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Enzymes

Enzymes. The Activation Energy Barrier. chemical reactions bond breaking and bond forming The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (E A ) form of heat from the surroundings. Catalysts.

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Enzymes

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  1. Enzymes

  2. The Activation Energy Barrier • chemical reactions • 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) • form of heat from the surroundings

  3. Catalysts • So what’s a cell got to do to reduce activation energy? • get help! … chemical help… ENZYMES Call in the ENZYMES! G

  4. Enzymes vocabulary substrate • reactant which binds to enzyme • enzyme-substrate complex: temporary association product • end result of reaction active site • enzyme’s catalytic site; substrate fits into active site active site products substrate enzyme

  5. Enzymes speed up metabolic reactions by lowering energy barriers without being consumed by the reaction • How • lowering the EA barrier • hasten reactions that would occur eventually

  6. Course of reaction without enzyme EA without enzyme EA with enzyme is lower LE 8-15 Reactants Free energy Course of reaction with enzyme DG is unaffected by enzyme Products Progress of the reaction

  7. LE 8-13 Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6 Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

  8. Catalysis in the Enzyme’s Active Site • In an enzymatic reaction, the substrate binds to the active site “lock and key” • The active site can lower an EA barrier by • Orienting substrates correctly • Straining substrate bonds • Providing a favorable microenvironment • Covalently bonding to the substrate

  9. 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 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. Substrates LE 8-17 Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products

  10. Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by: • General environmental factors, such as temperature and pH • Chemicals that specifically influence the enzyme • concentration

  11. Cofactors • Cofactors are nonprotein enzyme helpers • zinc, iron, and copper • Coenzymes are organic cofactors • vitamins

  12. A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Enzyme Inhibitors Normal binding LE 8-19 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

  13. Regulation of enzyme activity helps control metabolism • Allosteric regulation • protein’s function at one site is affected by binding of a regulatory molecule at another site • inhibit or stimulate an enzyme’s activity • made from polypeptide subunits • active and inactive forms

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

  15. Cooperativity • allosteric regulation that can amplify enzyme activity • binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate LE 8-20b Inactive form Stabilized active form Cooperativity another type of allosteric activation

  16. Feedback Inhibition • 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

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

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