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SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. CHAPTER 5: CHEMICAL REACTIONS IN THE LIVING CELL - PART I - “All living beings need some form of energy to maintain their complex structures and to stay alive …” Without a steady flow of energy through their bodies, living organisms would not able to maintain their complex carbon-based structures and life-supporting chemical activities. But what exactly is energy and what type of energy are we talking about? Since life forms do not operate with batteries to supply the necessary life energy, which energy is keeping the different forms of life going and alive? Definition: Energy Energy is per definition the ability to perfom work Work and energy are measured in the same units (= Joule); or in older text books in calorie (cal) energy is one of the two fundamental ideas in physics; the other one is matter A. Einstein taught us with his famous formula E = m x c2, that energy and matter are closely related and interconvertable Energy exists in many forms, but only some energy forms are tapped and used by biological organisms Form of energy Biological Use P Pootteennttiiaall Proton/ion gradients M e c h a n i c a l ( = k i n e t i c ) Mechanical (= kinetic) Movement/Flying C Chheem miiccaall Metabolism TThheerrm maall eenneerrggyy Body temperature E l e c t r o m a g n e t i c ( = l i g h t ) Electromagnetic (= light) Photosynthesis E Elleeccttrriiccaall Electrical organs M a g n e t i c Magnetic Orientation/Navigation G Grraavviittaattiioonn N Nuucclleeaarr M Maassss = these forms of energy are relevant to most biological organisms; = these forms of energy play a role and are used by some organisms All biological, including human life strictly depends upon the energy in the universe, of which the solar light energy is the most important one. Solar energy is utilized by photosynthesizing life forms, such as green plants and algae, to produce complex high energy carbon molecules, such as glucose and sucrose. Since the carbon molecules are taking up by all heterotrophic life forms, including fungi, animals and humans, as part of their food, literally all life on planet earth is dependent on the sun’s abundantly emitted solar energy. 1 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. The Rules of Thermodynamics or The Laws of Energy In order to understand how the different forms of life - as the truly most complex organized form of matter in the universe - are able to use different forms of energy to sustain their amazing life processes, such as growth, to movement, to flight and reproduction, it is important to first understand the nature of energy Our scientific understanding of energy came a long way and major contributions to our modern understanding have been made by the field of thermodynamics, which is an important sub-discipline of physics Thermodynamics is the scientific study of energy transformation that occurs between a defined collection of matter or a so-called system and its surroundings a system can be, e.g. a water turbine, the engine of a car or the living cells of biological organisms The field of thermodynamics lead to the discovery of two laws, the first and second law of thermodynamics or laws of energy What are the 2 laws of thermodynamics and what exactly do they say? First law of energy (= law of the conservation of energy) it states, that energy can be changed (= transformed) from one form into another, but it cannot be created or destroyed e.g. Pendulum experiment e.g. light emission in fire flies the total amount of energy and matter in the Universe remains constant, merely changing from one form to another S Seeccoonndd llaaw w ooff eenneerrggyy it states, that every system and its surroundings spontaneously tend toward a higher degree of disorder (= entropy); one prominent form of disorder is heat (= thermal energy) it also states that any form of energy conversion reduces the order in the universe and leads to an increase in disorder (= entropy), in many cases in form of released thermal energy (= heat) in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state 2 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Biological organisms and its cells are so-called open systems, which strictly underlay the 2 laws of thermodynamics open systems means that they exchange both matter and energy with its surroundings; a living organism takes up matter in form of food, oxygen and water and (after multiple transformations releases matter and energy in form of urea, water, CO2 and heat they follow the first law of energy since they don’t create energy de-novo but rather transform or convert pre-existing forms of energy into new forms of mostly chemical energy the follow the second law of energy since the energy conversion processes in living organisms do not occur with 100% efficiency “Biological organisms as open systems with enormously complex carbon-based structures obey the two laws of thermodynamics; they are not any different than any other energy-converting open systems, such as machines and engines …” Living cells are due to their permanent flow and conversions of energy and due to their highly organized structures in a thermodynamic sense open, low entropic systems. Open means that they permanently take up some for of energy. Low entropic means that parts of the taken up energy is used to create highly ordered (low entropy) structures. To perform their many tasks, cells require transfusions of energy from outside sources. In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. 3 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. “Living organisms are the strict opposite of disorder ..” Cells convert chemical energy (which is a form of potential energy), usually stored in the form of carbon-carbon (C – C) covalent bonds (as in glucose or a fatty acid) or in the form of phosphorus-oxygen (P – O) covalent bonds (as in the ATP molecule), into kinetic energy to accomplish their life processes, e.g. cell division, growth, biosynthesis, and active transport. This flow of energy maintains the organized structures, order and the life activities. Entropy wins when organisms cease to take in energy and die As a consequence of the permanent energy transformations, e.g. on our sun, in biological organisms, and in our intensively used cars and machineries, the entropy in the universe increases steadily. consider this: a car turns about 75% of the chemical energy of the molecules of gasoline into the unordered energy = heat; only 25% is transformed 4 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. in an ordered fashion into kinetic energy = driving force Organic molecules existing in or being taken up by a living cell possess potential energy due to the arrangement of their electrons in their chemical bonds. Organic molecules store energy in their arrangement of atoms. This chemical energy is permanently transformed into other forms of energy. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy. Some of the released energy is in most cases stored in other forms of chemical energy again (mostly in form of ATP), used to do work and the rest is dissipated as heat. - in many cases it is transformed into mechanical or kinetic energy, e.g. swimming, running, flying, sliding, etc. - in some cases it is transformed into light energy which can be seen as light emission (= bioluminsecence); e.g. in the bioluminescent abdomen of fire-flies or of the luminescent extensions of deep-sea fish - in some cases it is transformed into warmth/heat e.g. in brown fat tissue of polar inhabitants or in form of warm muscles after prolonged exercise Since cells and biological organisms create highly ordered structures from less ordered starting material, they increase the entropy in their surrounding In living systems, energy stored in organic molecules is released in a step-wise fashion along so-called metabolic pathways. The release of the energy stored in complex organic molecules is called catabolic. One type of catabolic process, fermentation, which leads to the partial degradation of sugars in the absence of oxygen. A more efficient and widespread catabolic process, cellular respiration, uses oxygen as a reactant to complete the breakdown of a variety of organic molecules. - Most of the processes in cellular respiration occur in mitochondria. The second law of thermodynamics also explains that energy transfers in a cell cannot be 100% efficient; some energy retrieved from the chemical reactions always escapes the cells in form of disordered energy, of which heat is the most common one. cells haven’t developed a mechanism to re-use this escaped heat energy for biological work. 5 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. 4.2. ENERGY TRANSFER IN BIOLOGICAL SYSTEMS, FREE ENERGY COUPLING & THE FUNDAMENTAL ROLE OF THE ATP MOLECULE TO LIFE There is thousands of chemical reactions and energy conversions happening every second in any cell of any form of life. During any chemical reaction, a starting molecule, called the rreeaaccttaanntt, is converted into a structurally different molecule, called the pprroodduucctt If the product is not removed, the reaction reaches a so-called equilibrium, which is unique for each chemical reaction. Each chemical reaction has an equilibrium constant (Keq). Chemists divide chemical reactions into 2 major reaction types which both occur and can be observed in a living cell: 11.. E Ennddeerrggoonniicc reactions These are chemical reactions where the energy content (Gibbs Free Energy) of the products is higher than the energy content of the reactants. They also require energy to be added to the reactions to start them and to keep them going. The most important endergonic chemical reactions in biological organisms are: 1. the formation of polypeptides and proteins from amino acids 2. the build-up of the nucleic acids DNA and RNA from precursor molecules (= nucleotides), and 3. the build-up of fat from the precursor molecule acetyl-CoA The rreeaaccttaannttss have less energy than the pprroodduucctt & extra energy must be supplied from the surrounding Another important example of an endergonic chemical reaction is the biological process called photosynthesis, which builds up high energy-containing glucose molecules from the simple precursor molecules carbon dioxide (CO2) and water (H2O) e.g. Photosynthesis solar energy Energy: + 6 CO2 + 6 H2O C6H12O6 + 6 O2 (glucose) rreeaaccttaannttss pprroodduuccttss 6 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. 22.. E Exxeerrggoonniicc reactions These are generally chemical reaction which release energy during the chemical process. The rreeaaccttaannttss contain more potential (= chemical) energy than the pprroodduuccttss since the equilibrium constant for an exergonic reaction is greater than 1, the concentration of products is greater than the concentration of reactants at equilibrium Important examples of exergonic chemical reactions are: e.g. Combustion wood or (cellulose) gasoline + 6 O2 6 CO2 + 6 H2O + energy (hydrocarbons) e.g. Cellular Respiration - Cellular respiration is similar to the combustion of gasoline in an automobile engine. - The overall process is: Organic compounds + O2 -> CO2 + H2O + energy - Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose. C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy (glucose) rreeaaccttaannttss pprroodduuccttss If 1 mol (180 g) of glucose reacts with oxygen under standard conditions, 686 kcal of energy is released. If glucose is simply burned in air, e.g. in a calorimeter, all or most of this energy is released as heat In the cell, however, this important exergonic chemical reaction is tightly coupled to the synthesis of ATP from ADP (see: free energy coupling in cellular chemical reactions) Unlike the explosive release of heat energy that would occur when H 2 and O2 combine, cellular respiration uses several small metabolic steps and an electron transport chain to break the fall of electrons to O2 into several steps. 7 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. E Enneerrggyy pprrooffiillee ooff cchheem miiccaall rreeaaccttiioonnss eexxeerrggoonniicc eennddeerrggoonniicc Energy Barrier Energy Barrier R Reeaaccttaanntt P Prroodduucctt R Reeaaccttaanntt P Prroodduucctt 8 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. S Sppoonnttaanneeoouuss aanndd nnoonn--ssppoonnttaanneeoouuss cchheem miiccaall rreeaaccttiioonnss Chemical reactions rarely start or ignite suddenly, but rather have to be “jumpstarted” by adding some form of “activation energy”, mostly in form of heat, to the reactants think of your car and the important role of the spark plugs in starting the combustion reaction in the combustion chambers of your car’s engine! your car wouldn’t go anywhere without that extra “energy push” given to the gasoline (= the reactant) in your combustion chambers! In order to become a new product, the molecules of the reactant have to overcome a the so-called energy potential barrier to make the transition toward the new energy state of the products of that chemical reaction A very low energy barrier of a distinct chemical reaction favors a spontaneous transition toward the new energy state; it favors a spontaneous chemical reaction If the energy barrier of a chemical reaction is high, a transition event is unlikely; if the reaction only occurs and proceeds under addition of external energy, e.g. heat, pressure, light, etc., chemists speak of a non-spontaneous chemical reaction Non-spontaneous, exergonic reaction C6H12O6 + 6 O2 6 CO2 + 6 H2O + eenneerrggyy in this chemical reaction 676 kcal/mole glucose of energy, mostly in form of heat, is given off during this reaction the released heat energy (= free enthalpy) donates the further required activation energy to keep the chemical reaction going until chemical equilibrium has been reached it is a typical example of an exothermic reaction (= release of heat) Spontaneous, endergonic reaction 2 N2O5 + eenneerrggyy 4 NO2 + O2 Dinitrogen pentoxide decomposes spontaneously under consumption of energy ergo: something other than the heat change is driving this chemical reaction; the other driving force is known in thermodynamics as Entropy (degree of disorder) 2 molecules of reactant (less disordered state) are transformed into 5 molecules of product (more disordered state), which in total represents a higher magnitude of disorder C Coonncclluussiioonn:: 22 m maajjoorr ffoorrcceess aarree ddrriivviinngg cchheem miiccaall rreeaaccttiioonnss 1. Difference in heat content (= Δ ΔH H) 2. Difference in disorder or Entropy (= Δ ΔS S) 9 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Chemists combine these two driving forces of chemical reactions in the concept of Gibbs Free Energy (G) G Giibbbbss FFrreeee E Enneerrggyy the Gibbs free energy is a number which gives the intrinsic potential energy (in kJ/mole) of a substance or a system this information experimentally retrieved from calorimetric measurements of individual reaction partners is used to determine whether a certain chemical reaction will occur spontaneously the change in Gibbs Free Energy between reactants and resulting products of a distinct chemical reaction is represented by G and called free energy change the ffrreeee eenneerrggyy cchhaannggee ((ΔΔG G)) is equal to the change in heat content or enthalpy (ΔH) minus the entropy change (ΔS): Δ ΔG G =Δ ΔH H – T xΔ ΔS S T= temperature in degrees Kelvin (= oK) A reaction that gives off Gibbs free energy is considered as exergonic: eexxeerrggoonniicc & spontaneous: ΔG = minus (-) the sign of ΔG is negative A reaction that consumes Gibbs free energy is referred to as endergonic: eennddeerrggoonniicc & non-spontaneous: ΔG = plus (+) the sign of ΔG is positive E Exxaam mpplleess ooff G Giibbbbss FFrreeee E Enneerrggyy cchhaannggeess ((G G)) ooff eennddeerrggoonniicc aanndd eexxeerrggoonniicc cchheem miiccaall rreeaaccttiioonnss INPUT OUTPUT non-spontaneous, endergonic (1) Glutamic acid + NH3 + energy Glutamine + H2O ΔG (Δ G == ++ 33..44 kkccaall//m moollee) 10 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. spontaneous, exergonic (2) Glucose-6-PO4 + H2O Glucose + PO4 + energy ΔG (Δ G == -- 33..33 kkccaall//m moollee) spontaneous, exergonic (3) ATP + H2O ADP + PO4 + energy ΔG (Δ G == -- 77..33 kkccaall//m moollee) FFrreeee eenneerrggyy ccoouupplliinngg iinn bbiioollooggiiccaall cceellllss (1) biological organisms couple endergonic with exergonic chemical reactions with the help of the energy-rich ATP molecule ATP + H2O ADP + PO4 + energy ΔG (Δ G == -- 77..33) ΔG (2) Glutamic acid + NH3 + energy Glutamine + H2O (Δ G == ++ 33..44) ---------------------------------------------------------------------------------------------Glutamic acid + NH3 + ATP Glutamine + PO4+ ADP + energy ΔG (Δ G == -- 33..99) the coupled reaction is ssppoonnttaanneeoouuss and eexxeerrggoonniicc ! burning is only one, very fast and uncontrolled, way to release energy (solely as heat and light) from chemical compounds in living organisms and cells, chemicals are burned in a slow, step by step and highly controlled way in a ‘biological burning process’, called cellular respiration during this process, part of the energy released during the exergonic breakdown of sugar molecules into water and CO2, is conserved into high-energy-containing molecules one of the most important molecules into which cells store chemical energy is ATP = adenosine-triphosphate (see part 3 of this UNIT for more details) every working cell simultaneously carries out thousands of endergonic and exergonic chemical reactions; the sum of which is called cellular metabolism With help of this cellular metabolism together with cellular respiration, living organisms stay capable to keep all their vital functions running e.g. eating, digesting, escaping predators, repair of damaged tissue or growing 11 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Role of the ATP molecule in cellular energetics All biological activities, such as muscle contraction, ion transport across cell membranes, or glucose uptake into cells, require energy which is collected from the exergonic degradation of nutritional sugar or other food sources. Part of the released (chemical) energy is conversed into the synthesis of the most important molecule of biological systems, into Adenosine Triphosphate (= ATP) ATP, adenosine triphosphate, is the pivotal molecule in cellular energetics, because in donates chemical (Gibbs free) energy to endergonic chemical reactions to allow them to proceed. The ATP molecule is the chemical equivalent of a loaded spring. The close packing of three negatively-charged phosphate groups is an unstable, energy-storing arrangement. Loss of the end (or gamma) phosphate group “relaxes” the “spring”. Whenever, the gamma phosphate of ATP is hydrolyzed to form ADP, 7.3 kcal or energy is released for every mol (see Table below) Table: Examples of the Standard Free Energy (G) of different biomolecules Phosphoenolpyruvate 1,3Diphosphoglycerate ATP Glucose -1-PO4 Glucose -6-PO4 14.8 kcal/mole 11.8 kcal/mole 7.3 kcal/mole 5.0 kcal/mole 3.3 kcal/mole Due to this relatively high energy donating capacity, ATP (= Adenosine-TrisPhosphate) is surely one of the most important molecules in biological organisms. It is of crucial importance in all biological energy transfer and coupling reactions It is not surprising that all living cell contain high (milli-molar) concentrations of ATP at any given time. The ATP molecule, is made up of adenine, ribose and three covalently linked phosphate groups. 12 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. TThhee cchheem miiccaall ssttrruuccttuurree ooff A Addeennoossiinnee--TTrriisspphhoosspphhaattee ((== A ATTP P)) A Addeenniinnee R Riibboossee γ β α 33 xx P h o Phosspphhaatteess During the diverse biological energy coupling reactions, the energy conserved in the three high-energy phosphate groups of the ATP molecule is released after cleavage of the last (or so-called gamma phosphate group) and used to drive coupled endergonic synthesis reactions within the cell the gamma (= γ) phosphate cleavage frees up approx. 31 kJ/mol of usable Energy (see Graphic below) 13 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. The ATP molecule & Exergonic cleavage of the γ-phosphate ΔG = - 7.3 kcal/mole γ-Phosphate H Adenosine Triphosphate (ATP) Pi Adenosine Diphosphate (ADP) Phosphate It is the chemical reaction (= hydrolysis reaction) of ATP which powers almost all forms of cellular work , such as: 1. muscle contraction ( moving, flying, swimming, crawling) 2. light perception ( seeing) 3. neural activities ( thinking) 4. transport of nutrients ( food resorption) 5. or light generation ( fire fly bioluminescence) • The transfer of the terminal phosphate group from ATP to another molecule which is called phosphorylation plays an important role in regulation of important cellular processes, such as insulin receptor functioning, cell communication and gene activation. This phosphate transfer changes the shape of the receiving molecule, (in most cases a protein) therefore performing work in form of transport, mechanical, or chemical. When the phosphate groups leaves the phosphorylated molecule again, the molecule returns to its alternate shape and its original biological function. • In a very special cellular process called protein phosphorylation, the gammaphosphate of ATP is transferred to certain amino acid residues of proteins, e.g. serine or tyrosine. This phosphorylation event changes the 3D structure of the affected protein which is usually accompanied with a change in protein function or in case of an enzyme with a change in enzymatic activity. - examples are: phosphorylation of myosin protein in muscle cells during contraction - phosphorylation of so-called receptor kinases after binding of hormones or 14 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. growth factors The chemical structure of ATP shows three phosphate groups which each contain a high amount of chemical reaction energy in their chemical bonds after cleavage of either one of these phosphate groups, the earlier conserved energy gets released in an exergonic reaction In cells, this exergonic ATP cleavage reaction is coupled with other endergonic biological reactions in a so-called free energy coupling reaction. Energy coupling reactions are the chemical driving force behind the many metabolic activities of cells in Glycolysis, one of the most important biological metabolic reactions (see UNIT 6 for more details), the exergonic energy released from the breakdown of glucose molecules is transferred and stored in the high-energy phosphate groups of ATP (see structural formula of ATP below) at other places in the cell, this ATP-conserved energy will be released again by a chemical process called hydrolysis; it is especially the third, so-called gamma-phosphate of the ATP which is usually hydrolyzed in this highly exergonic reaction 15 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. ATP is a renewable source of chemical energy, which cells can regenerate by two major mechanisms. The regeneration of ATP can happen via: 1. 2. new synthesis starting from ADP in mitochondria via a process called oxidative phosphorylation (see UNIT 6) fast regeneration of ATP from ADP in the presence of phosphocreatine, which is the major chemical ‘reserve fuel’ in many cells (e.g. skeletal muscle cells) “A working cell consumes and regenerates its entire pool of ATP approx. once every minute!!” Another important chemical reaction where energy coupling plays a fundamental role are the so-called reduction/oxidation reactions, which we will look up and discuss in more detail in the next chapter Reduction-Oxidation (= Redox) reactions The life activities of biological organisms are due to thousands of biochemical reactions, which are essentially energy transfers in form of moved electrons from one molecule to another In catabolic pathways electrons stored in food molecules are relocated between different chemical reaction partners, releasing energy that is used to synthesize ATP. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions. The most important chemical reactions in living organisms are indeed rreedductionooxxidation reaction or for short redox reactions! During redox reactions, outer shell electrons of functional groups of certain molecules are moved from one molecule to another 1. Removal of an electron (= e¯) from an atom or molecule is an ooxxiiddaattiioonn reaction Dehydrogenation (= removal of a hydrogen atom) is also an oxidation reaction Lactic acid Pyruvic acid + ee¯¯ 16 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. 2. Addition of an electron (= e¯) is a rreedduuccttiioonn reaction Hydrogenation (= addition of a hydrogen atom) is also a reduction reaction Pyruvic acid + ee¯¯ Lactic acid In reduction/oxidation reactions, one molecule (the reducing agent or reductant) is oxidized, and its electrons are passed on to another (usually neighboring) molecule (the oxidizing agent or oxidant), which becomes reduced Redox reactions play a major role in the most significant chemical reaction pathways and processes in living organisms, such as in photosynthesis, during glycolysis, in the Krebs cycle and in the mitochondrial electron transport chain e.g. glucose gets dismantled to CO2 and water during cellular respiration in a series of sequential redox reactions in the Krebs cycle, the molecule succinate is oxidized to fumarate under release of two electrons (and two protons) 17 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Conversion of succinate into fumarate (Krebs cycle, Mitochondrion) Redox reactions also occur when the movement of electrons is not complete but involve a change in the degree of electron sharing in covalent bonds. For example, in the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C-H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O-H). 18 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. However, the energy of most moved electrons within living cells is trapped and rearranged in a series of important and highly abundant biological compounds (redox molecules), such as NAD+ or FAD+. Reminder: The release of electrons from a molecule is commonly called oxidation; the molecule which donates these electron during the chemical reaction is called an electron donor or reductant. Conversely, the reception of electrons during a redox reaction is called reduction; the molecules which receives the electrons is called the electron acceptor or oxidant. Electron transfer during redox reactions requires both a donor and an electron acceptor; redox reactions are always coupled together during degradation of glucose it loses its electrons in form of hydrogen (H)atoms, while molecular oxygen (O2) gains electrons (again in form of H-atoms!); we say: glucose becomes oxidized, while O2 is reduced to water! at this point it may be easier to understand now when we always spoke about burning of glucose during cellular metabolism; since burning (on a molecular level) is nothing else than oxidation of a compound! When during cellular redox reactions, bonds shift from nonpolar to polar, the electrons move from positions equidistant between the two atoms for a closer position to oxygen, the more electronegative atom. Key Information: Oxygen is one of the most potent oxidizing agents. An electron looses energy as it shifts from a less electronegative atom to a more electronegative one. A redox reaction that relocates electrons closer to (the very electronegative) oxygen releases chemical energy that can do work. In biological systems the electrons are almost always removed from the covalent bonds of food molecules (e.g. glucose or fatty acids) in connection with a transfer of H-atoms from the involved molecules to either NAD+, NADP+ or FAD+ molecules, which we will study in more detail in the following chapter NAD+ NADP+ FAD = = = Nicotineamide Dinucleotide Nicotineamide Dinucleotide Phosphate Flavinadenine Dinucleotide Redox reactions of the Dinucleotides NAD+, NADP+ & FAD NAD+ + 2e¯ + 2 H+ NADP+ + 2e¯ + 2 H+ FAD + 2e¯ + 2 H+ NADH + H+ (NADH2) NADPH + H+ (NADPH2) FADH2 19 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Chemical structures & Redox reactions of NAD+ or NADP+ The removal of H-atoms in biological systems is accelerated by special proteins called Dehydrogenases. A protein with the capability to accelerate a certain chemical reaction is also called an enzyme. Dehydrogenase enzymes strip two hydrogen atoms from the fuel molecule (e.g., glucose or fatty acid), pass two electrons and one proton to NAD+ and release one proton (H+) into the surrounding water. H-C-OH + NAD+ 2 e-, 2 H+ C=O + NADH + H+ This electron and proton transfer changes the oxidized form (NAD+) to the reduced form (NADH + H+). NAD+ functions as the oxidizing agent in many of the redox steps during the catabolism of glucose and fatty acids. 20 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Chemical structure & Redox reactions of the FAD molecule FAD (oxidized form) 2 e- + 2 H+ FADH2 (reduced form) Summarized, Dehydrogenases transfer two H-atoms (2 protons and 2 electrons) with the help of the molecule NAD+ or FAD, which are closely attached to the protein. NAD+ and FAD are then referred to as the prosthetic group or the co-enzyme of that specific dehydrogenase. many enzymes are known to work or to be enzymatically active only in combination with their co-enzymes 21 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. many vitamines e.g. vitamine B12 or folic acid are co-enzymes of important cellular enzymes The NAD+ molecule within the dehydrogenase complex is the part of the enzyme which actually shuttles the electrons during the catalyzed redox reaction e.g. during several steps along glycolysis, NAD+ receives two hydrogen atoms (= including 2 electrons!) from glucose and becomes reduced to NADH + H+; glucose loses two electrons contained in the two H-atoms and is oxidized! In the course of this coupled redox process, NAD+ is loaded with energy; NADH + H+ carries this chemical energy over to specialized proteins located in the inner mitochondrial membrane The tightly packed proteins in the mitochondrial membrane are also called electron carrier proteins; they form a so-called electron transport chain (for more details see UNIT 6) one example of these proteins is cytochrome c reductase At the electron transport chain, NADH + H+ gives up its bound H-atoms (and electrons!) and regenerates to NAD+ again, while the first electron carrier protein of the electron transport chain receives the liberated electrons members of the electron transport chain are enzymes which all have specific so-called prosthetic groups (= co-enzymes), each with a slightly higher affinity for electrons than the uphill neighbor Therefore the released electrons from NADH + H+ begin a journey along a socalled electron cascade, while the H+ ions (= protons) are left behind and shuttled through the membrane into the mitochondrial matrix this separation of H+-ions from the electrons along this cascade and its accumulation in the mitochondrial matrix is of crucial importance for the cellular synthesis of ATP (see UNIT 6) At the end of this enzyme-bound electron cascade, the electrons finally are transferred to molecular oxygen (= O2), which is the final electron acceptor O2 gets reduced to water In summary, the many redox-steps along the breakdown (= oxidation) of glucose to CO2 and H2O release energy in amounts small enough to be utilizable by the cell; most of this energy is used to build up a gradient of H+-ions along the mitochondrial membrane for ATP synthesis if oxygen would be reduced all at one step with hydrogen, a chemical explosion would occur and the released energy in form of heat and light could not be used by the cell! 22 SAN DIEGO MESA COLLEGE SCHOOL OF NATURAL SCIENCES General Biology (BIOL 107): Instructor: Elmar Schmid, Ph.D. Other important cellular redox molecules, which means, molecules which play crucial roles in cellular redox processes are: 1. Ubiquinones (e.g. Q10) important redox molecule of the electron transport chain (ETC) located in the inner mitochondrial membrane 2. Plastoquinones important redox molecule of the electron transport chain (ETC) located between the light-tapping photosystems in the thylacoid membranes of plant chloroplasts 3. Cytochromes (e.g. cytochrome c) these are heme iron-containing proteins which are integrated or associated with biological membranes, such as the inner mitochondrial or the thylacoid membranes of the chloroplasts in mitochondria and chloroplasts, cytochromes are integral part of the ETCs or these two important cell organelles 4. Iron-Sulfur (Fe-S) proteins these type of redox molecules are found throughout the kingdoms of life they contain clusters of several iron and sulfur atoms which reversible take up 2 or 4 electrons during cellular redox reactions a prominent example belong this group of redox molecules is ferredoxin 4. Glutathione this molecule is the co-substrate of many enzymes involved in cellular detoxification and protection mechanisms it is a so-called tri-peptide and consists of three amino acids; it contains a cysteine, which (in its reduced form) has a characteristic so-called sulfhydryl- (SH-) group this functional group (like with NAD+/NADH + H+) can easily donate or accept electrons, depending on the cellular pH and environment in the case of a high demand of free electrons in the cell, e.g. to combat invaded or generated so-called free radicals, reduced glutathione (= GSH) becomes oxidized and forms a so-called disulphide bridge between two molecules of glutathione to become GS-SG. the glutathione redox system plays an important role in the cellular defense and protection against toxic molecules (e.g. in cigarette smoke) or irradiation (e.g. after intensive sun exposure) In the context of reduction-oxidation reactions in the cell we heard about biological molecules e.g. NAD, GSH, which are essential parts (as prosthetic groups or cosubstrates) of specific proteins, called enzymes 23