Ionophores & physiological uncouplers

1.Ionophores: The lipophilic substance that

promote transport of ions across membranes.

e.g. antibiotic valinomycin facilitates

the entry of H+ through the IMM.

2.Some endogenous compounds when increased act as

uncouplers: Bilirubin, Thyroxine & long chain free fatty acids

Bilirubin is, however, yet to be proved beyond doubt.

Thermogenine (UCP1):• It is considered as a physiological uncoupling

protein.• It is present in the brown adipose tissue of newly

born, some people and hibernating animals.• It allows protons to pass the mitochondrial

matrix without passing F0-F1 complex.• No ATP is formed and energy is released in the

form of heat.

B-Inhibitors of phosphorylation:

• Completely blocks F₀ so it inhibits ATP synthesis

• Example: oligomycin (antibiotic)

C- ATP/ADP transporters inhibitors

• This is achieved by the compound atractyloside

• Atracyloside is a plant toxin• Inhibits adenine nucleotide carrier• Adenine nucleotide carrier facilitates transport of

ATP and ADP• Thus blocking ADP supply prevents phosphorylation

SIGNIFICANCE OF UNCOUPLING• Hibernating animals also uncouple ATP synthesis to

generate heat (nonshivering thermogenesis).• In brown adipose tissue (which is very rich in

mitochondria), uncoupling protein (UCP) or “thermogenin,” forms a pathway for the flow of protons back into the matrix.

• This short circuits the proton gradient, generating heat.

• Some flowers also generate heat this way to voltalize fragrances that attract insects to fertilize their flowers.

Shuttle Pathways

• The inner mitochondrial membrane is impermeable to NADH. So the NADH produced in cytosol can’t directly enter the mitochondria.

• Two different “shuttles” are operative to do this job:– Glycerol 3-phosphate shuttle (transfers electrons to

FADH2 .

– Malate-aspartate shuttle (transfers electrons to NADH)

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Glycerol 3-phosphate shuttle:

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Malate-aspartate shuttle:

Aminotransferase

Aminotransferase

Cytosolic Malate dehydrogenase

Mitochondrial Malate dehydrogenase

a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e

A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )

C O O

C H 2

C H 2

C

C O O

O

C O O

C H 2

HC

C O O

N H 3+

C O O

C H 2

C H 2

HC

C O O

N H 3+

C O O

C H 2

C

C O O

O + +

Enzymes Of Biological Oxidation

All enzymes participating in biological oxidation belong to class oxidoreductase.

Major categories are:1. Oxidases2. Dehydrogenases3. Hydroperoxidases4. Oxygenases

Oxidases

• They catalyse the elimination of hydrogen from substrate, which is mostly used to form water.

• e.g. cytochrome oxidase, tyrosinase, monoamine oxidase & some flavoprotiens.

Dehydrogenases

Catalyse reversible transfer of hydrogen from on substrate to another

Thus bring about oxidation-reduction reactions.

Examples

• NAD+ : e.g. glycerol 3-phosphate dehydrogenase• NADP + : e.g. enoyl reductase• FMN : e.g. NADH dehydrogenase• FAD : e.g. succinate dehydrogenase• Cytochromes: e.g. all Cytochromes of ETC (b, c & c1)

except terminal cytochrome complex belong to this group.

Hydroperoxidases

• They prevent harmful effects of H2O2 produced by aerobic dehydrogenase.

• e.g. peroxidase and catalase

Oxygenases

• Catalyse direct incorporation of oxygen into the substrate molecule.

• e.g. Dioxygenases and Monooxygenases

Electron transport in Prokaryotes

• Lack mitochondria

• Set of e- carriers and enzymes of oxidative phosphorylation are bound to inner cell membrane.