An Introduction to Metabolism Chapter 8 An Introduction to Metabolism

Overview: The Energy of Life The living cell is a miniature chemical factory where thousands of reactions occur The cell extracts energy and applies energy to perform work Some organisms even convert energy to light, as in bioluminescence © 2011 Pearson Education, Inc.

Figure 8.1 Figure 8.1 What causes these two squid to glow? 3

Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics Metabolism is the totality of an organism’s chemical reactions Metabolism is an emergent property of life that arises from interactions between molecules within the cell © 2011 Pearson Education, Inc.

Organization of the Chemistry of Life into Metabolic Pathways A metabolic pathway begins with a specific molecule and ends with a product Each step is catalyzed by a specific enzyme- importance of protein synthesis! Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product © 2011 Pearson Education, Inc.

METABOLISM = Catabolism + Anabolism Catabolic pathways release energy by breaking down complex molecules into simpler compounds Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism Anabolic pathways consume energy to build complex molecules from simpler ones The synthesis of protein from amino acids is an example of anabolism © 2011 Pearson Education, Inc.

Forms of Energy Bioenergetics is the study of how organisms manage their energy resources Energy is the capacity to cause change Energy exists in various forms, some of which can perform work © 2011 Pearson Education, Inc.

Kinetic energy is energy associated with motion Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules Potential energy is energy that matter possesses because of its location or structure Chemical energy is potential energy available for release in a chemical reaction Energy can be converted from one form to another © 2011 Pearson Education, Inc.

Animation: Energy Concepts Right-click slide / select “Play” © 2011 Pearson Education, Inc. 9

A diver has more potential energy on the platform than in the water. Figure 8.2 A diver has more potential energy on the platform than in the water. Diving converts potential energy to kinetic energy. Animation Figure 8.2 Transformations between potential and kinetic energy. Climbing up converts the kinetic energy of muscle movement to potential energy. A diver has less potential energy in the water than on the platform. 10

The Laws of Energy Transformation Thermodynamics is the study of energy transformations A isolated system, such as that approximated by liquid in a thermos, is isolated from its surroundings In an open system, energy and matter can be transferred between the system and its surroundings (Organisms are open systems) © 2011 Pearson Education, Inc.

The First Law of Thermodynamics According to the first law of thermodynamics, the energy of the universe is constant Energy can be transferred and transformed, but it cannot be created or destroyed The first law is also called the principle of conservation of energy © 2011 Pearson Education, Inc.

The Second Law of Thermodynamics During every energy transfer or transformation, some energy is unusable, and is often lost as heat According to the second law of thermodynamics Every energy transfer or transformation increases the entropy (disorder) of the universe © 2011 Pearson Education, Inc.

(a) First law of thermodynamics (b) Second law of thermodynamics Figure 8.3 Heat Chemical energy Figure 8.3 The two laws of thermodynamics. (a) First law of thermodynamics (b) Second law of thermodynamics 14

Living cells unavoidably convert organized forms of energy to heat Spontaneous processes occur without energy input; they can happen quickly or slowly For a process to occur without energy input, it must increase the entropy of the universe © 2011 Pearson Education, Inc.

Biological Order and Disorder Cells create ordered structures from less ordered materials Organisms also replace ordered forms of matter and energy with less ordered forms Energy flows into an ecosystem in the form of light and exits in the form of heat © 2011 Pearson Education, Inc.

Figure 8.4 Figure 8.4 Order as a characteristic of life. 17

The evolution of more complex organisms does not violate the second law of thermodynamics Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases © 2011 Pearson Education, Inc.

Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously Biologists want to know which reactions occur spontaneously and which require input of energy To do so, they need to determine energy changes that occur in chemical reactions © 2011 Pearson Education, Inc.

Free-Energy Change, G A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell © 2011 Pearson Education, Inc.

Only processes with a negative ∆G are spontaneous The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), change in entropy (∆S), and temperature in Kelvin (T) ∆G = ∆H – T∆S Only processes with a negative ∆G are spontaneous Spontaneous processes can be harnessed to perform work © 2011 Pearson Education, Inc.

Free Energy, Stability, and Equilibrium Free energy is a measure of a system’s instability, its tendency to change to a more stable state During a spontaneous change, free energy decreases and the stability of a system increases Equilibrium is a state of maximum stability A process is spontaneous and can perform work only when it is moving toward equilibrium © 2011 Pearson Education, Inc.

• More free energy (higher G) • Less stable • Greater work capacity Figure 8.5a • More free energy (higher G) • Less stable • Greater work capacity In a spontaneous change • The free energy of the system decreases (G  0) • The system becomes more stable • The released free energy can be harnessed to do work Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change. • Less free energy (lower G) • More stable • Less work capacity 23

(a) Gravitational motion (b) Diffusion (c) Chemical reaction Figure 8.5b Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change. (a) Gravitational motion (b) Diffusion (c) Chemical reaction 24

Free Energy and Metabolism The concept of free energy can be applied to the chemistry of life’s processes © 2011 Pearson Education, Inc.

Exergonic and Endergonic Reactions in Metabolism An exergonic reaction proceeds with a net release (EXITING!) of free energy and is spontaneous An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous © 2011 Pearson Education, Inc.

Amount of energy released (G  0) Figure 8.6a (a) Exergonic reaction: energy released, spontaneous Reactants Amount of energy released (G  0) Energy Free energy Products Figure 8.6 Free energy changes (G) in exergonic and endergonic reactions. Progress of the reaction 27

Amount of energy required (G  0) Figure 8.6b (b) Endergonic reaction: energy required, nonspontaneous Products Amount of energy required (G  0) Energy Free energy Reactants Figure 8.6 Free energy changes (G) in exergonic and endergonic reactions. Progress of the reaction 28

Equilibrium and Metabolism Reactions in a closed system eventually reach equilibrium and then do no work Cells are not in equilibrium; they are open systems experiencing a constant flow of materials A defining feature of life is that metabolism is never at equilibrium A catabolic pathway in a cell releases free energy in a series of reactions (breaking bonds) and anabolic absorbs free energy (creating bonds). Closed and open hydroelectric systems can serve as analogies © 2011 Pearson Education, Inc.

(a) An isolated hydroelectric system Figure 8.7a G  0 G  0 Figure 8.7 Equilibrium and work in isolated and open systems. (a) An isolated hydroelectric system 30

(b) An open hydroelectric system Figure 8.7b (b) An open hydroelectric system G  0 Figure 8.7 Equilibrium and work in isolated and open systems. 31

(c) A multistep open hydroelectric system Figure 8.7c G  0 G  0 G  0 Figure 8.7 Equilibrium and work in isolated and open systems. (c) A multistep open hydroelectric system 32

A cell does three main kinds of work Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions A cell does three main kinds of work Chemical Transport Mechanical To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one Most energy coupling in cells is mediated by ATP © 2011 Pearson Education, Inc.

The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) is the cell’s energy shuttle ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups For the Cell Biology Video Space Filling Model of ATP (Adenosine Triphosphate), go to Animation and Video Files. © 2011 Pearson Education, Inc.

(a) The structure of ATP Figure 8.8a Adenine Phosphate groups Ribose Figure 8.8 The structure and hydrolysis of adenosine triphosphate (ATP). (a) The structure of ATP 35

Animation Adenosine triphosphate (ATP) Energy Inorganic phosphate Figure 8.8b Adenosine triphosphate (ATP) Animation Figure 8.8 The structure and hydrolysis of adenosine triphosphate (ATP). Energy Inorganic phosphate Adenosine diphosphate (ADP) (b) The hydrolysis of ATP 36

The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken. This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves For the Cell Biology Video Stick Model of ATP (Adenosine Triphosphate), go to Animation and Video Files. © 2011 Pearson Education, Inc.

How the Hydrolysis of ATP Performs Work The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction Overall, the coupled reactions are exergonic © 2011 Pearson Education, Inc.

Phosphorylated intermediate Figure 8.9 Glutamic acid conversion to glutamine (a) NH3 NH2 GGlu = +3.4 kcal/mol Glu Glu Glutamic acid Ammonia Glutamine (b) Conversion reaction coupled with ATP hydrolysis NH3 1 P 2 ADP NH2 ADP ATP P i Glu Glu Glu Glutamic acid Phosphorylated intermediate Glutamine GGlu = +3.4 kcal/mol (c) Free-energy change for coupled reaction Figure 8.9 How ATP drives chemical work: Energy coupling using ATP hydrolysis. NH3 NH2 ATP ADP P i Glu Glu GGlu = +3.4 kcal/mol GATP = 7.3 kcal/mol + GATP = 7.3 kcal/mol Net G = 3.9 kcal/mol 39

The recipient molecule is now called a phosphorylated intermediate ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant The recipient molecule is now called a phosphorylated intermediate © 2011 Pearson Education, Inc.

Protein and vesicle moved Figure 8.10 Transport protein Solute ATP ADP P i P P i Solute transported (a) Transport work: ATP phosphorylates transport proteins. Vesicle Cytoskeletal track Figure 8.10 How ATP drives transport and mechanical work. ATP ADP P i ATP Motor protein Protein and vesicle moved (b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed. 41

The Regeneration of ATP ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) The energy to phosphorylate ADP comes from catabolic reactions in the cell The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways © 2011 Pearson Education, Inc.

Energy from catabolism (exergonic, energy-releasing processes) Figure 8.11 ATP H2O Energy from catabolism (exergonic, energy-releasing processes) Energy for cellular work (endergonic, energy-consuming processes) Figure 8.11 The ATP cycle. ADP P i 43

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 © 2011 Pearson Education, Inc.

Sucrose (C12H22O11) Glucose (C6H12O6) Fructose (C6H12O6) Figure 8.UN02 Sucrase Sucrose (C12H22O11) Glucose (C6H12O6) Fructose (C6H12O6) Figure 8.UN02 In-text figure, p. 152 45

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 thermal energy that the reactant molecules absorb from their surroundings © 2011 Pearson Education, Inc.

Progress of the reaction Figure 8.12 A B C D Transition state A B EA Free energy C D Reactants A B Figure 8.12 Energy profile of an exergonic reaction. G  O C D Products Progress of the reaction 47

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 © 2011 Pearson Education, Inc.

Animation: How Enzymes Work Right-click slide / select “Play” © 2011 Pearson Education, Inc. 49

Course of reaction without enzyme EA without enzyme Figure 8.13 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy Course of reaction with enzyme G is unaffected by enzyme Figure 8.13 The effect of an enzyme on activation energy. Products Progress of the reaction 50

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 For the Cell Biology Video Closure of Hexokinase via Induced Fit, go to Animation and Video Files. © 2011 Pearson Education, Inc.

Enzyme-substrate complex Figure 8.14 Substrate Active site Figure 8.14 Induced fit between an enzyme and its substrate. Enzyme Enzyme-substrate complex (a) (b) 52

Catalysis in the Enzyme’s Active Site In an enzymatic reaction, the substrate binds to the active site of the enzyme The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate © 2011 Pearson Education, Inc.

Substrates enter active site. Figure 8.15-1 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme 54

Substrates enter active site. Figure 8.15-2 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site can lower EA and speed up a reaction. 3 Active site Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme Substrates are converted to products. 4 55

Substrates enter active site. Figure 8.15-3 1 Substrates enter active site. Substrates are held in active site by weak interactions. 2 Substrates Enzyme-substrate complex Active site can lower EA and speed up a reaction. 3 Active site is available for two new substrate molecules. 6 Figure 8.15 The active site and catalytic cycle of an enzyme. Enzyme 5 Products are released. Substrates are converted to products. 4 Products 56

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 © 2011 Pearson Education, Inc.

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 Optimal conditions favor the most active shape for the enzyme molecule Enzyme basics and Denaturation © 2011 Pearson Education, Inc.

Optimal temperature for typical human enzyme (37°C) Figure 8.16 Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Rate of reaction 20 40 60 80 100 120 Temperature (°C) (a) Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Figure 8.16 Environmental factors affecting enzyme activity. Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH (b) Optimal pH for two enzymes 59

Optimal temperature for typical human enzyme (37°C) Figure 8.16a Optimal temperature for typical human enzyme (37°C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77°C) Rate of reaction Figure 8.16 Environmental factors affecting enzyme activity. 20 40 60 80 100 120 Temperature (°C) (a) Optimal temperature for two enzymes 60

Optimal pH for pepsin (stomach enzyme) Figure 8.16b Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction Figure 8.16 Environmental factors affecting enzyme activity. 1 2 3 4 5 6 7 8 9 10 pH (b) Optimal pH for two enzymes 61

Cofactors 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.

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

(b) Competitive inhibition (c) Noncompetitive inhibition Figure 8.17 (a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition Substrate Active site Competitive inhibitor Enzyme Figure 8.17 Inhibition of enzyme activity. Noncompetitive inhibitor 64

The Evolution of Enzymes 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.

Two changed amino acids were found near the active site. Active site Figure 8.18 Two changed amino acids were found near the active site. Active site Figure 8.18 Mimicking evolution of an enzyme with a new function. Two changed amino acids were found in the active site. Two changed amino acids were found on the surface. 66

Concept 8.5: Regulation 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.

Allosteric Regulation of Enzymes 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.

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 © 2011 Pearson Education, Inc.

Active site (one of four) Figure 8.19 (a) Allosteric activators and inhibitors (b) Cooperativity: another type of allosteric activation Allosteric enzyme with four subunits Active site (one of four) Substrate Regulatory site (one of four) Activator Inactive form Stabilized active form Active form Stabilized active form Oscillation Animation Figure 8.19 Allosteric regulation of enzyme activity. Non- functional active site Inhibitor Inactive form Stabilized inactive form 70

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.

Identification of Allosteric Regulators 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.

Active form can bind substrate Figure 8.20a EXPERIMENT Caspase 1 Active site Substrate SH SH Known active form Active form can bind substrate Figure 8.20 Inquiry: Are there allosteric inhibitors of caspase enzymes? Allosteric binding site SH Allosteric inhibitor Hypothesis: allosteric inhibitor locks enzyme in inactive form Known inactive form 73

Allosterically inhibited form Inactive form Figure 8.20b RESULTS Caspase 1 Inhibitor Figure 8.20 Inquiry: Are there allosteric inhibitors of caspase enzymes? Active form Allosterically inhibited form Inactive form 74

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 © 2011 Pearson Education, Inc.

Positive and negative feedback http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter20/ animation__positive_and_negative_feedback__quiz_1_.html http://www.youtube.com/watch?v=CLv3SkF_Eag http://www.biology-online.org/4/1_physiological_homeostasis.htm http://www.youtube.com/watch?v=bCTKnEn7djU&feature=related

Initial substrate (threonine) Figure 8.21 Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Intermediate A Active site of enzyme 1 is no longer able to catalyze the conversion of threonine to intermediate A; pathway is switched off. Feedback inhibition Enzyme 2 Intermediate B Enzyme 3 Intermediate C Figure 8.21 Feedback inhibition in isoleucine synthesis. Isoleucine binds to allosteric site. Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine) 77

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 In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria © 2011 Pearson Education, Inc.

Figure 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. Figure 8.22 Organelles and structural order in metabolism. 1 m 79

Course of reaction without enzyme EA without enzyme Figure 8.UN03 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy G is unaffected by enzyme Course of reaction with enzyme Figure 8.UN03 Summary figure, Concept 8.4 Products Progress of the reaction 80

Figure 8.UN05 Figure 8.UN05 Appendix A: answer to Figure 8.16 legend question 81

An Introduction to Metabolism Chapter 8 An Introduction to Metabolism Questions prepared by Jung Choi Georgia Institute of Technology

Living Organisms and Order How do living organisms create macromolecules, organelles, cells, tissues, and complex higher-order structures? The laws of thermodynamics do not apply to living organisms. Living organisms create order by using energy from the sun. Living organisms create order locally, but the energy transformations generate waste heat that increases the entropy of the universe. Answer: c This question relates to Concept 8.1. 83

Free Energy, Enthalpy, and Entropy When sodium chloride (table salt) crystals dissolve in water, the temperature of the solution decreases. This means that, for dissociation of Na+ and Cl– ions, the change in enthalpy (H) is negative. the change in enthalpy (H) is positive, but the change in entropy is greater. the reaction is endergonic, because it absorbs heat. the reaction must be coupled to an exergonic reaction. the reaction cannot occur spontaneously. Answer: b This question relates to Concept 8.2. 84

Life and Chemical Equilibrium Are most chemical reactions at equilibrium in living cells? yes no only the exergonic reactions all reactions except those powered by ATP hydrolysis Answer: b This question relates to Concept 8.2. At equilibrium, the free energy change is zero, and no work can be done. A cell at equilibrium is dead! 85

Free Energy A reaction has a ∆G of 5.6 kcal/mol. Which of the following would most likely be true? The reaction could be coupled to power an endergonic reaction with a G of 8.8 kcal/mol. The reaction would result in a decrease in entropy (S) and an increase in the energy content (H) of the system. The reaction would result in an increase in entropy (S) and a decrease in the energy content (H) of the system. The reaction would result in products with a greater free- energy content than in the initial reactants. Answer: c This question relates to Concepts 8.2 and 8.3. 86

The hydrolysis of ATP: ATP  H2O → ADP  Pi is exergonic, with a G of 7.3 kcal/mol under standard conditions. What is the source of the 7.3 kcal/mol released in this reaction? breaking the terminal phosphate bond in ATP the increase in entropy from breaking apart ATP both the energy released from breaking the terminal phosphate bond and the increase in entropy the difference between the potential energy in the bonds of ATP and the water molecule, minus the potential energy in the bonds of ADP and Pi Answer: d 87

Rate of a Chemical Reaction The oxidation of glucose to CO2 and H2O is highly exergonic: G = –636 kcal/mole. Why doesn’t glucose spontaneously combust? The glucose molecules lack the activation energy at room temperature. There is too much CO2 in the air. CO2 has higher energy than glucose. The formation of six CO2 molecules from one glucose molecule decreases entropy. The water molecules quench the reaction. Answer: a This question relates to Concept 8.4. 88

Enzymes Firefly luciferase catalyzes the reaction luciferin  ATP ↔ adenyl-luciferin  pyrophosphate then the next reaction occurs spontaneously: adenyl-luciferin  O2 → oxyluciferin  H2O  CO2  AMP  light What is the role of luciferase? Luciferase makes the G of the reaction more negative. Luciferase lowers the transition energy of the reaction. Luciferase alters the equilibrium point of the reaction. Luciferase makes the reaction irreversible. all of the above Answer: b This question relates to Concept 8.4. 89

Enzyme-Catalyzed Reactions In the energy diagram below, which of the lettered energy changes would be the same in both the enzyme-catalyzed and uncatalyzed reactions? a b c d e Answer: c This question relates to Concept 8.4. 90

This figure could represent the rate of an enzyme-catalyzed reaction facilitated diffusion active transport any of the above Answer: d Concept: 8.4 Protein-mediated reactions, including transport across a membrane, will reach saturation when all available enzyme molecules or transport proteins are fully occupied by substrate. 91

If this is an enzyme-catalyzed reaction, how can the rate of this reaction be increased beyond the maximum velocity in this figure? Increase the substrate concentrations. Increase the amount of enzyme. Increase the amount of energy. any of the above There is no way to increase the rate of the reaction any further. Answer: b Concept 8.4 92

Enzyme Inhibitors Vioxx and other prescription nonsteroidal anti-inflammatory drugs (NSAIDs) are potent inhibitors of the cycloxygenase-2 (COX-2) enzyme. High substrate concentrations reduce the efficacy of inhibition by these drugs. These drugs are competitive inhibitors. noncompetitive inhibitors. allosteric regulators. prosthetic groups. feedback inhibitors. Answer: a This question relates to Concepts 8.4 and 8.5. Sources Copeland et al. Mechanism of Selective Inhibition of the Inducible Isoform of Prostaglandin G/H Synthase 1994, PNAS 91:11202-11206 Chan et al. Rofecoxib [Vioxx, MK-0966; 4-(4'-Methylsulfonylphenyl)-3-phenyl-2-(5H)-furanone]: A Potent and Orally Active Cyclooxygenase-2 Inhibitor. Pharmacological and Biochemical Profiles 1999, Pharmacology 290:551-560 93

Enzyme Regulation Adenosine monophosphate (AMP) activates the enzyme phosphofructokinase (PFK) by binding at a site distinct from the substrate binding site. This is an example of cooperative activation. allosteric activation. activation by an enzyme cofactor. coupling exergonic and endergonic reactions. Answer: b This question relates to Concept 8.5. 94

Which enzymes may translocate from the cytoplasm to associate with the cytoplasmic face of the plasma membrane in response to a signal? ion channels active transport proteins phospholipid hydrolases aTP synthases motor proteins Answer: c Concept 8.5 95