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An Introduction to Metabolism Ch. 8 AP Biology Ms. Haut Metabolic Pathways Catabolic Pathways Release energy by breaking down complex molecules into simpler ones Cellular respiration provides energy for cellular work C6H12O6 + 6O2 6CO2 + 6H2O + energy Energy released drives anabolic reactions Metabolic Pathways Anabolic Pathways Consume energy by building molecules Photosynthesis uses energy 6CO2 + 6H2O energy C6H12O6 + 6O2 Organisms Transform Energy Solar Energy (EK) Plants (glucose) Stored in chemical bonds (EP) Animals Break down Sugars; Some used (EK), some stored in chemical bonds (EP) Energy Kinetic energy is energy associated with motion Diving converts potential energy to kinetic energy. Climbing up converts kinetic energy of muscle movement to potential energy. In the water, the diver has less potential energy. Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules Potential energy is energy that matter possesses because of its location or structure On the platform, the diver has more potential energy. Chemical energy is potential energy available for release in a chemical reaction Energy can be converted from one form to another Laws of Thermodynamics First Law—Energy can be transferred, but never created or destroyed Second Law—Every energy transfer results in increased entropy (randomness in the universe) Some of the energy is converted to heat Reactions occur spontaneously Heat Chemical energy First law of thermodynamics CO2 H2O Second law of thermodynamics Free Energy Organisms live at the expense of free energy (portion of a system’s energy available for work) acquired from the surroundings Free energy is needed for spontaneous changes to occur Gibbs-Helmholtz Equation G = H - TS Free Total Temp energy energy (K) entropy enthalpy Can be used to determine if a reaction is spontaneous Spontaneous reactions occur in systems moving from instability to stability High energy Low energy Gibbs-Helmholtz Equation G = H - T S Measure of heat in the reaction In chemical reactions, reactions absorb energy to break bonds Energy is then released when bonds form between rearranged atoms of the product Key Importance of G Indicates amount of energy available for work Indicates whether a reaction will occur spontaneously (low G) G decreases as reaction approaches equilibrium G increases as reaction moves away equilibrium G = 0 when a reaction is in equilibrium Chemical Reactions Exergonic Endergonic Chemical products have lower G than reactants Products store more G than reactants Reaction releases energy Reaction requires energy input (absorbs) G = negative value G = positive value Spontaneous Non spontaneous In cellular metabolism, exergonic reactions drive endergonic reactions Rate of Reactions G indicates spontaneity not speed of reaction Spontaneous reactions will occur if it releases free energy (- G ), but may occur too slowly to be effective in living cells Can leave sucrose in sterile water for yrs. with hydrolysis occuring; add sucrase and reaction will hydrolyze in seconds Biochemical reactions require enzymes to speed up and control reaction rates ATP couples exergonic reactions to endergonic reactions A cell does three main kinds of work: Mechanical Transport Chemical To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one ATP Powers Cellular Work Unstable Bonds—can release energy when broken Energy transferred to another molecule (phosphorylated intermediate) with the phosphate Less stable More stable LE 8-11 Pi P Motor protein Protein moved Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP + Pi ATP Pi P Solute transported Solute Transport work: ATP phosphorylates transport proteins P NH2 Glu + NH3 + Pi Glu Reactants: Glutamic acid and ammonia Product (glutamine) made Chemical work: ATP phosphorylates key reactants The Regeneration of ATP • • ATP is a renewable resource that is regenerated by addition of a phosphate group to ADP The energy to phosphorylate ADP comes from catabolic reactions in the cell The chemical potential energy temporarily stored in ATP drives most cellular work ATP Energy for cellular work (endergonic, energyconsuming processes) Energy from catabolism (energonic, energyyielding processes) ADP + P i Enzymes Catalyst—chemical agent that speeds up a chemical reaction without being consumed by the reaction Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6 The Activation Energy Barrier Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) Activation energy is often supplied in the form of heat from the surroundings A B C D Transition state Free energy A B C D EA Reactants A B G < O C D Products Progress of the reaction Enzymes Catalytic proteins that speed up metabolic reactions by lowering energy barriers 1. Reactants must absorb energy to reach transition state (unstable) 2. Rxn occurs and energy is released as new bonds form to make products 3. G for overall rxn is difference b/w G of products and G of reactants Substrate Specificity of Enzymes Substrate—reactant that an enzyme acts Substrate binds to the active site on the enzyme Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction Induced Fit Model of Enzymatic Reactions Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Substrates Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products 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. How do Enzymes Work? Active site holds 2 or more reactants in the proper position to react Induced fit may distort chemical bonds so less thermal energy is needed to break them Active site may provide micro-environment that aids a reaction (localized pH) Side chains of amino acids in active site may participate in reaction Enzyme Activity A cell’s physical and chemical environment affects enzyme activity Each enzyme has optimal environmental conditions that favor the most active enzyme conformation Effects of Temperature Optimal temp. allows greatest number of molecular collisions without denaturing the enzyme Reaction rate when temperature Kinetic energy increases and collisions increases Beyond optimal temperature, reaction rate slows Too low, collisions b/w substrate and active site don’t occur fast enough Too high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds) Effects of pH Optimal pH range for most enzymes is pH 6 –8 Beyond optimal pH, reaction rate slows Too low (acidic) H+ ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme Too high (basic) OH- ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme Cofactors Small non-protein molecules that are required for proper enzyme catalysis Inorganic—Zn, Fe, Cu Coenzymes—vitamins Effects of Substrate Concentration The higher the [substrate], the faster the rate (up to a limit) If [substrate] high enough, enzyme is saturated with substrate Reaction rate depends on how fast the active site can convert substrate to product When reaction is saturated with substrate, you can speed up reaction rate by adding more enzyme Effects of Enzyme Inhibitors A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Competitive inhibitors— chemicals that resemble an enzyme’s normal substrate and compete with it for the active site Blocks active site from substrate If reversible, can be overcome by increasing substrate concentration A substrate can bind normally to the active site of an enzyme. Substrate Normal binding Active site A competitive inhibitor mimics the substrate, competing for the active site. Enzyme Competitive inhibitor Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibition Competitive inhibitor A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its Competitive inhibition active site no longer Noncompetitive inhibitor functions. Noncompetitive inhibition 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 Competitive Inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Effects of Enzyme Inhibitors Active site Enzyme Noncompetitive inhibitors—chemicals that bind to another part (allosteric site)of an enzyme Causes enzyme to change shape and prevents substrate from fitting in active site Essential mechanism in cell’s regulating metabolic reactions A substrate can bind normally to the A competitive active site of an inhibitor mimics the enzyme. substrate, competing for the active site. Normal binding Substrate Active site Competitive inhibitor Enzyme Normal binding Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. 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. Competitive inhibitor Allosteric site Competitive inhibition Noncompetitive inhibitor Noncompetitive inhibition 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 Noncompetitive inhibition Negative Feedback Metabolic Control often Depends on Allosteric Regulation Allosteric enzymes have 2 conformations, catalytically active and inactive Binding of an activator to the allosteric site stabilizes active conformation Binding of an inhibitor (noncompetitive) to the allosteric site stabilizes inactive conformation Control of Metabolism Initial substrate (threonine) Active site available Isoleucine used up by cell Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 can’t bind Intermediate B theonine pathway off Enzyme 3 Isoleucine binds to allosteric site Intermediate C Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine) In feedback inhibition, the end product of a metabolic pathway shuts down the pathway Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed Specific Localization of Enzymes Within the Cell Structures within the cell help bring order to metabolic pathways Some enzymes act as structural components of membranes Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria Mitochondria, sites of cellular respiration 1 µm