MEMBRANE-BOUND ELECTRON TRANSFER AND ATP SYNTHESIS (taken from

Chapter 18 of Stryer)

FREE ENERGY – MOST USEFUL THERMODYNAMIC

CONCEPT IN BIOCHEMISTRY

Living things require an input of free energy for 3 major purposes

 

1. Mechanical – Muscle contraction and other cellular movement

2. Active transport of molecules and ions

3. Synthesis of macromolecules and other biomolecules from simple precursors

 

First law of thermodynamics

“Energy can be neither created nor destroyed”

But, it can be converted from one form into another

Free energy for these processes comes from the environment Phototrophs - obtained by trapping light energy Chemotrophs – energy by oxidation of foodstuffs 

Free energy donor for most energy requiring processes is Adenosine triphosphate (ATP)

Large amounts of free energy is liberated when ATP is hydrolysed to ADP + Pi or AMP + PPi ATP is continuously formed and consumed Phototrophs harness the free energy in light to generate ATP - Photosynthesis  Chemotrophs form ATP by oxidation of fuel molecules – Oxidative phosphorylation

OXIDATIVE PHOSPHORYLATION

Glucose is converted to pyruvate

 

And under aerobic conditions undergoes oxidative decarboxylation to form AcCoA which is then oxidised to CO2 by the citric acid cycle

Stages of Catabolism

Citric Acid Cycle

GlycolysisActivated Carriers

These pathways along with fatty acid oxidation produce energy rich molecules NADH and FADH2 as well as small

amounts of ATP

 Chemotrophs derive energy from oxidation of fuel molecules and in aerobic organisms the ultimate electron acceptor is O2

Electron is not transferred directly

Electron is transferred through special carriers, Pyridine nucleotides

  Electron acceptor Electron donor

  NAD+ NADH

  FAD FADH2

 

Respiratory electron transfer is the transfer of electrons from the NADH and FADH2 (formed in glycolysis, fatty

acid oxidation and the citric acid cycle) to molecular oxygen, releasing energy.

Oxidative phosphorylation is the synthesis of ATP from ADP and Pi using this energy.

 

Both processes are located on the IMM

Mitochondrion

Outer membrane• Permeable (12000da)• Porin – 30-35kd pore forming protein

Inner membrane• Impermeable all ions and polar molecules• Possess family of transporter molecules (for

ATP/ADP , Pi , pyruvate, citrate , etc.). • Matrix side (N-negative), cytosolic side (P-

postive)

Mitochondria are the result of an Endosymbiotic event

Organelles contain their own DNA which encode 13 respiratory chain proteins

Many proteins encoded by cell nuclear DNA

Cells depend on organelle for oxidative phosphorylation , mitochondrion depend on cell for their very existence

Suggested that all existing mitochondria are derived from bacterial Rickettsia prowazekii

Oxidative phosphorylation is conceptually simple and mechanistically complex.

Flow of electrons from NADH and FADH2 to

O 2 occurs via protein complexes located in the

IMM

Leads to the pumping of protons from the matrix to the cytosol across the IMM.

ATP is synthesised when protons flow back into the matrix via a protein complex in the IMM.

An example of energy coupling via an electrochemical gradient across a membrane.

REDOX POTENTIAL AND FREE ENERGY CHANGES

The energy stored in ATP is expressed as the phosphoryl transfer potential which is given by G o for hydrolysis of ATP (-7.3kcal/mol)

The electron transfer potential of NADH is represented as Eo

the redox potential ( or reduction potential or oxidation-reduction potential) which is an electrochemical concept.

Redox potential is measured relative to the H+: H2 couple

which has a defined redox potential of 0V (Volts).

Redox couples

A negative redox potential means that a substance has a lower affinity for electrons than H2 .

A positive redox potential means a substance has a higher affinity for electrons than H2.

NAD+/ NADH at -0.32V is a strong reducing agent and poised to donate electrons

1/2 O2/ H2O at +0.82V is a strong oxidising reagent and

poised to accept electrons.

1/2 O2 + NADH +H+ H2O + NAD+

The difference (Eo = 1.14V) is equivalent to -52.6

kcal/mole.

Electrons can be transferrred between groups that are not in contact

THE RESPIRATORY ELECTRON TRANSFER CHAIN CONSISTS OF THREE PROTON PUMPS LINKED BY TWO MOBILE ELECTRON

CARRIERS

Electrons are transferred from NADH to O2 by a

chain of three large transmembrane respiratory chain protein complexes

I

II

III

IV

These are

a) Complex I also known as

NADH-Ubiquinone (UQ) oxidoreductase

NADH-Q reductase

b) Complex III also known as

Ubiquinol (UQH2)-Cytochrome c oxidoreductase

Cytochrome reductase 

c) Complex IV also known as

Cytochrome c- Oxygen oxidoreductase

Cytochrome oxidase

Q- coenzyme Q or ubiquinone

Complex I NADH-Q Oxidoreductase

NADH-Q reductase

NADH transfer of e- to flavin mononucleotide to produce FMNH2

e- from FMNH2 transferred to iron sulfur clusters

e- from iron sulfur (Fe-S) clusters shuttle to coenzyme Q (ubiquinone)

Results in pumping of 4 H+ out of matrix

NADH + Q + 5H+matrix NAD+ +QH2 + 4H+

cytosol

Complex IISuccinate Q reductase

FADH2 already part of complex, transfers electrons to Fe-S centres and then to Q

This transfer does not result in transport of protons

Complex IIIQ-cytochrome c Oxidoreductase

Q-cytochrome c Oxidoreductase Transfers e- from QH2 (2 e- ) cytochrome c

(1 e- ) via heme Mechanism known as Q cycle

QH2 + 2Cyt cox + 2H+matrix Q +2Cyt cred + 4H+

cytosol

Q Cycle

QH2

QH2 + 2Cyt cox + 2H+matrix Q +2Cyt cred + 4H+

cytosol

Complex IVCytochrome c Oxidase

Cytochrome c Oxidase

Proton transport by cytochrome c oxidase

Electrons are carried from Complex I to Complex III by UQH2, the hydrophobic quinol (reduced quinone)

diffuses rapidly within the IMM.

Electrons are carried from Complex III to Complex IV by cytochrome c, a small hydrophilic peripheral membrane protein located on the cytosolic or P side of the IMM.

Complex II (Succinate-UQ oxidoreductase) is membrane bound and contains the FADH2 as a

prosthetic group . So electrons from FADH2 feed in to

UQH2.

These respiratory chain complexes contain redox groups to carry the electrons being transferred through them. These are flavins, iron-sulfur clusters, haems and copper ions.

PROTON PUMPS AND THE ATP SYNTHASE

The free energy change of the reactions catalysed by Complexes I, III and IV is large enough for them to pump protons from the matrix or N side of the IMM to the cytosolic or P side of the IMM.

 

There is not enough energy released in Complex II, so no proton pumping occurs in this complex.

OXIDATION AND PHOSPHORYLATION ARE COUPLED BY A PROTON-MOTIVE FORCE

This is the chemiosmotic hypothesis put forward by Peter Mitchell in 1961.

Transfer of electrons from NADH (or FADH2) to

oxygen leads to the pumping of protons to the cytosolic side of the IMM.

The H+ concentration (pH) becomes higher (lower pH) on the cytosolic side, and an electrical potential (membrane potential) with the cytosolic side of the IMM positive is generated

So a proton-motive force (p) is generated which consists of both a pH and a .

Mitchell proposed that this proton-motive force drives the synthesis of ATP by another transmembrane protein complex, as the protons return back across the IMM through this protein complex.

 

This protein complex is called the ATPase (because like any enzyme it is reversible and was first discovered by it’s ability to hydrolyse ATP)

It’s preferred name is the ATP synthase.

Structure of ATPase

ATP synthase nucleotide binding sites are not equivalent

It is now thought that the proton-motive force induces a conformational change in the ATP synthase, which allows the release of tightly bound ATP (the product) from the enzyme, and thus catalyses ATP synthesis.

So this is an example of energy coupling via an activated protein conformation.

C-ringH+ in

H+ out

Rotates 360ºProducing 3 molecules ofATP

Thus 10 protons required

Requires 3 protons per molecule of ATP

THE COMPLETE OXIDATION OF GLUCOSE YIELDS ABOUT 30 ATP

Net Yield per glucose

 

Glycolysis 2 ATP

 

Citric Acid cycle 2 ATP (GTP)

 

Oxidative phosphorylation ~26 ATP

 

Most of the ATP is generated by oxidative phosphorylation

POWER TRANSMISSION BY PROTON GRADIENTS: A CENTRAL MOTIF OF

BIOENERGETICS

Proton gradients power a variety of energy-requiring processes i.e.

IT IS EVIDENT THAT PROTON GRADIENTS ARE A CENTRAL INTERCONVERTIBLE CURRENCY OF FREE ENERGY IN BIOLOGICAL SYSTEMS.

THE RATE OF OXIDATIVE PHOSPHORYLATION IS DETERMINED BY THE NEED FOR ATP

Under most physiologic conditions, electron transfer is tightly coupled to phosphorylation. Electrons do not usually flow through the electron transfer chain unless ADP is simultaneously phosphorylated to ATP.

Oxidative phosphorylation and thus electron transfer require a supply of

NADH

O2

ADP and Pi

 

The most important factor controlling the rate of oxidative phosphorylation is the level of ADP

Regulated by the energy charge.

This regulation of the rate of oxidative phosphorylation by the ADP level is called respiratory control.