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ELECTRON TRANSPORT CHAIN

AND OXIDATIVE PHOPHORYLATION

MEGHNA GARG R.NO: 3401

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CONTENTSINTRODUCTIONELECTRON ACCEPTORSLOCATION OF ETCCOMPLEXES OF ETC(I,II,III,IV)PROTON MOTIVE FORCEATP SYNTHASE

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INTRODUCTIONRespiration is the oxidative breakdown of

organic compound to release energy. Organic compounds: LIPIDS PROTEINS

CARBOHYDRATES Main aim of these various metabolic

reactions is to produce ATP. But how is ATP produced?

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NADH₂ AND FADH₂All the metabolic pathways lead to the

production of NADH₂ AND FADH₂ molecules. These compounds are highly reduced or energy rich.

These molecules are oxidized via the ETC chain.

Oxidation refers to the loss of electrons or hydrogen atoms.

The oxidation process releases large amount of negative free energy which then drives the synthesis of ATP from ADP. (C/a Oxidative phosphorylation).

BUT THE PROCESS IS NOT SO SIMPLE!

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Role of NAD/ NADP linked dehydrogenases : (removes two electrons from its substrate) Hydride ion(:H) proton(H⁺) (transferred to NAD⁺ or NADP⁺)

Reduced subs + NAD⁺ oxidised subs + NADHReduced subs + NADP⁺ oxidised subs + NADPHExample:αketo glutarate+ CoA + NAD⁺ Succinly CoA +

CO₂ + NADH + H⁺

NADH₂

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CONCEPT OF FLAVOPROTEINS :The FMN and FAD molecules are linked to

flavoproteins.( to the active site)The reduction potential of these molecules

depends on the interactions with local sites on the protein.

Unlike NAD or NADP molecule, can accept one or two electrons, thus :

FMN/FAD + e⁻ FMNH•/FADH• ORFMN/FAD + 2e⁻ FMNH₂/FADH₂

FADH₂

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LOCATION OF ETC

Outer membraneInner membrane (ETC)MatrixPermeability of the two membranesContents of the matrix

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COMPLEXES OF ETCThe ETC consists of five separate protein

complexes: Complex I , II, III, IV and V.The complexes I, II, III and IV are involved in

transportation of electrons to molecular oxygen.The complex V is involved in the synthesis of ATP.Each complex consists of certain prosthetic

groups which are the ‘electron carriers’ in that respective complex.

The electrons are sequentially passed from complex I to V.

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COMPLEX IAlso called as NADH Dehydrogenase.PROSTHETIC GROUPS: 1.) FMN 2.) FE-S center

( atleast six)IRON-SULFUR centers: in these centers, the

iron is present in association with inorganic sulfur atoms or with the sulfur atoms of cysteine residues of the protein.

These centers can range from simple structures to complex i.e one Fe atom cordinated to 4Cys residues

Or they can be complex like 2 Fe- 2S or 4Fe- 4S centers.

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PATHWAY FOR TRANSFER OF ELECTRON THROUGH COMPLEX I

Intermembrane space (P side)

Matrix( N side)

At the end there is net transfer of four protons from the matrix to intermembrane space.

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COMPLEX II

Succinate dehydrogenase complexHas 4 subunitsA: binding site for substrate and FADB: 3 Fe-S centersC:binding site for ubiquinone.D: contains heme b .(Not involved in electron transfer).Subunits A and B are present towards matrix and subunits C and D are embedded in membrane.

FAD

Fe-S

Heme bUbiquinone

Subs binding site Sub A(purple)

Sub B(brown)

Sub D(blue)

Sub C(green)

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PATHWAY OF ELECTRON TRANSFER THROUGH COMPLEX II

No transfer of protons from the matrix to intermembrane space.

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UBIQUINONEUbiquinone is a quinone derivative with a

long hydrophobic isoprenoid tale.It can accept one electron to form the

semiquinone form ( QH•) or it can accept two electron to form the ubiquinol (QH₂).

Besides Complexes I and II , other sources donates electrons directly to ubiquinone.

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PATHWAYS OF ELECTRON FLOW TO UBIQUINONE

Glycerol 3 phosphate genertaed during glycolysis from DHAP and also from fatty acids degradation.The fatty acyl CoA is generated during β oxidation of fatty acids.ETF: Electron Transport Flavoprotein

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CYTOCHROMESThe next complexes i.e. complex III and complex

IV are cytochromes.Each cytochrome consists of a heme group i.e.

iron encaged in a porphyrin ring .The Iron of heme group is readily convertible to

Fe⁺² from Fe⁺³ state.The cytochrome a has heme aCytochrome b has heme bCytochrome c has heme cBesides this each cytochrome differs in their light

absorption spectra.

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COMPLEX III

Also called as cyt bc1 complex.Has cyt b and cyt c.Cyt b has heme bCyt c has heme cBesides these reiske iron sulfur centers are present.( iron attached to histidine residues)

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FLOW OF ELECTRON VIA COMPLEX III

Follows a special Q cycle.At the end there is net transfer of four protons from the matrix to intermembrane space.

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COMPLEX IV

Also c/a cyt a a₃ complex.Has 13 subunits.Subunit I: has heme a, heme a₃, and Cu ion B.Subunit II: has 2 Cu ions , forms binuclear centre.Subunit III: role not clear but important for functioning of this complex.

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PROTON MOTIVE FORCE

Ten protons released , two components of p.m force

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In actively respiring mitochondria the measured electrical gradient is 0.15 to 0.20 volts

The pH difference is 0.75 units.The free energy change for pumping one

proton is 20kJ/mol of H⁺, thus for pumping ten protons the energy released is 200kJ/mol

ATP formation requires only 50kJ of energy.

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COMPLEX V

Also c/a ATP synthase.F₁ particle- 9 subunits.F₀ particle : 3 subunits.

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α ADP

β ADP

α empty

β empty

β ATP

α ATP

β subunits differ in their conformations

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ROTATIONAL CATALYSIS

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REFERENCESDavid.L.Nelson, Michael M.Cox, Lehninger

Principles of Biochemistry ( 4th edition), pp:1130.

Lubert Stryer et.al, Biochemistry (5th edition), pp:1514