Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers An enzyme is a catalytic protein Sucrose C 12 H 22 O 11 Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transition state CD A B EAEA Products CD A B  G < O Progress of the reaction Reactants C D A B Free energy The Activation Energy Barrier activation energy (E A ) Animation: How Enzymes Work Animation: How Enzymes Work

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Substrate Specificity of Enzymes enzyme substrate active site The enzyme binds to its substrate, forming an enzyme-substrate complex

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

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

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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Effects of Temperature and pH 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

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Enzyme Inhibitors Competitive inhibitors Noncompetitive inhibitors

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Allosteric Regulation of Enzymes Allosteric regulation Allosteric regulation may either inhibit or stimulate an enzyme’s activity

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

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

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 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)

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