Oxidative Phosphorylation I, Lecture 6 PDF

THE ELECTRON TRANSPORT CHAIN & OXIDATIVE PHOSPHORYLATION I Nikita Sahadeo, Ph.D. Biochemistry Unit Department Of Preclinical Sciences Faculty of Medical Sciences University of the West Indies January 2024 LEARNING OBJECTIVE List the components of the electron transport chain and explain the process...

THE ELECTRON TRANSPORT CHAIN & OXIDATIVE PHOSPHORYLATION I Nikita Sahadeo, Ph.D. Biochemistry Unit Department Of Preclinical Sciences Faculty of Medical Sciences University of the West Indies January 2024 LEARNING OBJECTIVE List the components of the electron transport chain and explain the process involved in oxidative phosphorylation ATP & ENERGY COUPLING Energy coupling Occurs when the energy produced by one reaction or system is used to drive another reaction or system. Endergonic Describes a reaction that absorbs (heat) energy from its environment. Exergonic Describes a reaction that releases energy (heat) into its environment. Free energy Gibbs free energy is a thermodynamic potential that measures the useful or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (isothermal, isobaric). ATP & ENERGY COUPLING A nucleoside consisting of the nitrogenous base adenine and the five-carbon sugar ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are labeled alpha, beta, and gamma. Together, these chemical groups constitute an energy powerhouse. The two bonds between the phosphates (phosphoanhydride bonds) when broken, release sufficient energy to power a variety of cellular reactions and processes. The bond between the beta and gamma phosphate is considered “high-energy” ATP & ENERGY COUPLING ATP is hydrolyzed into ADP in the following reaction: ATP+H2O→ADP+Pi+free energy ADP is combined with a phosphate to form ATP in the following reaction: ADP+Pi+free energy→ATP+H2O ∆G for the hydrolysis of one mole of ATP into ADP and Pi is −7.3 kcal/mole (−30.5 kJ/mol) ∆G for the hydrolysis of one mole of ATP in a living cell is almost double the value at standard conditions: 14 kcal/mol (−57 kJ/mol) To harness the energy within the bonds of ATP, cells use a strategy called energy coupling. WHAT IS OXIDATIVE PHOSPHORYLATION? The process by which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 via an electron transport chain (ETC) NADH and FADH2 are both produced by the breakdown of carbohydrates, fats and amino acids; ATP synthesis driven by proton-motive force generated by the (ETC), referred to as the chemiosmotic hypothesis. THE FUNDAMENTAL CONCEPTS OF O.P. The process of oxidative phosphorylation involves understanding fundamental concepts: I. electronegativity II. the sources of reduced NADH and FADH2 III. the anatomy of the mitochondrion THE REDUCING AGENTS NADH & FADH 2 ATP is used as free-energy currency by coupling its (spontaneous) dephosphorylation with a (nonspontaneous) biochemical reaction to give a net release of free energy (i.e., a net spontaneous reaction) NADH Coupled reactions are also used to generate ATP by phosphorylating ADP. The nonspontaneous reaction of joining ADP to inorganic phosphate to make ATP is coupled to the oxidation reaction of NADH or FADH2 The oxidation reaction for NADH has a larger, but negative, free energy change than the positive free energy change required for the formation of ATP from ADP and phosphate. This set of coupled reactions is referred to as oxidative phosphorylation. FADH2 ATP YIELD OF ONE GLUCOSE MOLECULE DURING AEROBIC RESPIRATION THE MITOCHONDRIA The mitochondrion consists of inner and outer membranes The inner membrane has inward-facing fold-like projections known as cristae that vastly increase the surface area of the membrane to maximize the amount of energy production. The protein complexes involved in the electron transport chain are studded along this membrane. The inner membrane envelops the matrix, which houses mitochondrial DNA, ribosomes, and a multitude of enzymes and metabolites. The space between the inner and outer membrane is known as the intermembrane space; this is the site of hydrogen ion deposition for the protein complexes in the electron transport chain. The increased hydrogen ion (H+ ion) concentration (and effectual decreased pH) generate a membrane potential across the inner mitochondrial membrane. COUPLING THE OXIDATIVEPHOSPHORYLATION REACTIONS Synthesis of ATP is coupled with the oxidation of NADH and the reduction of O2. There are three key steps in this process: Electrons are transferred from NADH, through a series of electron carriers, to O2. The electron carriers are proteins embedded in the inner mitochondrial membrane. Transfer of electrons by these carriers generates a proton (H+) gradient across the inner mitochondrial membrane. When H+ spontaneously diffuses back across the inner mitochondrial membrane, ATP is synthesized. The large positive free energy of ATP synthesis is overcome by the even larger negative free energy associated with proton flow down the concentration gradient. Proton motive force refers to the energy obtained from the electrochemical gradient created by several of the electron carriers. Protons can re-enter the matrix only via proton specific channels (FO) The proton-motive force produced provides the energy for ATP synthesis; catalyzed by the F1 complex The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP ATP SYNTHASE It is proposed: the 3 active β sites take turns catalysing ATP synthesis; a subunit starts in the β-ADP conformation that binds ADP and Pi; the conformation changes to the β-ATP form that tightly binds and stabilises ATP; the conformation then changes to β-empty, which has low affinity for ATP; the newly synthesised ATP leaves the enzyme; another round of catalysis begins when this subunit again assumes the β-ADP form and binds ADP and Pi. Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient CYTOCHROMES Proteins with iron-containing prosthetic group Mitochondria has classes a, b and c; distinguished by differences in their light absorption spectra Iron exists in association with inorganic S or with the S atoms of Cys residues All iron-sulfur proteins participate in one electron transfer; one iron atom is reduced or oxidized COMPLEX I (NADH DEHYDROGENASE) The exergonic transfer of a hydride ion from NADH to FMN, then via a series of Fe-S centers to the Fe-S protein N-2. Electrons then transfer from N-2 to Q to form QH2 which diffuses into the lipid bilayer The endergonic expulsion of four protons (per pair of electrons) from the matrix to the intermembrane space COMPLEX II (SUCCINATE DEHYDROGENASE) The only membrane-bound enzyme of the TCA cycle Electrons pass from succinate to FAD, then through Fe-S centres to Q COMPLEX III (CYTOCHROME C REDUCTASE) Couples the transfer of electrons from ubiquinol (QH2) to cytochrome c to proton expulsion to the intermembrane space QH2 is oxidized to Q and two molecules of cytochrome c are reduced Cytochrome c is a soluble protein in the intermembrane space; its haem accepts an electron from Complex III and donates it to the binuclear Cu center of Complex IV COMPLEX IV (CYTOCHROME C OXIDASE) Carries electrons from cytochrome c to molecular O2, reducing it to H2O, via CuA, heme a and the FeCu centre Four electrons, four substrate protons are used to give two molecules of H2O Simultaneously, four protons are pumped from the matrix to the intermembrane space A benzoquinone; lipid-soluble, small, hydrophobic; resides in the membrane Accepts: - one electron to form the semiquinone radical, QH - two electrons to form ubiquinol, QH2 Acts at the junction between two electron donors (Complexes I and II) and one electron acceptor (Complex III) Couples electron flow to proton movement UBIQUINONE (COENZYME Q)