Kuliah 24 Nov

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KULIAH BIOKIMIA

24 NOVEMBER 2011


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How is the oxidation

of NADH (ETC)coupled to the

phosphorylation ofADP (ATP synthesis)?


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In 1961, Peter Mitchell proposed that

electron transport and ATP synthesis arecoupled by a proton gradient acrossthe inner mitochondrial membrane


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Electron Transport Via theRespiratory Chain Creates a

Proton GradientWhich Drives

the Synthesis of ATP

The flow of electrons through the

respiratory chain generates ATP bythe process ofoxidativephosphorylation


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In Peter Mitchells model, the transfer ofelectrons through the respiratory chainleads to the pumping of protons fromthe matrix to the cytosolic side of the

inner mitochondrial membrane.

The H+ concentration becomes lower inthe matrix, and an electrical field with

the matrix side negative is generated


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Peter Mitchell Proposed chemiosmotic hypothesis

revolutionary idea at the time

1961 | 1978

1920-1992

proton motive force


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Chemiosmosis is the diffusion of ions across a

membrane.More specifically, it relates to the generation ofATP by the movement of hydrogen ions across a

membrane.

Hydrogen ions (protons) will diffuse from an areaof high proton concentration to an area of lower

proton concentration. Peter Mitchell proposed that

an electrochemical concentration gradient ofprotons across a membrane could be harnessed tomake ATP. He likened this process to osmosis, the

diffusion of water across a membrane, which iswhy it is called chemiosmosis.


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ATP Synthase, also known as Complex V, is alarge membrane protein that plays a key role

in the energy metabolism of all knownorganisms.

Located in the inner membrane of themitochondria, ATP Synthase is involved with

the electron transport chain within the samemembrane.

ATP SYNTHASE (COMPLEX V)


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ATP synthase has a total of > 20 protein subunitsin most organisms!

ATP synthase consists ofF1 and F0 subunit.

the F1 subunit contains the catalytic activity ofthe ATP synthase.

The F1 subunit consists 9 subunits of 5 different

types polypeptide chains (3, 3,1, 1, and1)

ATP Synthase Is Composed ofaProton-Conducting Unit (F0) and a

Catalytic Unit (F1)


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The knoblike portion of F1 is a flattenedsphere 8 nm high and 10 nm across .

A central shaft (gamma) runs through the

center of these six subunits, as an alpha-helical coil.

The delta and epsilon subunits are attachedto the gamma subunit and help link the F1domain to the Fo domain. This central stalk

rotates at about 50-100 times per second.

Struktur F1


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Fo consists of three subunits: a, b, and c.

The two b subunits associate firmly with thealpha and beta subunits of F1, holing themfixed to the membrane.

The membrane-embedded cylinder of csubunits (10-14 c) is attached to the shaft ofF1.

As protons move through the membrane viaFo, the cylinder and shaft rotate, and the beta

subunits of F1 change conformation as thegamma subunit associates with each in turn


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The F0 and F1 subunits are connected

in two ways, by the central stalkand by an exterior column.

The exterior column consists of one asubunit, two b subunits, and the dsubunit.


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How does the flow of protons drive thesynthesis of ATP?

The results of isotopic-exchangeexperiments revealed that ATP does notleave the catalytic site unless protons flowthrough the enzyme.

Thus, the role of the proton gradient is notto form ATP but to release it from thesynthase.


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Paul Boyer proposed a binding-changemechanism for proton-driven ATP synthesis.

This proposal states that changes in theproperties of the three beta in F1 subunits

allow sequential : ADP and Pi binding, thenATP synthesis, and ATP release.


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How do the conformations of the Beta subunits of F1change?

The amino acid sequences of all three beta subunits areidentical, but their conformations differ, mainly because of

the association of the gamma subunit with just one of thethree.

The ATP-ADP binding sites of the three beta subunits differand are labelled beta-ATP, beta-ADP, and beta-empty.

This difference in binding is critical to the protein'smechanism.


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For every three protons that flow through ATPSynthase, the complex rotates one position (dueto the rotation of the gamma subunit) and oneATP molecule is formed.

The three beta subunits interact in such a way

that when one subunit releases ATP and becomebeta-empty, its neighbor to one side mustassume the beta-ADP conformation and theneighbor one the other side must assume the

beta-ATP conformation. Thus, for each complete rotation, three ATP are

synthesized and released.

The image below shows this rotaion.


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How does proton flow

through F0 drive the rotationof the subunit?

Proton Flow Around the c RingPowers ATP Synthesis


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The proton-gradient-driven rotation of the c ringdrives the rotation of the subunit, which in turnpromotes the synthesis of ATP through the binding-

change mechanism. Recall that the number ofc subunits in the c ring

appears to range between 10 and 14.

This number is significant because it determines thenumber of protons that must be transported togenerate a molecule of ATP.

Each 360-degree rotation of the subunit leads to thesynthesis and release of 3 molecules of ATP.

Thus, if there are 10 c subunits in the, each ATP

generated requires the transport of 10/3 = 3.33protons.

For simplicity, we will assume that 3 protons mustflow into the matrix for each ATP formed, but we mustkeep in mind that the true value may differ.


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1. Sebutkan dan jelaskan inhibitor-inhibitordalam proses electron transport chain

2. Sebutkan dan jelaskan inhibitor-inhibitor

dalam proses oxidative phosphoryllation


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H+

H+

O2+

QC

32ATP2

Pyruvate fromcytoplasm

Electrontransportsystem

ATPsynthase

H2O

CO2

Krebscycle

IntermembranespaceInner

mitochondrialmembrane

1. Electrons areharvested and carriedto the transportsystem.

2. Electronsprovide

energyto pumpprotons acrossthemembrane.

3. Oxygen joinswith protons toform water.

2H+

NADH

NADH

Acetyl-CoA

FADH2

ATP

4. Protons diffuse back indown their concentrationgradient, driving the

synthesis of ATP.Mitochondrial

matrix

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H+

H+

O2

H+

e-

e-

e-

e-


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Cellular respiration

2 ATP 2 ATP ~36 ATP+ +


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Summary of cellularrespiration

Where did the glucose come from?

Where did the O2 come from?

Where did the CO2 come from?

Where did the CO2 go?

Where did the H2O come from? Where did the ATP come from?

What else is produced that is not listedin this equation?

Why do we breathe?

C6H12O6 6O2 6CO2 6H2O ~40 ATP+ + +


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ETC backs up

nothing to pull electrons down chain NADH & FADH2cant unload H

ATP production ceases

cells run out of energy

and you die!

Taking it beyond What is the final electron acceptor in Electron

Transport Chain?

O2

So what happens if O2unavailable?

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