Reactive Oxygen Species (ROS) and Oxidative Stress_KRR2023_Post.pptx

Reactive Oxygen Species (ROS) and Oxidative Stress (Chapter 25) K. Russell Randall, Ph.D. Learning Objectives: 1. Describe free radicals and how they are formed 2. Identify the major reactive oxygen species (ROS) and explain how they are formed in the cell 3. Describe the effects of ROS on...

Reactive Oxygen Species (ROS) and Oxidative Stress (Chapter 25) K. Russell Randall, Ph.D. Learning Objectives: 1. Describe free radicals and how they are formed 2. Identify the major reactive oxygen species (ROS) and explain how they are formed in the cell 3. Describe the effects of ROS on various biomolecules 4. Describe lipid peroxidation reactions and their impact on biological systems 5. Describe the role of oxidative stress in damage and disease 6. Identify the major antioxidant enzymes, vitamins and biomolecules that provide protection against ROS formation and damage Free Radicals What are Free Radicals? By definition, a free radical is any atom (e.g., oxygen, nitrogen, iron) with an unpaired electron in the outermost shell and is capable of independent existence.  Very short half-lives  Abstract or donate an electron in order to achieve stability  A chain reaction  Can only be quenched when 2 radicals react together Free Radicals What are Free Radicals?  A free radical is easily formed when a covalent bond between entities (atoms) is broken, and one electron remains with each newly formed atom.  The resulting free radicals may participate in further reactions or may combine to reform the original compound  Free radicals are formed as necessary Free Radicals intermediates in a variety of normal biochemical reactions, but when generated in excess or not appropriately controlled, radicals can cause major damage to macromolecules.  A prominent feature of radicals is that they have extremely high chemical reactivity - explains not only their normal biological activities, but how they inflict damage on cells.  The high chemical reactivity is due to the presence of unpaired electron(s). http://www.ehowtogetridofstretchmarks.com/wp-content/uploads/2012/02/The-effects-of-free-radicals-on-skin-and-how-they-form.jpg Relatively inert at body temperature Approx. 90% of our O2 usage is committed to oxidative phosphorylation Oxygen Approx. 10% used for enzymatic hydroxylation and oxidation reactions <1% converted to reactive oxygen species (ROS) – reactive forms of O2 ROS has some beneficial effects, but are also the source of chronic damage to tissue biomolecules What are Reactive Oxygen Species?  Radicals of most concern in biological systems are derived from oxygen - reactive oxygen species (ROS).  Highly reactive ions due to the presence of unpaired valence shell electrons  In the natural, ground state form, oxygen is a biradical  Sequential reduction of molecular oxygen (equivalent to sequential addition of electrons) leads to formation of a group of reactive oxygen species: Reactive Oxygen Species  ROS are the most damaging radicals in biological systems  ROS are toxic by-products of life in an aerobic environment.  ROS forms as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling.  The electron transport chain is the main source of ROS https://teachmephysiology.com/wp-content/uploads/ 2017/03/etc.jpg Sources of Reactive Oxygen Species? Nitric oxide synthases Myeloperoxidase Xanthine oxidase Superoxide Superoxide Anion (O2-): Formed when oxygen (O2) acquires an additional electron- leaves the molecule with only one unpaired electron.  Within the mitochondria O2- is continuously being formed.  One of the major sites of superoxide generation is coenzyme Q (CoQ) in the mitochondrial electron transport chain (The one-electron reduced form of CoQ (CoQH•) is free within the membrane and can transfer an electron to dissolved O 2)  The rate of formation depends on the amount of oxygen flowing through the mitochondria at any given time. NADPH oxidase Hydrogen Peroxide Hydrogen Peroxide (H2O2): produced in vivo by many reactions. H2O2 and lipid peroxides are generated enzymatically as major reaction products by several oxidases present in peroxisomes, mitochondria, and the ER.  Superoxide readily undergoes dismutation with the formation of hydrogen peroxide – occurs spontaneously or catalyzed by superoxide dismutase (SOD) H2O2 can be converted to highly damaging hydroxyl radicals or be catalyzed and excreted harmlessly as water. Superoxide Dismutase Hydroxyl Radical Hydroxyl Radical species; short-lived (OH·): Most reactive and damaging This type of free radical can be formed from O2- and H2O2 via the Haber-Weiss reaction.  The interaction of transition metals, e.g., copper or iron, and H2O2 also produces OH(Fenton reaction).  These reactions are significant because these substrates (i.e., Fe, peroxide) are found within the body and can easily interact. • Generation of the Hydroxyl Radical Other free radicals include: 1. Reactive Nitrogen Species (RNS) Nitric Oxide NO Peroxynitrite ONOO2. Transition Metals: With the exception of Zinc, transition metals serve to catalyze the Haber-Weiss and Fenton Redox Reactions. • Iron Fe2+, Fe3+ • Copper • Zinc Cu1+, Cu2+ Zn2+ The Role of Free Radicals and ROS in Cell Damage and Disease Interaction of free radicals and ROS with bases in DNA, unsaturated fatty acids in cell membranes and plasma lipoproteins, as well as proteins can lead to heritable mutations, Free-radical and ROS–mediated cellular injury Lipid Peroxidation http://www.jnanobiotechnology.com/content/figures/14773155-5-1-5.jpg  The total body free radical burden can be estimated by measuring the products of lipid peroxidation  Lipid peroxidation - the oxidative deterioration of lipids containing any number of carbon-carbon double bonds.  The process whereby free radicals “steal” electrons from the lipids in our cell membranes, thereby initiating a free radical attack on the cell, resulting in cell damage and increased production of free radicals.  Lipid peroxidation (LPO) may result in breakdown of the plasma and organelle membranes, eventually causing cell damage and death.  Lipid peroxidation is used as an indicator of oxidative stress in tissues and cells Lipid Peroxidation: radical chain reaction a free Reactive oxygen species readily attack the polyunsaturated fatty acids of the fatty acid membrane, initiating a selfpropagating chain reaction. ROS targets the carbon-carbon double bond of polyunsaturated fatty acids, where the double bond on the carbon weakens the carbonhydrogen bond. http://www.biotek.com/assets/tech_resources/10592/fig4.jpg The carbon-carbon double bond allows for easy dissociation of the https://encryptedtbn3.gstatic.com/images? q=tbn:ANd9GcSOzrPCUkQ_kqg8MGobd0ogGWzhweFniumUL19dhwL4YAWvz sCK During times of environmental stress ROS levels can increase dramatically, resulting in significant damage to cell structures. This accumulative action can result into a situation known as oxidative stress. The Role of Oxidative Stress in Cell Damage and Disease Oxidative damage caused by ROS has been implicated in the aging process as well as in a growing list of diseases. Erythrocytes (red blood cells), in particular, experience oxidative stress due to their periodic Implication of oxidative stress in pathology http://www.intechopen.com/source/html/38469/media/ image1.png Antioxidant Protection Antioxidants - compounds such as vitamins and glutathione that protect cells against the damaging effects of ROS and RNS.  Oxidative stress and subsequent cellular damage occurs when there is an imbalance between antioxidants and ROS. http://oem.bmj.com/content/60/8/612/F1.medium.gif Antioxidant Protection Antioxidants assist in preventing ROS damage by scavenging free radicals. http://www.examiner.com/images/blog/EXID18534/images/ 3(5).jpg http://agendokter.info/wp-content/uploads/2010/03/antioxidants-free-radical-body.jpg Antioxidant Protection Antioxidant Scavenging Enzymes: Cells are normally able to defend themselves against ROS damage using enzymes. 1. The chief cellular defense strategy against oxidative damage by ROS is the enzyme superoxide dismutase (SOD) 2. Another important enzyme that scavenges the hydrogen peroxide formed by superoxide dismutase and by other processes is catalase. 3. Glutathione Peroxidase & Glutathione Reductase  Glutathione may well be the most important intracellular defense against damage by ROS  A sufficient pool of reduced glutathione is necessary to help cells cope with oxidative stress  GSH reacts with peroxides, which might otherwise lead to oxidative damage. Glutathione-Mediated Defense Against Oxidative Stress When glutathione is oxidized, the reducing power of NADPH is used to regenerate GSH. GSH in Erythrocytes  Major function of reduced glutathione (GSH) in erythrocytes – eliminate H2O2 and organic hydroperoxides, which can irreversibly damage hemoglobin and cleave C-C bonds in the phospholipid tails of cell membranes.  The unchecked buildup of peroxides results in premature cell lysis Dehydrogenase – key role in protections against ROS RBCs in individuals deficient in glucose-6-phosphate dehydrogenase (G6PD) are particularly sensitive to oxidative damage. Glucose 6-phosphate dehydrogenase deficiency causes drug-induced hemolytic anemia Nonenzymatic Antioxidants - Antioxidant Vitamins Vitamin E The roles of vitamins E and C in reducing lipid peroxides, and stabilization of the tocopheroxyl radical by delocalization of the unpaired electron. Vitamin E (α-tocopherol) - the most widely distributed antioxidant in nature - lipid-soluble antioxidant vitamin - plays a vital role in protecting membranes from oxidative damage. Its primary activity is to trap peroxy radicals in cellular membranes. Nonenzymatic Antioxidants - Antioxidant Vitamins Ascorbic Acid (vitamin-C): water-soluble vitamin Humans cannot synthesize their own ascorbic acid - it must be obtained through the diet http://reducefreeradicals.com/wpcontent/uploads/2012/01/freeradicals.jpg Examples of good sources of ascorbic acid in nature include: Citrus Fruits, Mango, Kiwi, Guava, Strawberries, Broccoli, Brussel sprouts, Cabbage, Cauliflower, Potatoes and Sweet potatoes, Sweet Peppers, Parsley, Watercress Nonenzymatic Antioxidants - Antioxidant Vitamins Carotenoids: - lipid-soluble vitamin - term applied to β-carotene (the precursor of vitamin A) and similar compounds (e.g., zeaxanthin and lutein – macular carotenoids) - lutein and zeaxanthin shown to decrease incidence of age-related macular degeneration https://media.springernature.com/lw685/springer-static/image/chp %3A10.1007%2F978-3-030-46459-2_12/MediaObjects/ 481152_1_En_12_Fig1_HTML.png Compartmentation of Free-Radical Defenses ROS formation in mitochondria and mitochondrial defenses Nitric oxide synthases Myeloperoxidase Xanthine oxidase • G6PD • Bilirubin