Class Note for BIOC 460 at UA
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Bloc 460 Dr Miesfeld Fall 2008 Lecture 21 Introduction to Metabolism Bioenergetics Solar energy Key Concepts Energy conversion in biological systems Metabolic redox reactions Review of thermodynamic principles and coupled reactions The adenylate system and the Energy Charge of the cell KEY CONCEPT QUESTIONS How is energy from the sun converted to chemical energy What is reaction coupling and why is it importantin metabolic pathways ENERGY CONVERSION IN BIOLOGICAL SYSTEMS Essentially all biological processes on this planet are directly or indirectly affected by oxidation reduction reactions in photosynthetic organisms that convert solar energy into chemical energy Photosynthetic organisms use this chemical energy to sustain life during the daylight hours and to produce carbohydrates from 002 that can be stored as metabolic fuel for use at night All other organisms obtain chemical energy from their environment which in many cases means consuming the organic materials produced by photosynthetic organisms and using them as metabolic fuel for aerobic respiration Bioenergetics is a term that describes the processes involved in these energy conversion reactions in living systems One branch of biochemistry is devoted to understanding the molecular mechanisms that control these processes Living organisms need an constant input ofenergy to put off death as long as possible The reason for this is that life is dependent on the maintenance ofa highly ordered Alive steady state called homeostasis which requires energy An organism that is at equilibrium with its environment is no longer alive indeed organisms must maintain a steady state that is far from equilibrium in order to survive For example the concentration ofglucose needed to sustain life in a saguaro cactus is much higher inside the cactus than it is in the surrounding desert and it requires the process of photosynthesis to provide the energy needed to keep it this way gure 1 Similarly the concentration of sodium chloride Figure 1 Dead glucoselinszde lglUCoselinside is lower inside the cells ofa humpback whale than it is in the isgrenierthan iseqiinlro surrounding ocean In this case it is the whale39s diet of glucoselomsme glucoseloumue shrimp and plankton that provide the chemical energy required to maintain a safe intracellular sodium chloride concentration When an organism can no longer maintain Dead homeostasis using these energy conversion processes the intracellular concentration of water essential ions and I I macromolecules begin to equilibrate with the surroundings y and the organism dies The reason all living organisms need an input of energy which can be stored is to delay reaching equilibrium with the environment as long as possible Energy conversion in living systems is required for three types ofwork that maintain the steady state 1 chemical work in the form of macromolecular biosynthesis oforganic molecules 2 osmotic work to maintain a concentration of intracellular salts and organic molecules that is different than the extracellular milieu and 3 mechanical work in the form of agellar rotation or 1 of 12 pages 1 Bioc 460 Dr Miesfeld Fall 2008 muscle contraction The conversion of hydrogen to helium by thermonuclear reactions in the sun and the subsequent release of energy in the form of visible light is called solar energy and it is the ultimate power source for life on earth figure 2 Solar energy provides all of the energy required for the two types of mm organisms that inhabit this planet photosynthetic autotrophs Thermonuclearfusion and heterotrophs Photosynthetic autotrophs use solar W7 energy to oxidize H20 and generate chemical energy that is 4H T We used to maintain homeostasis during daylight hours sunlight 39 Photosynthetic autotrophs also use this chemical energy to convert atmospheric C02 into carbohydrate C5H1206 which 7 Ph I otosynthetlc is a storage form of energy used at night The process of H20 1 photosynthesis autotroph oxidizing H20 to capture chemical energy and generate 02 is g xa called photosynthesis whereas the conversion of C02 to x iteration C5H1206 is carbon fixation The most abundant i Jozl photosynthetic autotrophs in the biosphere are vascular Clo plants single cell algae and photosynthetic bacteria 2 Heterotrophs which includes all nonphotosynthetic organisms are dependent in one way or another on if OZ39CGH ZOG photosynthetic autotrophs as a source of chemical energy Aerobic l carbohydrate which is used as metabolic fuel for aerobic quot respiration respiration lmportantly the production of 02 by photosynthetic autotrophs through the oxidation of H20 is critical for aerobic respiration because 02 is the terminal electron acceptor in this process Some bacteria derive redox energy from compounds in the soil and are considered heterotrophs although they are not reliant on photosynthetic autotrophs 01L RIG Heterotroph Metabolic redox reactions Both photosynthesis and aerobic respiration interconvert energy using a series of linked oxidationreduction reactions in which electrons are transferred from a molecule of higher electrochemical potential to one of lower electrochemical potential Qxidation is the Loss of electrons and Reduction is the gain of electrons OIL RIG A series of linked oxidation reduction reactions often called redox reactions transfer electrons from one compound to another in sequential fashion Since electrons do not exist free in solution a reduced compound becomes oxidized when it transfers an electron to an oxidized M g z compound that becomes reduced figure 3 The 139 Wor l 2 211mm importance of redox reactions in biochemical processes is that chemical work can be performed Reduced oxldlzed T Reduced using the energy made available by electron transfer 4 ml quot The initiating biochemical event required for all quot 939 subsequent energy conversion processes in our oxidation reduction biosphere is the absorption of light energy by I Mam educt39bquot pigment molecules such as chlorophyll present in e photosynthetic organisms Light absorption causes X E photooxidation of chlorophyll which results in the OXidiZEd Reduced 0 d d transfer of an electron from the chlorophyll molecule 1 wbi k39 X39 39ze to an acceptor molecule which then passes the 2 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 electron to another acceptor molecule of lower electrochemical potential The redox reactions of aerobic respiration are fundamentally similar to that of photosynthesis except in this case oxidation ofglucose by enzymemediated Figure 4 reactions transfers 24 e to the 24 e 10 NAD 52215 60 coen mes nicotinamide adenine 2 2 dinuczlgotide NAD and flavin adenine 2 FAD dinucleotide FAD Oxidation of NADH o and FADH2 provides redox energy for the 10 NADHH quot process of oxidative phosphorylation 6CO2 lxl 2 FADHZ 120 I l lZHZO which synthesizes 32 ATP for each 24 e l 24 9 glucose molecule oxidized gure 4 gz The electrons are passed through l 32 ATP electron carrier molecules associated Biosymhetic thesiled 3 with the mitochondrial inner membrane reactions and are ultimately donated to 02 as the I Protein nal electron acceptor to produce 12 conformational H20 The biochemistry of aerobic gradients changes respiration glycolysis and the citrate cycle and oxidative phosphorylation electron transport system and ATP synthesis are described in later lectures Review of thermodynamic principles To better understand how energy is utilized in biological systems we need to review basic thermodynamic principles in the context of a system and its surroundings The system de nes the collection of matter in a de ned space whereas the surroundings is everything else The system and surroundings together constitute the universe Biological systems are open systems as both matter nutrients and waste products and energy primarily heat are freely exchanged with the surroundings Three laws of thermodynamics have been de ned The first law of thermodynamics states that in any physical or chemical change that occurs the total amount of energy in the universe remains the same even though the form of energy can change Put another way energy can neither be created or destroyed only transformed The second law of thermodynamics states that in the absence of an energy input all natural processes in the universe tend toward disorder randomness and moreover that the measure of this disorder called entropy is always increasing in the universe The Gibbs free energy value is the third bioenergetic principle m we need to review This is usually expressed as a change in free energy and relates to the spontaneity of a chemical reaction In fact all three of these terms can be tied together and are related to the equilibrium constant Keq of a reaction Motorized gtIll 1 cl r Flcclrical loads for igniting sample l hcrmmnulci39 First Law of Thermodynamics The rst law of thermodynamics states that energy cannot be created or destroyed only converted from one form to another In a closed system such as a bomb calorimeter in which a compound is combusted by a spark under constant pressure in the presence of pure oxygen completely oxidized the amount of heat exchange between the reaction chamber and a surrounding waterjacket is a measure ofthe insulated container 0 inlcl Bomb reaction clmmbel39l Fine wiru in conlart with sample Lup holding sample W atcr 3 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 change of enthalpy AH figure 5 A reaction that gives off heat is called exothermic and AH has a negative value whereas a reaction that absorbs heat is called endothermic and AH is a positive number The enthalpy in a chemical compound is reflected in the number and type of chemical bonds Therefore depending on how a chemical reaction changes the number and type of bonds in the reactants and product the reaction Figure 5 will either be exothermic or endothermic For 1 gram Ofglucose example the combustion of 1 gram of glucose is an 39 39 Bomb Aerobic exothermic reaction and produces heat C02 and calorimeter respiration H20 The temperature increase of the surrounding water is a measurement of the potential energy of l l the glucose as reflected in the number and type of chemical bonds Complete oxidation occurs by igniting a spark in the presence of pure 02 which in this example converts glucose C6H1206 to C02 and H20 in an exothermic reaction that raises the temperature of the surrounding water 375 C We 2 can write an equation for this reaction in terms of chemical reactants and products in which the heat produced is at constant pressure Ignition heat f glucose work 1 liter of water Heat Work CsH1205 6 02 69 6 C02 6 H20 Heat Heat A Calorie C is a unit of energy that was originally defined by the amount of heat energy required to raise 1 kilogram of water from 145 C to 155 C using a calorimetry device Energy can also be expressed in the international unit of measurement the Joule J in which 1 Calorie 4184 kJ Note that in nutritional sciences calorie with a capital quotCquot actually refers to kilocalories kcal so 1 kCal For example as Shown ln LonEntropy Freezercompartment figure 6 the energy potential of 1 gram of glucose is 157 kJ regardless of the route taken to fully oxidize it bomb calorimeter or a 0 C E mouse 39 i l l 39 Second Law of Thermodynamics lquot Electricity The second law of thermodynamics states that L HighEntropy Themputofenergy all natural processes in the universe tend Liquid 200C in theform of towards disorder randomness in the absence phase 1 72 7quot 39 of energy input This concept of disorder is U istrueinalivngcell defined by entropy S a measure of disorder m J The more disorder in a system the higher the value of entropy The second law of l thermodynamics is used in biochemistry to HigherEntropy determine the directionality of a reaction that Gas r is its spontaneity Irreversible reactions such phase 391 100 C as an ice cube melting at room temperature is a an example of an increase in entropy ASuniverse gt O The H20 molecules in the ice 2 4 0T 12 pages Bloc 460 Dr Miesfeld Fall 2008 crystals are highly ordered through hydrogen bonding whereas the H20 molecules in liquid water are much more disordered many freedoms ofrotation lce melting at room temperature is spontaneous however the process can be reversed by the input ofenergy namely the electricity used by a freezer compartment to lower the temperature ofan ice cube tray figure 7 Once the water is frozen solid in the freezer compartment the input of electricity restrains the ice crystals from melting again Similarly the metabolic energy required for sustaining life restrains the natural tendency ofthe molecules within the organism to become disordered as dictated by the second law ofthermodynamics Two examples of increased entropy in living systems are the degradation of biomolecules into a larger number of smaller components and random mutations in DNA that result in disorganized genetic information Gibbs Free Energy G and the Equilibrium Constant Keg In 1878 an American theoretical physicist name J Willard Gibbs described a way to determine if a chemical reaction is favorable or unfavorable under constant pressure and temperature using a function he called free energy The Gibbs free energy term G is de ned as the difference between the enthalpy ofthe system H and the entropy S at a given temperature T Since the absolute values for G H and 8 cannot be easily determined but the change between two states can the most useful Gibbs free energy equation is AG AH TAS lmportantly if the AG value for a reaction is less than zero AGltO then the reaction is favorable and is exergonic however if the AG value is greater than zero AGgtO then the reaction is unfavorable and endergonic A reaction in which AG is equal to zero AGO is at equilibrium meaning that the rate of formation of products is equal to the rate of formation of reactants no net product or reactant formation is occurring This relationship between free energy enthalpy and entropy can be seen in the formation and breakage of chemical bonds in which energy is released when bonds are formed and energy is absorbed when bonds are broken At constant temperature and pressure the equilibrium constant Keq is a measure ofthe directionality of a reaction beginning with equal concentrations of all reactants and products For example in a reaction in which the reactantsA and B are converted to the products C and D and a b c and dare the moles of each reactant and product the Keq is de ned by the concentration of each reactant and product denotes concentration in moles when the reaction has reached equilibrium using the relationship aAbBcCdD Keq Egalc wld AeqlalBeqlb lfthe Keq gt 0 it means that the reaction proceeds toward product formation left to right as written whereas a Keq lt 0 means the reaction favors the formation of reactant right to left as written The relationship between the change in free energy and the reactant and product concentrations under initial conditions was defined by Gibbs using the following equation in which AGu refers to the standard free energy change kJmol R is the gas constant 8315 JmolK and T is the absolute temperature AG AG RT ln Ci Di Ail Bi 5 of 12 pages Bloc 460 Dr Miesfeld Fall 2008 The concentrations of reactants and products in this equation are given as the initial concentrations Ai Bi etc which in biological systems refers to the steady state concentration in living cells homeostasis As biochemists we rede ne both the actual AG and the standard change in free energy AG to AG39 and AG respectively which refers to free energy changes under physiological conditions pH 7 and the concentration of H20 as 555M The importance ofthe free energy values in biochemistry is that it can be used to predict ifa reaction is favorable or unfavorable given the characteristic AG 39 value for the reaction and the concentrations of each reactant and product The AG 39 value is determined by setting up the reaction under standard physiological conditions pH7 555M H20 298 K 1 atm 1 M of solute and then allowing it to proceed to equilibrium at which time the concentrations of all reactants and products are measured Aeq Beq etc Once the reaction has reached equilibrium the change in free energy is zero AG39 0 and AG 39 can be calculated directly from the Keq o AG 39 RT In ceg Deg AeqllBeql AG 39 RT In Keq The values of AG 39 and Keq can both be used to describe the spontaneity ofa reaction in that a favorable reaction with a Keq gt 0 corresponds to a AG 39 lt 0 which denotes an exergonic reaction Similarly an unfavorable reaction with a Keq lt 0 will have a AG39 gt O and is endergonic There are two important points to make about AG39 and AG 39 values First based on Gibbs equation the actual free energy change of a reaction inside a living cell can be favorable AG39 lt0 even ifthe characteristic AG 39 for the reaction is unfavorable AG gt0 under equilibrium conditions This is because metabolic flux in living cells the ow of metabolites through metabolic pathways can maintain the steady state concentrations of reactants and products far from their equilibrium concentrations This is often accomplished using coupled reactions that take advantage ofthe high free energy content potential energy of ATP Moreover coupled reactions quickly remove products since these same metabolites serve as reactants in coupled reactions lmportantly the natural log ln ofa number that is less than 1 is a negative number Therefore even if the standard free energy change is unfavorable AG 39 gt0 the overall AG39 can still be favorable ifthe mass action ratio ie productsreactants is less than 1 since the ln ofthis number will be negative lfthe concentration of products is very small compared to the concentration of reactants then the ln mass action ratio value will be very negative AG39 AG 39 RT ln products actual reactantsactuai Exergonic and endergonic reactions are coupled in metabolism This brings us to the question of how unfavorable endergonic reactions in living systems are able to occur The answer is that endergonic reactions are coupled to exergonic reactions such that the overall change in free energy is favorable exergonic The thermodynamic basis for coupling endergonic and exergonic reactions is that the AG39 for two reactions that share a common intermediate the product of the rst reaction is a reactant in the second reaction is equal to the sum ofthe AG39 values for the two separate reactions as shown below 6 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Enzyme 1 Reaction 1 A 69 B AG391 4 kJmol endergonic Enzyme 2 Reaction 2 B 69 C AG397 10 kJmol exerqonic Net reaction A 69 C AG391 AG39z AG393 6 kJmol exergonic Although the first reaction is unfavorable AG39gtO the formation of product C occurs because the combined free energies of reactions 1 and 2 are favorable AG39ltO One way to think about how reaction 2 affects reaction 1 in this example is to realize that by quickly converting B into C the concentration of B is not able to reach its normal equilibrium concentration This causes the equilibrium of reaction 1 to be shifted toward more product formation in order to replace the B that is continually being consumed by reaction 2 This coupling is the driving force of metabolic flux One of the most common types of coupled reactions is one in which the Figure 8 phosphoanhydride bond energy in Adenosine triphosphate ATP ATP is used to drive an unfavorable reaction ATP contains two phosphoanhydride bonds sometimes 30 5kJmol sawmoi referred to as quothigh energy phosphate bondsquot P that can be used as a 0 fr po source of free energy figure 8 Note lY l B l 1 however that the term high energy 0 039 0 phosphate bond can be misleading Phosphoanhydride bonds since phosphoanhydride bonds are no Ribose ring different than any other chemical bond and adhere to the laws of thermodynamics The change in standard free energy AG 39 for cleavage of the phosphoanhydride bond between the 5 and v phosphate of ATP is 305 kJmol and that of the phosphoanhydride bond between the 0 and 5 phosphates is 323 kJmol Importantly while hydrolysis reactions were used to calculate these AG 39 values it is not the cleavage ofthese phosphoanhydride bonds that provides energy for Glutamine synthetase complex coupled metabolic reactions bond cleavage actually or 77 requires energy but rather the transfer of a phosphate aquot or adenylate AMP group to a reactant to generate a highly reactive intermediate ATPcoupled reactions take place within the active site of an enzyme which accelerates the rate of a reaction by providing an ideal chemical environment for product formation In these enzymemediated coupled reactions the phosphorylated or adenylated chemical intermediates are the compounds that function as the shared intermediate in the two reactions The conversion of the amino acid glutamate to glutamine by the enzyme glutamine synthetase is a 7 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 good example of an ATPcoupled reaction that can be broken down into two reactions The first reaction is endergonic AG 39 142 kJmol and the second reaction is exergonic AG 39 305 kJmol to give an overall favorable AG of163 kJmol Glutamate NH4 69 glutamine AG 39 142 kJmol ATP 69 ADP Pi AG 39 305 kJmol Glutamate NH4 ATP 69 glutamine ADP Pi AG 39 163 kJmol A second type of ATPcoupled reaction is exempli ed by the enzyme fatty acylCoA synthetase which catalyzes a coupled reaction that generates an enzymelinked acyladenylate intermediate Fatty acylCoA synthetase with ATP and the release of pyrophosphate PPi The adenylyl group is then replaced with coenzyme A to generate the products fatty acylCoA and AMP An added feature of coupled adenylation transfer reactions is that PR is rapidly hydrolyzed to 2 P by the enzyme inorganic pyrophosphatase which drops the overall AG 39 for the reaction by another 192 kJmol This is important because the formation of a fatty acylCoA such as palmitoylCoA is a highly endergonic reaction with a AG 39 of 31 4 kJmol In this case both phosphoanhydride bonds of ATP are ultimately required to drive this reaction to product formation Palmitic acid CoA SH 69 palmitoylCoA AG 39 314 kJmol ATP 69 AMP PPi AG 39 305 kJmol PR 69 2 Pi AG 39 192 kJmol Palmitic acid ATP palmitoylCoA AMP 2 Pi AG 39 183 kJmol Steadystate substrate concentrations are also a contributing factor to metabolic flux as they reflect the mass action ratio which determines the actual AG of a reaction Remember that the overall AG for a metabolic pathway is indeed negative inside the cell as predicted by thermodynamic principles ie life is favorable as long as energy is available to prevent the organism from reaching equilibrium with the environment With that in mind let s see how the actual free energy change AG of a reaction is affected by the steadystate concentrations of substrates and products using coupled reactions in the glycolytic pathway The AG of the phosphoglucoisomerase reaction is positive AG 17 kJmol However since the product of the reaction fructose6P is quickly converted to fructose16 bisphosphate by the next enzyme in the pathway phosphofructokinase1 which is an ATPcoupled reaction AG 142 kJmol then the concentration of fructose6P is kept low and the phosphoglucoisomerase reaction is pulled to the right under actual conditions AG39 29 kJmol This can be shown by calculating AG39 using the steadystate concentrations of glucose6P and fructose6P in erythrocytes at 37 C as shown below Glucose6P 69 fructose6 P AG 17 kJmol glucose6Pactual 83 x 10395 M fructose6Pactual 14 x 10395M 8 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 AG39 AG RT In mass action ratio AG39 17 kJmol RT In fructose6Pactua glucose6 Pactua AG39 17 kJmol 000831 kJmol K 310 K ln14 x10395M 83 x 105 M AG39 17 kJmol 46 kJmol AG39 29 kJmol for the reaction Glucose6 P fructose6 P As seen in figure 9 the availability of ATP for the phosphofructokinase1 reaction turns an unfavorable set of reactions AG 39gt0 into a favorable quotminipathwayquot by altering the mass action ratio AG39lt0 In the absence of ATP the AG 39 values for minipathway shows that if the reactions are allowed to go to equilibrium then glucose6P formation is favored However by adding ATP which contributes phosphoryl transfer energy to the phosphofructokinase1 reaction then the stead state level of fructolse6P is decreased M and fructose16BP is in the absence of ATP In the presence of ATP favored I I Glucose6p Glucose6 P Note that it is not always possible to PhosphogucoisomeraselT AG 39 17 kJmol PhosphoglumBomemSeiT AG 3929 kJmOI measure steadystate levels of metabolites in Fructose P ATP FrUCtose39639P cells especially if they are phosphofmmkme AGOr163 kJmol Phosphofructokinasel i AG39 142 kJmol inSIde organelles such as ADP mitochondria or Fructose 16 bisphosphate Fructose 16bisphosphate chloroplast glycolysis takes place in the cytosol Therefore the AG39 for a reaction cannot be determined and one must assume that the overall reactions in a pathway are favorable even if AG 39 is not favorable Nevertheless often by simply adding up the AG 39 values for a set of linked reactions the total AG 39 is indeed favorable and even if it isn39t you know that it must be in nature for the reactions to proceed The adenylate system and the Energy Charge of the cell Since ATP plays such an important role in the cell as a source of free energy for coupled reactions and mechanical work its levels need to be maintained within a fairly narrow range to avoid a metabolic catastrophe This is done by interconverting ATP ADP and AMP using several key phosphoryl transfer reactions that together constitute the adenylate system To see why the adenylate system is important consider that a 70 kg person requires 100 moles of ATP every day based on the energy content of food The molecular weight of ATP is 507gmol which means we hydrolyze as much as 50 kg of ATP every day Rather than synthesizing our own weight in ATP on a daily basis it is much more efficient to recycle adenylate forms by reforming ATP from ADP AMP and Pi The most common way is for ADP Pi to be converted to ATP by the enzyme ATP synthase which is a component of the aerobic respiration and photosynthesis pathways Both aerobic respiration and photosynthesis require an input of energy in order to drive the ATP synthase reaction forward ie metabolic fuel and light respectively Note that since some 9 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 reactions lead to production of AMP rather than ADP for example the adenylation transfer reaction catalyzed by fatty acyICoA synthetase AMP must first be phosphorylated to generate ADP This reaction is catalyzed by the enzyme adenylate kinase which transfers the yphosphate from ATP to AMP to generate 2 ADP AG 39 0 kJmol The 2 ADP generated by adenylate kinase can then be phosphorylated to 2 ATP by ATP synthase AG 39 for 2 ATP 61 kJmol to give the net reaction of AMP 2 Pi gt ATP as shown below Adenylate kinase AMP ATP 2 ADP ATP synthase 2 ADP 2 Pi ATP ATP Net AMP 2 Pi ATP Since ATP is the high energy form of the adenylate system then the ratio ofthe concentration of ATP to the concentration of ADP and AMP in the cell at any given time can be used as a measure ofthe energy state ofthe cell This relationship can be expressed in terms of the Energy Charge EC which takes into account the number of phosphoanhydride bonds available for work Energy Charge EC ATP 05 ADP ATP ADP AMP If the adenylate system components were present in the cell at the same concentration such that ATP ADP AM P Figure 10 then EC 05 However most cells are found to have an Phy ig n39ggi a EC in the range of 07 to 09 which means that the ATP is higherthan ADP or AMP gure 10 For example under steady state conditions the EC value in rat hepatocytes is 08 based on adenine nucleotide concentrations of ATP 34 mM ADP 13 mM AMP 03 mM Percent of total Energy Charge EC ATP 05 ADP ATP ADP AMP Enerqychame Energy Charge EC 34 mM 05 13 mM 08 34 mM 13 mM 03 mM The EC of a cell is maintained between 07 and 09 by regulating metabolic ux through pathways that generate and consume ATP Photosynthetic autotrophs use sunlight as their source of energy for ATP production whereas heterotrophs use nutrients present in their diet as a source of metabolic fuel in the form of carbohydrate protein and lipid to synthesize ATP Most organisms use stored metabolic fuels as a source of energy when other forms of energy are not readily available Extracting energy from metabolic fuels is the function of catabolic pathways 10 of 12 pages which convert energyrich compounds into energy depleted compounds and in the process generate reduced coenzymes NADH NADPH and FADH2 as well as ATP figure 11 These reduced coenzymes and ATP are then used for the biosynthesis of biomolecules through anabolic pathways In general catabolic pathways are degradative processes that obtain energy from compounds using redox reactions to generate ATP while anabolic pathways are biosynthetic processes that utilize energy from redox reactions and ATP to restrain entropy and maintain order lmportantly regulatory processes control the activity of key enzymes in catabolic and anabolic metabolic pathways in order to stabilize the EC and maintain homeostasis For example when EC levels decrease due to increased rates of aerobic respiration or sustained flux through anabolic Bioc 460 Dr Miesfeld Fall 2008 Figure 1 1 Carbohydrates Proteins Metabolic Lipids Fuel Nucleic acids Catabolic Pathways Anabolic Pathways 73 C02 Simple sugars H20 Amino acids NH3 Fatty acids Nucleotide bases pathways then enzymes responsible for ATP synthesis become activated and flux through catabolic pathways is increased figure 12 In most organisms this means degrading stored metabolic fuel in the form of carbohydrate or lipid Alternatively when EC levels are elevated due to Figure 12 photosynthesis or high levels of l nutrients following a meal then enzymes that control flux through anabolic pathways are activated to r l l l Catabolism gt Anabolism Y gtl Y s lg iv5 gtATP I E Catabolism Anabolism lAD Pr i Catabolism lt Anabolism take advantage ofthe available ATP and replenish stored metabolic fuel 11 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 ANSWERS TO KEY CONCEPT QUESTIONS How is energy from the sun converted to chemical energy Life on earth is made possible by the biochemical reactions of photosynthesis carbon xation and aerobic respiration which together convert solar energy into ATP and NADPH which is used to synthesize carbohydrates from 002 and H20 Aerobic organisms such as ourselves consume carbohydrates as a chemical source of energy and metabolize them in the presence of 02 to from 002 and H20 All organisms depend directly or indirectly on energy derived from thermonuclear fusion reactions on the sun to prevent for as long as possible reaching equilibrium with the environment a high entropy state called death What is reaction coupling and why is it important in metabolic pathways Reaction coupling permits energetically unfavorable reactions to be more favorable in the context ofa pathway Coupling ATP hydrolysis to a phosphoryl transfer reaction is one example of reaction coupling that takes place in the same enzyme active site The net AG for an ATP coupled reaction is often highly negative for example the phosphorylation of glucose by the enzyme hexokinase Another type of reaction coupling is when two enzymes in pathway are energetically linked through a shared common intermediate Since the actual change in free energy AG is the sum ofthe change in standard free energy AG and RTnmass action ratio in which the mass action ratio is productactuallsubstrateactual depletion ofa reaction product by its metabolism as a substrate in the coupled reaction results in a reduction in AG since In ofa mass action ratio lt1 is a negative number Therefore even though the AG for a reaction is a positive number based on the reaction reaching equilibrium in a test tube under ideal conditions the actual AG is a negative number because RTnmass action ratio is a negative number due to lowerthan quotexpectedquot product in the cell due to its function as a substrate in a linked reaction ofthe pathway 12 of 12 pages
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