Biochemistry: Bioenergetics, Mitochondrial Electron Transport, and Oxidative Phosphorylation PDF

1A BIOCHEMISTRY BIOENERGETICS, MITOCHONDRIAL ELECTRON TRANSPORT & OIXDATIVE PHOSPHORYLATION D...

1A BIOCHEMISTRY BIOENERGETICS, MITOCHONDRIAL ELECTRON TRANSPORT & OIXDATIVE PHOSPHORYLATION DR. B. JANDOC OUTLINE I. Overview II. Standard State III. Free Energy IV. ATP as an Energy Carrier V. Electron Transport (Respiratory) Chain VI. Oxidative Phosphorylation VII. Coupling of Phosphorylation to Respiration VIII. Superoxide Metabolism IX. Case Study: Ischemia and Reperfusion BIOENERGETICS, MITOCHONDRIAL ELECTRON TRANSPORT & OXIDATIVE PHOSPHORYLATION I. OVERVIEW A. Free Energy Change (ΔG)  2 varieties:  Bioenergetics o ΔG: predicts the change in free energy → direction of the o Transfer and utilization of energy in biologic systems reaction o Concerns only the initial and final energy states of reaction o ΔG° (Standard Free Energy): energy change when reactants components and products are 1moL/L o Predicts if a process is possible 1. Sign of ΔG predicts the Direction of a Reaction  Changes in Free Energy  Negative ΔG: net loss of enery → reaction goes spontaneously o Measure of the energetic feasibility of a chemical reaction → exergonic o Predicts whether a reaction will take place  Kinetic o Measures how fast the reaction occurs II. STANDARD STATES (for which bioenergetics is evaluated)  pH = 7  Temperature = 25°C (288°K)  All solute concentration = 1M  All gases = 1 atmospheric pressure  Positive ΔG: net gain of energy → reaction does not go III. FREE ENERGY spontaneously → endergonic  energy available to do work o Energy must be added to the system to make the process  Direction and extent to which a chemical reaction proceeds is proceed determined by the degree to which 2 factors change during the reaction o Enthalpy (ΔH)  Measure of the change in heat content of the reactants and products  energy absorbed / released in a reaction  H = E + PV (E = internal energy; P = pressure; V = volume) o Entropy (ΔS)  Measure of the change in randomness or disorder of reactants and products o Free Energy (G)  Amount of useful work that can be obtained from a system at constant temperature, pressure, and volume  Zero ΔG: the reactants are in equilibrium (negative ΔG →  Energy is needed to maintain entropy level (TΔS) reaction continues → equilibrium)  ΔG = ΔH – TΔS Trans 1 | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 10  1A BIOCHEMISTRY BIOENERGETICS, MITOCHONDRIAL ELECTRON TRANSPORT & OIXDATIVE PHOSPHORYLATION DR. B. JANDOC 2. ΔG of the Forward and Back Reactions  Equal in magnitude but opposite in signs  Forward reaction = -5000cal/mol; back reaction =+5000cal/mol  If reaction A↔B allowed to go to equilibrium at constant 3. ΔG depends on the Concentration of the Reactants and temperature and pressure → overall ΔG=0 Products  At constant temperature and pressure  If Keq = 1 then ΔG° = 0 A↔B ( reaction at equilibrium)  If Keq > 1 then ΔG° < 0 A←→B ( favour forward reaction) 4. Sign of ΔG can be Different from ΔG°  If Keq < 1 then ΔG° > 0 A←→B ( favour backward  A reaction with +ΔG° can proceed in the forward reaction reaction; gain of energy but no loss of energy) (negative overall ΔG) if the ratio of products to reactants ([B]/[A]) is sufficiently small (ratio of reactants to products is large) 3. ΔG° of Two Reactions are Additive (Coupled Reaction) B. Standard Free Energy Change (ΔG°)  Equal to the ΔG under standard conditions (reactants and 4. ΔG of a Pathway are Additive products kept at 1mol/L concentrations, ln is zero)  As long as the sum of the ΔG of the individual reaction is  ΔG = ΔG° + 0 negative → pathway can potentially proceed as written even if sum of the individual component reactions have a +ΔG  Actual rate of reactions depend on the activity of the enzyme that catalyse the reaction IV. ATP AS AN ENERGY CARRIER  Reactions or processes that have large + ΔG are made possible by coupling the endergonic reaction with a second spontaneous process with large - ΔG (ATP hydrolysis) A. Reactions are Coupled Through Common Intermediates  2 chemical reactions have a common intermediate when they 1. ΔG° is Predictive Only Under Standard Conditions occur sequentially so that the product of the first reaction is a  Composed of constants (R,T,Keq) → cannot predict the direction substrate for the second of a reaction under physiologic conditions  Not altered by changes in concentrations of products and substrates  Many coupled reactions use ATP to generate a common 2. Relationship Between ΔG° and Keq intermediate  Reaction is at equilibrium  Other reactions lead to ATP synthesis by transfer of phosphate  Ratio of product and reactant is constant from an energy-rich intermediate to ADP Trans 1 | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 10  1A BIOCHEMISTRY BIOENERGETICS, MITOCHONDRIAL ELECTRON TRANSPORT & OIXDATIVE PHOSPHORYLATION DR. B. JANDOC B. Energy Carried by ATP  Nucleoside monophosphate kinase  ΔG° of ATP: -7300cal/mol for each of the 2 terminal phosphate N-P + ATP → N-P-P + ADP groups → high energy phosphate compound  Very high-energy phosphate compounds (High Group-Transfer V. ELECTRON TRANSPORT (RESPIRATORY) CHAIN Potential): contain energy higher than ATP ”Respiratory” – utilization of molecular oxygen as the electron acceptor PHASES:  Phase I  Low Energy Phosphate Compounds o Oxidation of fuels o 90% vessel wall occlusion → cessation of blood flow anterior transmural left ventricular infarction o CK-MB Ischemia  At 4hrs: slightly increased  Myocardial metabolism shifts to anaerobic glycolysis (produce  At 12 hours: 4 fold increase only 1/10 ATP) o Plasma cholesterol: moderately increased  Decreased flow of substrates to the myocardium o Plasma triglycerides: within normal range  Decreased removal of metabolic products → accumulate intracellular metabolites → increased intracellular osmotic  Treatment pressure → cell swelling affecting cell membrane permeability o Patient was seen early (within 4 hours)  Affected myocardium will exhibit:  By 6 hours → irreversible damage o ATP depletion Streptokinase o Lactic acid* accumulation o Tissue plasminogen activator (t-PA) by catheterization + o Severe acidosis (H accumulation*) o Advise: cholesterol lowering regimen; stop smoking  *inhibit glycolysis at the level of glyceraldehyde-3- phosphate Discussion  Marked reduction of inotropism Transmural M.I.  Cessation of synthesis of nor regenerated by terminal electron  Etiology: caused by occlusive or near-occlusive thrombus lying in transport chain (no oxygen) close proximity to an atherosclerotic plaque  Depression of oxidative phosphorylation (↓ATP, ↑ADP)  Dx: o Adenine nucleotide pool depletion coincides with the o Clinical history development of irreversible cellular damage o ECG o Serial CK-MB measurements Reperfusion Injury o Troponin T and I  Cell death  Overall aim of treatment:  Damage to plasma membrane during the period of ischemia → o Prevent death from cardiac arrhythmias ++ altered permeability → Ca influx → unregulated enzyme o Limit infarction size (intracoronary administration of t-PA) inhibition / activation  Long-term treatment  Free radicals: o Measures to decrease plasma cholesterol o OH - o Diet  Cause: lipid peroxidation; DNA strand breakage; o Drugs that inhibit HMG-CoA reductase - oxidation of protein SH group o Superoxide Atherosclerosis  Generated by transfer of single electron to oxygen  Predisposing factors: catalysed by: cytochrome P450, xanthine oxidase, o Increased LDL respiratory burst oxidase (NADPH oxidase in PMN o Increased cholesterol leukocytes o Decreased HDL REFERENCE o Smoking o Hypertension  Notes from Dr. Jandoc and lecture as “ Trans 1 | ABACCO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 10 of 10