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Enzymes Biological catalysts Increase rate of reactions by lowering activation energy (E A ) Spontaneous reactions can take a long time! Need enzymes to.

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Presentation on theme: "Enzymes Biological catalysts Increase rate of reactions by lowering activation energy (E A ) Spontaneous reactions can take a long time! Need enzymes to."— Presentation transcript:

1 Enzymes Biological catalysts Increase rate of reactions by lowering activation energy (E A ) Spontaneous reactions can take a long time! Need enzymes to speed reactions for cell survival

2 Activation Energy (E A ) Needed to destabilize bonds of reactants

3 LE 8-14 Transition state CD A B EAEA Products CD A B  G < O Progress of the reaction Reactants C D A B Free energy Could raise temp. to break bonds

4 Why don’t cells rely on increases in temperature to break bonds? Because proteins could be denatured causing cell damage.

5 LE 8-15 Course of reaction without enzyme E A without enzyme  G is unaffected by enzyme Progress of the reaction Free energy E A with enzyme is lower Course of reaction with enzyme Reactants Products

6 LE 8-13 Sucrose C 12 H 22 O 11 Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6 Example:

7 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

8 LE 8-16 Substrate Active site Enzyme Enzyme-substrate complex

9 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

10 LE 8-17 Enzyme-substrate complex Substrates Enzyme Products 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. Active site (and R groups of its amino acids) can lower E A 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. Substrates are converted into products. Products are released. Active site is available for two new substrate molecules.

11 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?

12 LE 8-18 Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) Temperature (°C) Optimal temperature for two enzymes Rate of reaction Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) pH Optimal pH for two enzymes 0 Rate of reaction What is your normal body temp.?

13 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

14 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

15 LE 8-19 Substrate Active site Enzyme Competitive inhibitor Normal binding Competitive inhibition Noncompetitive inhibitor Noncompetitive inhibition A substrate can bind normally to the active site of an enzyme. 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. Example of allosteric effect

16 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

17 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

18 LE 8-20a Allosteric enzyme with four subunits Regulatory site (one of four) Active form Activator Stabilized active form Active site (one of four) Allosteric activator stabilizes active form. Non- functional active site Inactive form Inhibitor Stabilized inactive form Allosteric inhibitor stabilizes inactive form. Oscillation Allosteric activators and inhibitors

19 LE 8-20b Substrate Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Cooperativity another type of allosteric activation Stabilized active form Inactive form

20 Shift from regulation of one enzyme to regulation of an enzymatic pathway

21 Feedback Inhibition End product of a metabolic pathway shuts down the pathway Prevents over-production of unneeded molecules

22 LE 8-21 Active site available Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Enzyme 2 Intermediate A Isoleucine used up by cell Feedback inhibition Active site of enzyme 1 can’t bind theonine pathway off Isoleucine binds to allosteric site Enzyme 3 Intermediate B Enzyme 4 Intermediate C Enzyme 5 Intermediate D End product (isoleucine)

23 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

24 LE 8-22 Mitochondria, sites of cellular respiration 1 µm

25 LE 8-22 It’s nice to get so much attention!


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