Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers

1. 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

2. LE 8-13 Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6

3. 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

4. LE 8-14 B A C D Transition state EA B A Free energy D C Reactants B A DG < O C D Products Progress of the reaction

5. 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 Animation: How Enzymes Work

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

7. Substrate Specificity of Enzymes • The reactant that an enzyme acts on is called the enzyme’s substrate • The enzyme binds to its substrate, forming an enzyme-substrate complex • The active site is the region on the enzyme where the substrate binds • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

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

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

10. 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. • 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 Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products

11. 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

12. Effects of Temperature and pH • Each enzyme has an optimal temperature in which it can function • Each enzyme has an optimal pH in which it can function

13. LE 8-18 Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) Rate of reaction 40 0 20 60 80 100 Temperature (°C) Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction 2 3 6 7 9 10 0 1 4 5 8 pH Optimal pH for two enzymes

14. Cofactors • Cofactors are nonprotein enzyme helpers • Coenzymes are organic cofactors

15. 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

16. 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

17. Concept 8.5: Regulation of enzyme activity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated • To regulate metabolic pathways, the cell switches on or off the genes that encode specific enzymes

18. Allosteric Regulation of Enzymes • Allosteric regulation is the term used to describe cases where a protein’s function at one site is affected by binding of a regulatory molecule at another site • Allosteric regulation may either inhibit or stimulate an enzyme’s activity

19. Allosteric Activation and Inhibition • Most allosterically regulated enzymes are made from polypeptide subunits • Each enzyme has active and inactive forms • The binding of an activator stabilizes the active form of the enzyme • The binding of an inhibitor stabilizes the inactive form of the enzyme

20. LE 8-20a Allosteric activator stabilizes active form. Allosteric enzyme with four subunits Active site (one of four) 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

21. Cooperativity is a form of allosteric regulation that can amplify enzyme activity • In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits

22. 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

23. 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

24. LE 8-21 Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell 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)

25. Specific Localization of Enzymes Within the Cell • Structures within the cell help bring order to metabolic pathways • Some enzymes act as structural components of membranes • Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria

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