Transcript Enzymes
Enzymes Biological catalysts Increase rate of reactions by lowering activation energy (EA) Spontaneous reactions can take a long time! Need enzymes to speed reactions for cell survival Activation Energy (EA) • Needed to destabilize bonds of reactants LE 8-14 A B C D Free energy Transition state A B C D Could raise temp. to break bonds EA Reactants A B DG < O C D Products Progress of the reaction Why don’t cells rely on increases in temperature to break bonds? Because proteins could be denatured causing cell damage. LE 8-15 Free energy Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Course of reaction with enzyme DG is unaffected by enzyme Products Progress of the reaction LE 8-13 Example: Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6 Structure & Function of Enzyme DRAW • Enzymes bind substrate molecules (the reactant) • Substrates bind to active site on enzyme • Binding induces conformational change in enzyme-better ”fit” for substrate • Active sites are highly specific and discriminatory i.e. sucrase does not accept lactose LE 8-16 Substrate Active site Enzyme Enzyme-substrate complex How does enzyme lower activation energy of reaction? – Orients substrates for optimal interaction –Strains substrate bonds –Provides a favorable microenvironment -May covalently bond to the substrate LE 8-17 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. Environmental Conditions Affect Enzyme Function ? Temperature: cold-->decreased chance of bumping into substrate hot--> good chance of substrate interaction but chance of denaturation at some point pH->change in charge (H+ or OH-) can denature proteins and substrate Examples of pH sensitive enzymes? LE 8-18 Optimal temperature for typical human enzyme What is your normal body temp.? 0 Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) 40 60 Temperature (°C) 20 80 100 Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) 0 1 2 3 4 Optimal pH for trypsin (intestinal enzyme) 5 pH Optimal pH for two enzymes 6 7 8 9 10 Cofactors • Non-protein enzyme helpers (like metal, Fe) •Coenzymes •organic cofactors (con-enzyme A) •Vitamins •e.g. Vitamin K: required for blood clotting & Required in certain carboxylation reactions Regulation of Enzymes Enzyme Inhibitors • Competitive inhibitor – binds to active site of enzyme – blocks substrate binding by competition •Noncompetitive inhibitor – binds to another part of enzyme – causes enzyme to change shape – prevents active site from binding substrate –Allosteric effect DRAW LE 8-19 A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Competitive 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. Example of allosteric effect Noncompetitive inhibitor Noncompetitive inhibition Allosteric Regulation of Enzymes • Where protein function at one site is affected by binding of a regulatory molecule at another site • May inhibit or stimulate enzyme activity Allosteric Activation and Inhibition • Most allosterically regulated enzymes are made from polypeptide subunits • active and inactive forms • binding of activator stabilizes active form of enzyme • binding of inhibitor stabilizes inactive form of enzyme LE 8-20a Allosteric enzyme with four subunits Regulatory site (one of four) Active site (one of four) Activator Active form Oscillation Nonfunctional active site Allosteric activator stabilizes active form. Inactive form Stabilized active form Allosteric inhibitor stabilizes inactive form. Inhibitor Allosteric activators and inhibitors Stabilized inactive form LE 8-20b Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate Inactive form Stabilized active form Cooperativity another type of allosteric activation Shift from regulation of one enzyme to regulation of an enzymatic pathway Feedback Inhibition • End product of a metabolic pathway shuts down the pathway • Prevents over-production of unneeded molecules LE 8-21 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) Metabolic pathways are often localized in cell • Cellular structures organize and concentrate components of enzymatic pathways – e.g. organelles (mitochondria, chloroplast, lysosomes) – Pathways: respiration, photosynthesis, hydrolysis LE 8-22 Mitochondria, sites of cellular respiration 1 µm LE 8-22 It’s nice to get so much attention!