ENZYMES:An Introduction to CatalysisActivation Energy and Rates of Chemical ReactionsSlide 4Catalysts and Free EnergyEnzymesHow Do Enzymes Work? Enzymes work by binding to SubstrateHow Do Enzymes Work?The Steps of Enzyme CatalysisDo Enzymes Act Alone?Enzyme mechanism? The Active SiteEnzyme RegulationSlide 13What Limits the Rate of Catalysis?Slide 15Slide 16Physical Conditions Affect Enzyme FunctionSlide 18Slide 19Rate of Enzyme-Catalyzed ReactionsSlide 21© 2011 Pearson Education, Inc.ENZYMES:•Enzyme: a biological catalyst (Catalysts speed up the rate of a reaction).–with the exception of some RNAs (ribozymes) that catalyze their own splicing, all enzymes are proteins–enzymes can increase the rate of a reaction by a factor of up to 1020 over an uncatalyzed reaction–some enzymes are so specific that they catalyze the reaction of only one stereoisomer; others catalyze a family of similar reactions•The rate of a reaction depends on its activation energy, Ea. •an enzyme provides an alternative pathway with a lower activation energy•Many enzymes have names that end with –ase  catalase, hexokinase, glucokinase,© 2011 Pearson Education, Inc.An Introduction to Catalysis •Catalysis may be the most fundamental of protein functions. •Reactions take place when:–Reactants collide in precise orientation –Reactants have enough kinetic energy to overcome repulsion between the electrons that come in contact during bond formation •Enzymes perform two functions: 1. Bring substrates together in precise orientation so that the electrons involved in the reaction can interact2. Decrease the amount of kinetic energy reactants must have for the reaction to proceedEnzymes are located at specific sites within the cell, which intern determines its function.© 2011 Pearson Education, Inc.Activation Energy and Rates of Chemical Reactions•The activation energy (Ea) of a reaction is the amount of free energy required to reach the intermediate condition, or transition state. •Reactions occur when reactants have enough kinetic energy to reach the transition state. –The kinetic energy of molecules is a function of their temperature.•Thus, reaction rates depend on:–The kinetic energy of the reactants –The activation energy of the particular reaction (the free energy of the transition state)© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Catalysts and Free Energy•Catalysts lower the activation energy of a reaction by lowering the free energy of the transition state. •Catalysts do not change ΔG and are not consumed in the reaction. A catalyst is a substance that lowers the activation energy (energy barrier) of a reaction and increases the rate of the reaction.© 2011 Pearson Education, Inc.Enzymes•Enzymes are protein catalysts and typically catalyze only one reaction. •Most biological chemical reactions occur at meaningful rates only in the presence of an enzyme.Enzymes:1. Bring reactants together in precise orientations2. Stabilize transition states •Protein catalysts are important because they speed up the chemical reactions that are required for life.© 2011 Pearson Education, Inc.How Do Enzymes Work? Enzymes work by binding to Substrate•In an enzyme-catalyzed reaction, SubstrateSubstrate, S:, S: a reactant•Active siteActive site:: the small portion of the enzyme surface where the substrate(s) becomes bound by non-covalent forces, ex; hydrogen bonding, electrostatic attractions, van der Waals attractionsE + SESenzyme-substratecomplexThree-dimensional shape of the enzyme determines the specificityLock-and-key vs. Induced fit models:E + S  ES → E + P© 2011 Pearson Education, Inc.How Do Enzymes Work?•Enzymes bring substrates together in specific positions that facilitate reactions, and are very specific in which reactions they catalyze. •Substrates bind to the enzyme’s active site.•Many enzymes undergo a conformational change when the substrates are bound to the active site; this change is called an induced fit.•Interactions between the enzyme and the substrate stabilize the transition state and lower the activation energy required for the reaction to proceed.© 2011 Pearson Education, Inc.The Steps of Enzyme Catalysis•Enzyme catalysis has three steps: 1. Initiation: Substrates are precisely oriented as they bind to the active site.2. Transition state facilitation: Interactions between the substrate and active site R-groups lower the activation energy. 3. Termination: Reaction products are released from the enzyme.© 2011 Pearson Education, Inc.Do Enzymes Act Alone?•Some enzymes require cofactors to function normally. These are either metal ions or small organic molecules called coenzymes. •Most enzymes are regulated by molecules that are not part of the enzyme itself. •Vitamins  coenzymes?““Doctors prescribe vitamins and millions of people take them. Doctors prescribe vitamins and millions of people take them. But it needs a great deal of biochemical understanding to know But it needs a great deal of biochemical understanding to know why they are needed and how the organism uses them.”why they are needed and how the organism uses them.”© 2011 Pearson Education, Inc.Enzyme mechanism? The Active SiteSome important questions to ask about enzyme mode of action:• Which amino acid residues are in the active site and catalyze the reaction?• What is the spatial relationship of the essential amino acids residues in the active site?• What is the mechanism by which the essential amino acid residues catalyze the reaction?© 2011 Pearson Education, Inc.Enzyme Regulation•Covalent modification•Competitive inhibition occurs when a molecule similar in size and shape to the substrate competes with the substrate for access to the active site.© 2011 Pearson Education, Inc.•Allosteric regulation occurs when a molecule causes a change in enzyme shape by binding to the enzyme at a location other than the active site. - Allosteric regulation can activate or deactivate the enzyme.© 2011 Pearson Education, Inc.What Limits the Rate of Catalysis?•Enzymes are saturable; in other words, the rate of a reaction is limited by the amounts of substrate present and available enzyme.–The speed of an enzyme-catalyzed reaction increases linearly at low substrate concentrations.–The increase slows as substrate concentration increases–The reaction rate reaches maximum speed at high substrate concentrations.•Most