Biochem lecture 26- Electron Transport Chain

What are the three roles that play key roles in aerobic metabolism?

Citric acid cycle, electron transport, and oxidative phosphorylation

Electron Transport

Series of oxidation-reduction reactions by which the electrons derived from oxidation of nutrients are passed to oxygen

Oxidative phosphorylation

Process for generating ATP and depends on the creation of the pH gradient within the mitochondrion as a result of the electron transport

What is the role of electron transport in eukaryotic cells?

Occurs in the mitochondria, O2 is the final acceptor of electrons and glycolysis occurs outside the mitochondria in the cytosol

Electron Transport Chain

Series of intermediate carries the transfer electrons from NADH and FADH2 to O2 (takes place in the inner mitochondrial membrane)

proton gradient

Difference between the hydrogen ion concentration in the mitochondrial matrix and that in the inter-membrane space.

-Protons pumped “uphill”

-low H+ in matrix, high H+ in inter membrane space

-gives rise to most of the ATP production associated with the complete oxidation of glucose

Chemiosmotic coupling

Connects ETC with OxPhos and the protons flow back to the matrix via ATP synthase

What does oxidative phosphorylation use the proton gradient for?

To preform the work of generating ATP

What does the electron transport chain include to function?

Four multi enzyme systems and two electron carries, coenzyme Q and cytochrome c

Where does the electron transport chain take place?

In the inner mitochondrial membrane

What creates a pH gradient?

When protons are pumped across the inner membrane into the inter-membrane space

Standard reduction potentials

Provide a basis for comparison among redox reactions and help predict the sequence of reactions in the electron transport chain

How do you measure reduction potentials?

The reference point is the half-cell on the right where H+ is in aqueous solution in equilibrium with hydrogen gas, the sample that is texted is in the other half-cell (left)

When is the reduction potential negative?

When electrons flow TOWARDS the reference half-cell and away from the sample half-cell

When is the reduction potential positive?

When the sample half-cell ACCEPTS electrons from the reference half-cell

How do you determine if electrons are more likely to be transferred from NADH to coenzyme Q or vice versa?

It is determined by measuring a reduction potential for the carriers, the molecule with a HIGH reduction potential tends to be reduced if it is paired with a molecule with a lower reduction potential

How does reduction potential relate to Gibbs free energy change?

DeltaG*’ represents the free-energy change of a reaction under biochemical standard conditions

How is the deltaG*’ of a redox reaction calculated?

DeltaG*’= -nFdeltaE*’

N= number of moles of electrons transferred

F= faradays constant (96.458 kJ V^-1 mol^-1)

DeltaE*’= total voltage for the two half reactions

What is the rule about calculating deltaG*’?

If the reduction potential is positive, then deltaG*’ is NEGATIVE and the reaction will occur spontaneously

NADH-CoQ Oxidoreductases

Catalyzes the transfer of electrons from NADH to Coenzyme Q also known as Ubiquinone

-includes several proteins that contain iron-sulfur clusters and flavoproteins that oxidize NADH

What does complex I lead to?

It leads to the proton pumping across the inner mitochondrial membrane

How do electron carries differ from one another?

NADH carries both electrons and H atoms while Fe-S protein only carry electrons

What is special about CoQ?

It is a mobile electron carrier within within the inner mitochondrial membrane and its electrons gets transferred during complex II

What is stage 1 of complex I?

The transfer of electrons from NADH to the flavin portion of the falvoprotein

What is stage 2 of complex I?

Reduced FMN is reoxidized and the oxidized form of the iron-sulfur protein is reduced

What is stage 3 of complex I?

The reduced iron-sulfur protein donates its electrons to CoQ which is reduced to CoQH2

What is the deltaG*’ indicated during complex I?

It indicated that the reaction is strongly EXERGONIC and release enough energy to drive the phosphorylation of ADP to ATP (which does not happen during ETC)

Succinate-coenzyme Q Oxidoreductase (complex II)

Transfers electrons from succinate to CoQ

What are some characteristics on complex II?

-energy from this reaction insufficient to drive ATP production

-EXERGONIC

-NO H+ pumped out of the matrix during this step

-succinate-CoQ Oxidoreductase

What is stage 1 of complex II?

Succinate is oxidized to fumarate by succinate dehydrogenase, FAD is the initial electron carrier for complex II which is covalently bounded to the enzyme

What is stage 2 of complex II?

FADH2 is reoxidized as another iron-sulfur protein is reduced

What is stage 3 of complex II?

The reduced FE-S transfers electrons to CoQ

Cytochromes

-They all contain a heme group

-they transfer electrons, not H+

-cytochrome c is the mobile electron carrier

-Complex II and III contain b cytochromes

-complex III have cytochrome c1

-complex IV has cytochromes a

How are a, b and c cytochromes different?

They are different based on the side chain depending on the heme group

-b cytochromes have the same heme structure as globins

-a and c cytochromes have different side chains for the heme groups

In each heme group what is iron reduced to?

It is reduced to Fe (II) and reoxidized to Fe (III)

CoQH2-cytochrome c Oxidoreductase

Catalyzes the oxidation of reduced coenzyme Q with eventual reduction of cytochrome c in multi-step processes

What are some characteristics of complex III?

-H+ ions are transported across inner mitochondrial membrane

-Highly exergonic to make ATP

-two cytochrome c molecules required per CoQ

-includes two b-type cytochromes, cytochrome c1 and several iron-sulfur proteins

Q cycle

-Provides the link between two-electron transfers and one-electron transfers

-involves cyclic flow of electrons from CoQH2 to other components of complex III

What are the three forms of coenzyme Q

Quinone (oxidized), semiquinone, and hydroquinone (fully reduced)

Cytochrome c oxidase

Catalyzes the transfer of electrons from cytochrome c to O2 with formation of water

What are some characteristics of complex IV?

-results in proton pumping

-contains cytochromes a and a3 as well as Cu2+ ions that are involved in electron transport

-the link to molecule oxygen in aerobic metabolism

Where are respiratory complex located?

They are integral membrane proteins revising in the inner mitochondrial membrane

Where is CoQ in ETC?

Soluble in hydrophobic interior of membrane

Summary of electron transport chain

-complex I: NADH-CoQ Oxidoreductase

-complex II: Succinate-CoQ Oxidoreductase (FADH2 pathway)

-complex III: CoQH2-cytochrome c Oxidoreductase

-complex IV: cytochrome oxidase

-mobile electron carriers: CoQ and cytochrome c