ATP as energy currency Mitochondria and the electron transport chain

organization Inhibitors of the electron transport chain Oxidative phosphorylation and the uncoupling proteins Inherited defects in oxidative phosphorylation. The role of mitochondria in apoptosis

Δ Gº -7.3 kcal/mol/bond

Electron transport chain (ETC) is a system of electron transport that uses respiratory O2 to finally produce ATP (energy)

ETC is located in the inner mitochondrial membrane &is the final common pathway of metabolism

Energy-rich molecules as glucose or fatty acids are metabolized by a series of metabolic reactions yielding CO2 and H2O .

The metabolic intermediates of these reactions give electrons to specialized co-enzymes NAD and FAD to form NADH and FADH2 which donate a pair of electrons to specialized set of electrons carriers, named the electron transport chain, ECT (Respiratory chain).

As electrons are passed down the electron transport chain, they lose much of their energy. Part of this energy can be taken and stored by production of ATP from ADP and inorganic phosphate Pi (Oxidative phosphorylation).

The remainder of the free energy not trapped as ATP is released as heat

Energy-rich molecules as glucose, fatty acids & amino acidsEnergy-rich molecules as glucose, fatty acids & amino acidsare metabolized by a series of metabolic reactions yielding COare metabolized by a series of metabolic reactions yielding CO2 2 and Hand H22OO

Energy is produced as ATP or heatEnergy is produced as ATP or heat

Energy-rich Energy-rich reducedreduced

CoenzymesCoenzymesNADH & NADH & FADH2FADH2

OXIDATIVEOXIDATIVEPHOSPHORYLATIONPHOSPHORYLATION

Diet Carbohydrates Glycogen (liver & Sk. Diet Carbohydrates Glycogen (liver & Sk. Ms.)Ms.)

Glucose Glucose

GLYCOLYSIS (in cytoplasm)

Pyruvate

in mitochondria

Acetyl CoA Citric Acid Cycle (in mitochondria)

NADH & FADH2

Electron transport chain (flow of electrons)

Formation of ATPATP(oxidative phosphorylation)

CATABOLISM OF CATABOLISM OF CARBOHYDRATESCARBOHYDRATES

The transport of a pair of electrons from NADH (and FMNH2) to oxygen via the electron transport chain produces energy which is more than

sufficient to produce 3 ATPs from 3 ADP and 3 Pi.

The transport of a pair of electrons from FADH2 to oxygen via the ETC produces sufficient energy to produce 2 ATPs from 2ADPs.

Energy released from NADH & FADH2 entering ETCEnergy released from NADH & FADH2 entering ETC

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIAOXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIAOXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIAOXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA

Electron transport chain is formed from five separate enzyme complexes called complexes I, II, III, IV and V Complexes I, II, III and IV contain parts of the electron transport chain, while complex V catalyzes ATP synthesis (phosphrylation).

Each carrier of the electron transport chain can receive electrons from a donor and can subsequently donate electrons to the next carrier in the chain.

The electrons finally combine with O2 and proton (H+) to form H2O.

This requirement for O2 makes the electron transport process the respiratory chain .

Electron transport chainElectron transport chain

Inhibitors of ETC are compounds that prevent the passage of electrons by binding to a component of the chain and subsequently blocking the oxidation/reduction reactions.

As ETC and oxidative phosphorylation are tightly coupled, inhibition of the ECT also inhibits ATP synthesis.

Inhibitors of Electron transport chainInhibitors of Electron transport chain

Transfer of electrons across Transfer of electrons across electron transport chainelectron transport chain

FREE ENERGY RELEASEDFREE ENERGY RELEASED

Transport of protons (HTransport of protons (H++) ) across the inner mitochondrial across the inner mitochondrial membrane from the matrix to membrane from the matrix to

the intermembrane space.the intermembrane space.

This creates an electrical This creates an electrical gradient with more +ve charge gradient with more +ve charge

on the outside of the membrane on the outside of the membrane than on the inside and a pH than on the inside and a pH gradient with lower pH on gradient with lower pH on

outside.outside.

How the free energy generated by the transport of electrons How the free energy generated by the transport of electrons by ECT is used to produce energy (ATP)by ECT is used to produce energy (ATP)

Coupling of ECT to phosphorylation of ADP to ATP Coupling of ECT to phosphorylation of ADP to ATP

Protons reenter (goes back) the Protons reenter (goes back) the mitochondrial matrix by passing mitochondrial matrix by passing

through a channel in the complex V through a channel in the complex V (ATP synthase complex) giving (ATP synthase complex) giving

an energy that is required for the an energy that is required for the synthesis of ATP from ADP and Pi.synthesis of ATP from ADP and Pi.

((phosphorylationphosphorylation))

Proton PumpProton Pump

Electron transport Electron transport is coupled to coupled to the phosphorylationphosphorylation of ADP by the transport of protons (H+) across the inner mitochondrial membrane from

the matrix to the intermembrane space.

This creates an electrical gradient with more +ve charge on the outside of the membrane than on the inside and a pH gradient with lower pH on

outside.

Protons reenter (goes back) the mitochondrial matrix by passing through a channel in the complex V (ATP synthase complex) giving an energy that

are required for the phosphorylation of ADP to ATPthe phosphorylation of ADP to ATP.

Oligomycin binds to ATP synthase closing the H+ channel preventing reentry of protons to the matrix & thus preventing phosphorylation of ADP to ATP Accordingly, electron transport chain (ETC) is stopped (as ETC & phosphorylation are coupled)

Oxidative Phosphorylation (in mitochondria)Oxidative Phosphorylation (in mitochondria)

OxidationOxidation: electron flow in electron transport chain (with production of energy)

PhosphorylationPhosphorylation: phosphorylation of ADP to ATP

PHOSPHORYLATIOPHOSPHORYLATION

OXIDATION

NADH

ATPATP

Uncoupling proteins (UCPs) are located in the inner mitochondrial membrane leading to proton leak as they allow protons to reenter the mitochondrial matrix with no no accompanying synthesis of ATP (no phosphorylation of ADP to ATP).

No energy is utilized for the process of ATP synthesis, although ETC is functioning….i.e. The process of ETC is not coupled to posphorylation.

However, energy is released in the form of heat.

Uncoupling proteins (UCP)Uncoupling proteins (UCP)

UCP1 UCP1 (also called thermogenin) is responsible for the activation of fatty acid oxidation and heat production in thebrown fat of mammals.

Brown fat uses 90% of energy of ETC for thermogenesis in response to cold at birth & during arousal in hibernatingAnimals (by help of UCPl1).

Humans have little of the brown fat (except in the newborn

Uncoupling proteins (UCP)Uncoupling proteins (UCP)

Synthetic uncouplers are compounds that can uncouple ETC & Synthetic uncouplers are compounds that can uncouple ETC &

phosphorylation by increasing the permeability of the inner phosphorylation by increasing the permeability of the inner

mitochondrial membrane to protons (thus will not reenter through ATP mitochondrial membrane to protons (thus will not reenter through ATP

synthase) synthase)

Examples:

2,4-dintrophenol 2,4-dintrophenol An uncoupler that causes electron transport to proceed at arapid rate without phosphorylation & thus energy is released as heat rather than being used to synthesize ATP

High dose of aspirin (salicylates) High dose of aspirin (salicylates) uncouples oxidative phosphorylation causing fever (observed with toxicoverdose of aspirin)

Mitochondrial DNA (mtDNA) Mitochondrial DNA (mtDNA) is maternally inherited as mitochondria of sperm cell do not enter the fertilized ova.

Mitochondrial DNA (mtDNA) codes for 13 polypeptide 13 polypeptide (of total 120) required for oxidative phosphorylation.

(while the remaining are synthesized in the cytosol & are transported into the mitochondria).

Defects of oxidative phosphorylation usually results from alteration in mtDNA alteration in mtDNA (mutation rate 10 times more than that of nuclear DNA).

Tissues with greater ATP requirement Tissues with greater ATP requirement (as CNS, sk.ms. & heart muscles, kidney & liver) are most affected by defects in oxidative phosphorylation.

Examples for diseases caused by mutations in mtDNA:Examples for diseases caused by mutations in mtDNA: 1- Mitochondrial myopathiesMitochondrial myopathies 2- Leber hereditary optic neuropathy Leber hereditary optic neuropathy

Mitochondria and apoptosis

initiated by the formation of pores in the outerMitochondrial membrane

pores allow cytochrome c to leave and enter the cytosol

+proapoptotic factors, activates a family of proteolytic enzymes ,the caspases

cleavage of key proteins and resulting in the morphologic and biochemical changes characteristic of apoptotic cell death