The Oxidative Phosphorylation

Objectives 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

Adenosine Triphosphate (ATP) Δ Gº -7.3 kcal/mol/bond

The tight control of the cellular respiration and phosphorylation is known as the respiratory control

Electron Transport Chain (ETC) 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

Electron Transport Chain (ETC) 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 acids are metabolized by a series of metabolic reactions yielding CO2 and H2O Energy is produced as ATP or heat

Metabolic breakdown of energy-yielding molecules Energy-rich reduced Coenzymes NADH & FADH2 PHOSPHORYLATION OXIDATIVE

Diet Carbohydrates Glycogen (liver & Sk. Ms.) Glucose GLYCOLYSIS (in cytoplasm) Pyruvate in mitochondria Acetyl CoA Citric Acid Cycle (in mitochondria) NADH & FADH2 Electron transport chain (flow of electrons) Formation of ATP (oxidative phosphorylation) CATABOLISM OF CARBOHYDRATES

Energy released from NADH & FADH2 entering ETC 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.

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA The transfer of hydrogen and ATP / ADP between the cytosol and the matrix need special carriers or shuttles to move both across the inner mitochondrial membranes

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA

OXIDATIVE PHOSPHORYLATION OCCURS IN THE MITOCHONDRIA All components of the electron transport chains are protein in nature, except Coenzyme Q. The coenzyme Q and cytochrome C are the mobile components of the electron transport chain The ATP synthase is formed of Fo/ F1 . Fo is a transmembrane domain while F1 is on the in matrix side of the inner mitochondrial membrane. The isolated ATP synthase enzyme has ATPase activity leading to hydrolysi of ATP into ADP + Pi

Electron transport chain 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 .

Inhibitors of Electron 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.

How the free energy generated by the transport of electrons by ECT is used to produce energy (ATP) Coupling of ECT to phosphorylation of ADP to ATP Transfer of electrons across electron transport chain FREE ENERGY RELEASED 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 is required for the synthesis of ATP from ADP and Pi. (phosphorylation)

Proton Pump Electron transport is coupled to the phosphorylation 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 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)

ATP PHOSPHORYLATION OXIDATION NADH Oxidative Phosphorylation (in mitochondria) Oxidation: electron flow in electron transport chain (with production of energy) Phosphorylation: phosphorylation of ADP to ATP

Uncoupling proteins (UCP) 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 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 protein 1 (UCP1) UCP1 (also called thermogenin) is responsible for the activation of fatty acid oxidation and heat production in the brown fat of mammals. Brown fat uses 90% of energy of ETC for thermogenesis in response to cold at birth & during arousal in hibernating Animals (by help of UCPl1). Humans have little of the brown fat (except in the newborn

Uncoupling proteins (UCP) Uncoupler proteins are present in the inner mitochondrial memberane . In the human body there are two types of adipocytes : the brown adipocytes and the white adipocytes. The brown adipocytes play an important role in newborns. The uncouplers lead to liberation of energy in the form of heat. There are synthetic uncouplers as 2,4dinitrophenol and salicylates in high doses leads to liberation of energy in the form of heat

Synthetic Uncouplers Synthetic uncouplers are compounds that can uncouple ETC & phosphorylation by increasing the permeability of the inner mitochondrial membrane to protons (thus will not reenter through ATP synthase) Examples: 2,4-dintrophenol An uncoupler that causes electron transport to proceed at a rapid rate without phosphorylation & thus energy is released as heat rather than being used to synthesize ATP High dose of aspirin (salicylates) uncouples oxidative phosphorylation causing fever (observed with toxic overdose of aspirin)

Inherited defects in oxidative phosphorylation Mitochondrial DNA (mtDNA) is maternally inherited as mitochondria of sperm cell do not enter the fertilized ova. Mitochondrial DNA (mtDNA) codes for 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 (mutation rate 10 times more than that of nuclear DNA). 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: 1- Mitochondrial myopathies 2- Leber hereditary optic neuropathy

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