OXIDATIVE PHOSPHORYLATION •UNCOUPLERS •SHUTTLES •ENZYMES OF OXIDATION
Aug 13, 2015
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.