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Transcript
Enzymes They do everything. All of the things. What did that enzyme do? It did a thing! Enzymes All end in –ase Sugars end in –ose They are like a lock and key Enzymes are specific! ◦ They only attach to one thing (or a mimic/blocker) ◦ They only do one thing ◦ Example – the enzyme that gives me red hair doesn’t also break down my breakfast in my stomach More boring enzyme facts! They are reusable! When they break we can break them down and build new enzymes Can build hard bonds, strengthen existing ones, speed up reactions, make things more powerful There is a Daft Punk reference in that I’m not going to subject you too. The Laws of Thermodynamics OH COME ON MAN THAT WAS SO LAST YEAR Law 1 – energy can’t be created nor destroyed only transformed (we gotta break down food) Law 2 – The amount of available energy will always decrease after each reaction (we gotta eat food) Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction LE 8-13 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 LE 8-14 A B C D Transition state A B Free energyC D EA Reactants A B G < O C D Products Progress of the reaction How Enzymes Lower the EA Barrier • Enzymes catalyze reactions by lowering the EA barrier • Enzymes do not affect the change in free-energy (∆G); instead, they hasten reactions that would occur eventually LE 8-15 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy Course of reaction with enzyme G is unaffected by enzyme Products Progress of the reaction Substrate Specificity of Enzymes • The reactant that an enzyme acts on is called the enzyme’s substrate • The active site is the region on the enzyme where the substrate binds LE 8-16 Substrate Active site Enzyme-substrate complex Enzyme Enzyme Database Catalysis in the Enzyme’s Active Site • The active site can lower an EA barrier by o Orienting substrates correctly o Straining substrate bonds o Providing a favorable microenvironment o Covalently bonding 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. Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by: o General environmental factors, such as temperature and pH o Chemicals that specifically influence the enzyme Factors Affecting Enzyme Function Enzyme concentration Substrate concentration Temperature pH Salinity Activators Inhibitors catalase Factors affecting enzyme function Enzyme concentration ◦ as enzyme = reaction rate more enzymes = more frequently collide with substrate ◦ reaction rate levels off reaction rate substrate becomes limiting factor not all enzyme molecules can find substrate enzyme concentration Factors affecting enzyme function Substrate concentration ◦ as substrate = reaction rate more substrate = more frequently collide with enzyme ◦ reaction rate levels off reaction rate all enzymes have active site engaged enzyme is saturated maximum rate of reaction substrate concentration #jokes Factors affecting enzyme function Temperature ◦ Optimum T° greatest number of molecular collisions human enzymes = 35°- 40°C body temp = 37°C ◦ Heat: increase beyond optimum T° increased energy level of molecules disrupts bonds in enzyme & between enzyme & substrate H, ionic = weak bonds denaturation = lose 3D shape (3° structure) ◦ Cold: decrease T° molecules move slower decrease collisions between enzyme & substrate LE 8-18 Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) Rate of reaction 0 40 20 60 Temperature (°C) 80 100 Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction 0 1 2 3 4 5 pH Optimal pH for two enzymes 6 7 8 9 10 Enzymes and temperature Different enzymes function in different organisms in different environments reaction rate human enzyme hot spring bacteria enzyme 37°C temperature 70°C (158°F) Enzyme Inhibitors • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective pH What’s happening here?! pepsin trypsin Oh what the heck that was in German, dude We don’t want pepsin in your intestines or trypsin in your stomach Because they are pH activated any pepsin that gets out turns off and any trypsin that gets in is turned off Factors affecting enzyme function pH ◦ changes in pH adds or remove H+ disrupts bonds, disrupts 3D shape disrupts attractions between charged amino acids affect 2° & 3° structure denatures protein ◦ optimal pH? most human enzymes = pH 6-8 depends on localized conditions pepsin (stomach) = pH 2-3 trypsin (small intestines) = pH 8 0 1 2 3 4 5 6 7 8 9 10 11 Factors affecting enzyme function Salt concentration ◦ changes in salinity adds or removes cations (+) & anions (–) disrupts bonds, disrupts 3D shape disrupts attractions between charged amino acids affect 2° & 3° structure denatures protein ◦ enzymes intolerant of extreme salinity Dead Sea is called dead for a reason! Compounds which help enzymes Activators Fe in hemoglobin ◦ cofactors non-protein, small inorganic compounds & ions Mg, K, Ca, Zn, Fe, Cu bound within enzyme molecule ◦ coenzymes non-protein, organic molecules bind temporarily or permanently to enzyme near active site many vitamins NAD (niacin; B3) FAD (riboflavin; B2) Coenzyme A Mg in chlorophyll Compounds which regulate enzymes Inhibitors ◦ ◦ ◦ ◦ ◦ molecules that reduce enzyme activity competitive inhibition noncompetitive inhibition irreversible inhibition feedback inhibition Competitive Inhibitor Inhibitor & substrate “compete” for active site ◦ penicillin blocks enzyme bacteria use to build cell walls ◦ disulfiram (Antabuse) treats chronic alcoholism blocks enzyme that breaks down alcohol severe hangover & vomiting 5-10 minutes after drinking Overcome by increasing substrate concentration ◦ saturate solution with substrate so it out-competes inhibitor for active site on enzyme Non-Competitive Inhibitor Inhibitor binds to site other than active site ◦ allosteric site ◦ allosteric inhibitor regulation of enzyme function keeps enzyme inactive some anti-cancer drugs inhibit enzymes involved in DNA synthesis stop DNA production stop division of more cancer cells cyanide poisoning irreversible inhibitor of Cytochrome C, an enzyme in cellular respiration stops production of ATP ◦ causes enzyme to change shape conformational change active site is no longer a functional binding site Irreversible inhibition Inhibitor permanently binds to enzyme ◦ competitor permanently binds to active site ◦ allosteric permanently binds to allosteric site permanently changes shape of enzyme nerve gas, sarin, many insecticides (malathion, parathion…) cholinesterase inhibitors doesn’t breakdown the neurotransmitter, acetylcholine Allosteric regulation Conformational changes by regulatory molecules ◦ inhibitors keeps enzyme in inactive form ◦ activators keeps enzyme in active form Conformational changes Allosteric regulation Metabolic pathways A EF FG G A BB CCDDE enzyme enzyme enzyme enzyme enzyme enzyme 1 2 3 4 5 Chemical reactions of life are organized in pathways divide chemical reaction into many small steps efficiency intermediate branching points control = regulation 6 Efficiency Organized groups of enzymes ◦ if enzymes are embedded in membrane they are arranged sequentially Link endergonic & exergonic reactions Whoa! All that going on in those little mitochondria! Feedback Inhibition Regulation & coordination of production ◦ product is used by next step in pathway ◦ final product is inhibitor of earlier step allosteric inhibitor of earlier enzyme feedback inhibition ◦ no unnecessary accumulation of product 1 2 3 4 5 ABCDEFG enzyme X enzyme enzyme enzyme enzyme enzyme 6 allosteric inhibitor of enzyme 1 Feedback inhibition Example ◦ synthesis of amino acid, isoleucine from amino acid, threonine ◦ isoleucine becomes the allosteric inhibitor of the first step in the pathway as product accumulates it collides with enzyme more often than substrate does 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. Noncompetitive inhibitor Noncompetitive inhibition Cooperativity Substrate acts as an activator ◦ substrate causes conformational change in enzyme induced fit ◦ favors binding of substrate at 2nd site ◦ makes enzyme more active & effective hemoglobin Hemoglobin 4 polypeptide chains can bind 4 O2; 1st O2 binds now easier for other 3 O2 to bind
