F REE E NERGY AND M ETABOLISM The concept of free energy can be applied to the chemistry of life’s processes © 2011 Pearson Education, Inc.

E XERGONIC AND E NDERGONIC R EACTIONS IN M ETABOLISM An exergonic reaction proceeds with a net release of free energy and is spontaneous An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous © 2011 Pearson Education, Inc.

F IGURE 8.6 (a) Exergonic reaction: energy released, spontaneous (b) Endergonic reaction: energy required, nonspontaneous Reactants Energy Products Progress of the reaction Amount of energy released (  G  0) Reactants Energy Products Amount of energy required (  G  0) Progress of the reaction Free energy

F IGURE 8.6 A (a) Exergonic reaction: energy released, spontaneous Reactants Energy Products Progress of the reaction Amount of energy released (  G  0) Free energy

F IGURE 8.6 B (b) Endergonic reaction: energy required, nonspontaneous Reactants Energy Products Amount of energy required (  G  0) Progress of the reaction Free energy

F IGURE 8.7 C (c) A multistep open hydroelectric system  G  0

C ONCEPT 8.4: E NZYMES 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 © 2011 Pearson Education, Inc.

F IGURE 8.UN02 Sucrase Sucrose (C 12 H 22 O 11 ) Glucose (C 6 H 12 O 6 ) Fructose (C 6 H 12 O 6 )

T HE A CTIVATION E NERGY B ARRIER 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 thermal energy that the reactant molecules absorb from their surroundings © 2011 Pearson Education, Inc.

F IGURE 8.12 Transition state Reactants Products Progress of the reaction Free energy EAEA  G  O A B C D A B C D A B C D

H OW E NZYMES L OWER THE E A B ARRIER 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 © 2011 Pearson Education, Inc.

Animation: How Enzymes Work Right-click slide / select “Play”

F IGURE 8.13 Course of reaction without enzyme E A without enzyme E A with enzyme is lower Course of reaction with enzyme Reactants Products  G is unaffected by enzyme Progress of the reaction Free energy

S UBSTRATE S PECIFICITY OF E NZYMES 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 © 2011 Pearson Education, Inc.

F IGURE 8.14 Substrate Active site Enzyme Enzyme-substrate complex (a) (b)

C ATALYSIS IN THE E NZYME ’ S A CTIVE S ITE In an enzymatic reaction, the substrate binds to the active site of the enzyme 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 © 2011 Pearson Education, Inc.

F IGURE Substrates Substrates enter active site. Enzyme-substrate complex Substrates are held in active site by weak interactions. 1 2 Enzyme Active site

F IGURE Substrates Substrates enter active site. Enzyme-substrate complex Substrates are held in active site by weak interactions. Active site can lower E A and speed up a reaction Substrates are converted to products. 4 Enzyme Active site

F IGURE Substrates Substrates enter active site. Enzyme-substrate complex Enzyme Products Substrates are held in active site by weak interactions. Active site can lower E A and speed up a reaction. Active site is available for two new substrate molecules. Products are released. Substrates are converted to products

E FFECTS OF L OCAL C ONDITIONS ON E NZYME A CTIVITY An enzyme’s activity can be affected by General environmental factors, such as temperature and pH Chemicals that specifically influence the enzyme © 2011 Pearson Education, Inc.

E FFECTS OF T EMPERATURE AND P H Each enzyme has an optimal temperature in which it can function Each enzyme has an optimal pH in which it can function Optimal conditions favor the most active shape for the enzyme molecule © 2011 Pearson Education, Inc.

F IGURE 8.16 Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Temperature (°C) (a) Optimal temperature for two enzymes Rate of reaction pH (b) Optimal pH for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme)

F IGURE 8.16 A Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Temperature (°C) (a) Optimal temperature for two enzymes Rate of reaction

F IGURE 8.16 B Rate of reaction pH (b) Optimal pH for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme)

IB Q UESTIONS ? Define enzyme and active site Explain enzyme-substrate specificity Explain the effects of temperature, pH and substrate concentration on enzyme activity Define denaturation Explain the production of lactase in the production of lactose free milk.

C OFACTORS Cofactors are nonprotein enzyme helpers Cofactors may be inorganic (such as a metal in ionic form) or organic An organic cofactor is called a coenzyme Coenzymes include vitamins © 2011 Pearson Education, Inc.

E NZYME I NHIBITORS 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 Examples of inhibitors include toxins, poisons, pesticides, and antibiotics © 2011 Pearson Education, Inc.

F IGURE 8.17 (a) Normal binding(b) Competitive inhibition (c) Noncompetitive inhibition Substrate Active site Enzyme Competitive inhibitor Noncompetitive inhibitor

T HE E VOLUTION OF E NZYMES Enzymes are proteins encoded by genes Changes (mutations) in genes lead to changes in amino acid composition of an enzyme Altered amino acids in enzymes may alter their substrate specificity Under new environmental conditions a novel form of an enzyme might be favored © 2011 Pearson Education, Inc.

F IGURE 8.18 Two changed amino acids were found near the active site. Active site Two changed amino acids were found in the active site. Two changed amino acids were found on the surface.

C ONCEPT 8.5: R EGULATION OF ENZYME ACTIVITY HELPS CONTROL METABOLISM Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes © 2011 Pearson Education, Inc.

A LLOSTERIC R EGULATION OF E NZYMES Allosteric regulation may either inhibit or stimulate an enzyme’s activity Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site © 2011 Pearson Education, Inc.

A LLOSTERIC A CTIVATION AND I NHIBITION 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 © 2011 Pearson Education, Inc.

F IGURE 8.19 Regulatory site (one of four) (a) Allosteric activators and inhibitors Allosteric enzyme with four subunits Active site (one of four) Active form Activator Stabilized active form Oscillation Non- functional active site Inactive form Inhibitor Stabilized inactive form Inactive form Substrate Stabilized active form (b) Cooperativity: another type of allosteric activation

F IGURE 8.19 A Regulatory site (one of four) (a) Allosteric activators and inhibitors Allosteric enzyme with four subunits Active site (one of four) Active form Activator Stabilized active form Oscillation Nonfunctional active site Inactive form Inhibitor Stabilized inactive form

F IGURE 8.19 B Inactive form Substrate Stabilized active form (b) Cooperativity: another type of allosteric activation

Cooperativity is a form of allosteric regulation that can amplify enzyme activity One substrate molecule primes an enzyme to act on additional substrate molecules more readily Cooperativity is allosteric because binding by a substrate to one active site affects catalysis in a different active site © 2011 Pearson Education, Inc.

I DENTIFICATION OF A LLOSTERIC R EGULATORS Allosteric regulators are attractive drug candidates for enzyme regulation because of their specificity Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses © 2011 Pearson Education, Inc.

F IGURE 8.20 Caspase 1 Active site Substrate SH Known active form Active form can bind substrate Allosteric binding site Allosteric inhibitor Hypothesis: allosteric inhibitor locks enzyme in inactive form Caspase 1 Active formAllosterically inhibited form Inhibitor Inactive form EXPERIMENT RESULTS Known inactive form

F IGURE 8.20 A Caspase 1 Active site Substrate SH Known active form Active form can bind substrate Allosteric binding site Allosteric inhibitor Hypothesis: allosteric inhibitor locks enzyme in inactive form EXPERIMENT Known inactive form

F IGURE 8.20 B Caspase 1 Active form Allosterically inhibited form Inhibitor Inactive form RESULTS

F EEDBACK I NHIBITION 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 © 2011 Pearson Education, Inc.

F IGURE 8.21 Active site available Isoleucine used up by cell Feedback inhibition Active site of enzyme 1 is no longer able to catalyze the conversion of threonine to intermediate A; pathway is switched off. Isoleucine binds to allosteric site. Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine)

S PECIFIC L OCALIZATION OF E NZYMES W ITHIN THE C ELL Structures within the cell help bring order to metabolic pathways Some enzymes act as structural components of membranes In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria © 2011 Pearson Education, Inc.

F IGURE 8.22 Mitochondria The matrix contains enzymes in solution that are involved in one stage of cellular respiration. Enzymes for another stage of cellular respiration are embedded in the inner membrane. 1  m