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Transcript
10/27/2009 An Introduction to Enzymes and Metabolism. Modified from Campbell Overview: The Energy of Life The living cell Is a miniature factory where thousands of reactions occur Converts energy in many ways Metabolism Is the totality of an organism’s chemical reactions Arises from interactions between molecules 1 10/27/2009 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 2 Enzyme 3 D C B Reaction 1 Reaction 2 Starting molecule Reaction 3 Product Catabolic pathways Break down complex molecules into simpler compounds Release energy Anabolic pathways Build complicated molecules from simpler ones Consume energy 2 10/27/2009 Forms of Energy Energy Is the capacity to cause change Exists in various forms, of which some can perform work The First Law of Thermodynamics According to the first law of thermodynamics Energy can be transferred and transformed Energy cannot be created or destroyed Exergonic and Endergonic Reactions in Metabolism An exergonic reaction Proceeds with a net release of free energy and is spontaneous 3 10/27/2009 Reactants Free enerrgy Amount of energy released (∆G <0) Energy Products Progress of the reaction (a) Exergonic reaction: energy released Figure 8.6 An endergonic reaction Is one that absorbs free energy from its surroundings and is nonspontaneous Free energy y Products Amount of energy released l d (∆G>0) Energy Reactants Progress of the reaction (b) Endergonic reaction: energy required 4 10/27/2009 Equilibrium and Metabolism Reactions in a closed system Eventually reach equilibrium 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. Figure 8.7 An analogy for cellular respiration ∆G < 0 ∆G < 0 ∆G < 0 Figure 8.7 (c) A multistep open hydroelectric system. Cellular respiration is analogous to this system: Glucose is broken down in a series of exergonic reactions that power the work of the cell. The product of each reaction becomes the reactant for the next, so no reaction reaches equilibrium. 5 10/27/2009 ATP powers cellular work by coupling exergonic reactions to endergonic reactions A cell does three main kinds of work Mechanical Transport Chemical Energy coupling Is a key feature in the way cells manage their energy resources to do this work The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) Is the cell’s energy shuttle Provides energy for cellular functions Adenine N O O -O - - O Phosphate groups Figure 8.8 O O C N HC O O O O NH2 C N CH2 - O H H OH OH H H CH C N Ribose 6 10/27/2009 Energy is released from ATP When the terminal phosphate bond is broken P P P Adenosine triphosphate (ATP) H2 O P Figure 8.9 + i P Energy P Adenosine diphosphate (ADP) Inorganic phosphate How ATP Performs Work ATP drives endergonic reactions By phosphorylation, transferring a phosphate to other molecules The three types yp of cellular work Are powered by the hydrolysis of ATP P P i Motor protein Protein moved (a) Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP + ATP P P Solute P i i Solute transported (b) Transport work: ATP phosphorylates transport proteins P Glu + NH3 Reactants: Glutamic acid and ammonia Figure 8.11 NH2 Glu + P i Product (glutamine) made (c) Chemical work: ATP phosphorylates key reactants 7 10/27/2009 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) Energy for cellular work (endergonic, energyconsuming processes) ADP + P Figure 8.12 i Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers A catalyst Is a chemical agent g that speeds p up p a reaction without being consumed by the reaction An enzyme Is a catalytic protein 8 10/27/2009 The Activation Barrier Every chemical reaction between molecules Involves both bond breaking and bond forming Hydrolysis Is an example of a chemical reaction H CH2OH CH2OH O H O H H H HO OH + O CH2OH H Sucrase H2O H HO H OH H OH CH2OH O H H OH H OH CH2OH O HO OH H HO H CH2OH OH H Sucrose Glucose Fructose C12H22O11 C6H12O6 C6H12O6 Figure 8.13 The activation energy, EA Is the initial amount of energy needed to start a chemical reaction Is often supplied in the form of heat from the surroundings in a system 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 9 10/27/2009 How Enzymes Lower the EA Barrier An enzyme catalyzes reactions By lowering the EA barrier Free energ gy 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 Progress of the reaction Figure 8.15 Substrate Specificity of Enzymes The substrate Is the reactant an enzyme acts on The enzyme Binds to its substrate substrate, forming an enzymeenzyme substrate complex 10 10/27/2009 The active site Is the region on the enzyme where the substrate binds Induced fit of a substrate Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction Catalysis in the Enzyme’s Active Site In an enzymatic reaction The substrate binds to the active site 11 10/27/2009 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates Enzyme-substrate complex 6 Active site Is available for two new substrate Mole. 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, t i the th substrates b t t • stressing and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction. 4 Substrates are Converted into Products. The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate Effects of Temperature and pH Each enzyme Has an optimal temperature in which it can function 12 10/27/2009 Has an optimal pH in which it can function Optimal pH for pepsin (stomach enzyme) Rate of rea action Optimal pH for trypsin (intestinal enzyme) 3 4 2 1 (b) Optimal pH for two enzymes 0 5 6 7 8 9 Figure 8.18 Cofactors Cofactors Are nonprotein enzyme helpers Coenzymes Are organic cofactors Enzyme Inhibitors Competitive inhibitors Bind to the active site of an enzyme, competing with the substrate 13 10/27/2009 Noncompetitive inhibitors Bind to another part of an enzyme, changing the function A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor Figure 8.19 (c) Noncompetitive inhibition Regulation of enzyme activity helps control metabolism A cell’s metabolic pathways Must be tightly g y regulated g Allosteric Regulation of Enzymes Allosteric regulation Is the term used to describe any case in which a protein’s function at one site is affected by binding of a regulatory molecule at another site 14 10/27/2009 They change shape when regulatory molecules bind to specific sites, affecting function Allosteric activater Allosteric enyzme Active site stabilizes active from with four subunits (one of four) Regulatory site (one of four) Activator Active form Stabilized active form Allosteric activater stabilizes active form Oscillation NonInhibitor functionalInactive form Figure 8.20 Stabilized inactive form active site (a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again. Feedback Inhibition In feedback inhibition The end product of a metabolic pathway shuts down the pathway Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Intermediate A Feedback inhibition Active site of enzyme 1 no longer binds threonine; pathway p y is switched off Enzyme 2 Intermediate B Enzyme 3 Intermediate C Isoleucine binds to allosteric site Enzyme 4 Intermediate D Enzyme 5 Figure 8.21 End product (isoleucine) 15 10/27/2009 16