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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers.

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Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers."— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 C 12 H 22 O 11 Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6

3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 (E A ) Activation energy is often supplied in the form of heat from the surroundings

4 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

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Enzymes Lower the E A Barrier Enzymes catalyze reactions by lowering the E A barrier Enzymes do not affect the change in free-energy (∆G); instead, they hasten reactions that would occur eventually Animation: How Enzymes Work Animation: How Enzymes Work

6 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

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Enzyme-substrate complex

9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catalysis in the Enzyme’s Active Site In an enzymatic reaction, the substrate binds to the active site The active site can lower an E A barrier by – Orienting substrates correctly – Straining substrate bonds – Providing a favorable microenvironment – Covalently bonding 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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) 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

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cofactors Cofactors are nonprotein enzyme helpers Coenzymes are organic cofactors

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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.

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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

21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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

23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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)

25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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