Chapter 8 An Introduction to Metabolism PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Organization of the Chemistry of Life into Metabolic Pathways A metabolic pathway has many steps That begin with a specific molecule and end with a product That are each catalyzed by a specific enzyme Enzyme 1 A Enzyme 3 D C B Reaction 1 Starting molecule Enzyme 2 Reaction 2 Reaction 3 Product Energy can be converted from one form to On the platform a diver Diving converts potential another has more potential energy energy to kinetic energy Figure 8 2 Climbing up converts kinetic energy of muscle movement to potential energy In the water a diver has less potential energy The Laws of Energy Transformation Chemical energy a First law of thermodynamics Energy can be transferred or transformed but Neither created nor destroyed For example the chemical potential energy in food will be converted to the kinetic energy of the cheetah s movement in b The Second Law of Thermodynamics Heat co2 H2O b Second law of thermodynamics Every energy transfer or transformation increases the disorder entropy of the universe For example disorder is added to the cheetah s surroundings in the form of heat and the small molecules that are the by products of metabolism Biological Order and Disorder 50 m Free Energy Stability and Equilibrium At maximum stability the system is at equilibrium More free energy higher G Less stable Greater work capacity In a spontaneously change The free energy of the system decreases G 0 The system becomes more stable The released free energy can be harnessed to do work Less free energy lower G More stable Less work capacity a Gravitational motion Objects move spontaneously from a higher altitude to a lower one Figure 8 5 b c Chemical reaction In a Diffusion Molecules cell a sugar molecule is in a drop of dye diffuse broken down into simpler until they are randomly dispersed molecules Exergonic Reaction Proceeds with a net release of free energy and is spontaneous Free energy Reactants Amount of energy released G 0 Energy Products Progress of the reaction a Exergonic reaction energy released Endergonic Reaction Is one that absorbs free energy from its surroundings and is non spontaneous Free energy Products Amount of energy released G 0 Energy Reactants Progress of the reaction b Endergonic reaction energy required Equilibrium and Metabolism Reactions in a closed system eventually reach equilibrium G 0 a A closed hydroelectric system Water flowing downhill turns a turbine that drives a generator providing electricity to a light bulb but only until the system reaches equilibrium G 0 Cells in our body experience a constant flow of materials in and out preventing metabolic pathways from reaching equilibrium G 0 b An open hydroelectric system Flowing water keeps driving the generator because intake and outflow of water keep the system from reaching equlibrium The Structure and Hydrolysis of ATP ATP adenosine triphosphate Is the cell s energy shuttle Provides energy for cellular functions Adenine NH2 N O O O O O O O O N CH2 N CH C N O H Phosphate groups C HC O O C H H H OH OH Ribose How ATP Performs Work P P P Adenosine triphosphate ATP H2O P i Inorganic phosphate P P Energy Adenosine diphosphate ADP The three types of cellular work are powered by the hydrolysis of ATP P i P Motor protein Protein moved a Mechanical work ATP phosphorylates motor proteins Membrane protein ADP ATP P P P Solute i Solute transported b Transport work ATP phosphorylates transport proteins P Glu NH3 Reactants Glutamic acid and ammonia NH2 P i Glu Product glutamine made c Chemical work ATP phosphorylates key reactants i ATP hydrolysis can be coupled to other reactions Endergonic reaction G is positive reaction is not spontaneous NH2 Glu Glutamic acid NH3 Ammonia G 3 4 kcal mol Glu Glutamine Exergonic reaction G is negative reaction is spontaneous ATP Figure 8 10 H2O ADP Coupled reactions Overall G is negative together reactions are spontaneous P G 7 3 kcal mol G 3 9 kcal mol The Regeneration of ATP Catabolic pathways Drive the regeneration of ATP from ADP and phosphate ATP hydrolysis to ADP P i yields energy ATP synthesis from ADP P i requires energy ATP Energy from catabolism exergonic energy yielding processes Figure 8 12 Energy for cellular work endergonic energyconsuming processes ADP P i The energy profile for an exergonic reaction A B C D Free energy Transition state A B C D EA Reactants A B C D G O Products Progress of the reaction Figure 8 14 How Enzymes Lower the EA Barrier An enzyme catalyzes reactions By lowering the EA barrier Free energy Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants G is unaffected by enzyme Course of reaction with enzyme Products Figure 8 15 Progress of the reaction The active site is the region on the enzyme where the substrate binds Substate Active site Enzyme a Catalysis in the Enzyme s Active Site Induced fit of a substrate Enzyme substrate complex b The catalytic cycle of an enzyme 1 Substrates enter active site enzyme changes shape so its active site embraces the substrates induced fit Substrates Enzyme substrate complex 6 Active site s available for wo new substrate molecules Enzyme 5 Products are Released Figure 8 17 Products 2 Substrates held in active site by weak interactions such as hydrogen bonds and ionic bonds 3 Active site and R groups of its amino acids can lower EA and speed up a reaction by acting as a template for substrate orientation stressing the substrates and stabilizing the transition state providing a favorable microenvironment participating directly in the catalytic reaction 4 Substrates are Converted into Products Effects of Temperature and pH Each enzyme has an optimal temperature in which it can function Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic Rate of reaction heat tolerant bacteria 0 20 40 Temperature C a Optimal temperature for two enzymes 80 100 Has an optimal pH in which it can function Optimal pH for pepsin stomach enzyme Rate of reaction Optimal pH for trypsin intestinal enzyme 0 1 2 3 4 b Optimal pH for two enzymes 5 6 7 8 9 Enzyme Inhibitors Competitive inhibitors bind to the active site of an enzyme competing with the substrate A substrate can bind normally to the active site of an enzyme Substrate Active site Enzyme a Normal binding A competitive inhibitor mimics the substrate competing for the active site Figure 8 19 b
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