An Introduction to Metabolism

Transcription

1 Chapter 8 1 An Introduction to Metabolism PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2 An organism s metabolism transforms matter and energy, subject to the laws of thermodynamics Metabolism is the totality of an organism s chemical reactions 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 Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product

3 Catabolic pathways release energy 3 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 Bioenergetics is the study of how organisms manage their energy resources

4 Forms of Energy 4 Energy is the capacity to cause change Energy exists in various forms, some of which can perform work Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5 5 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 Free energy is a measure of a system s instability, its tendency to change to a more stable state Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animation: Energy Concepts

6 A diver has more potential energy on the platform than in the water. Diving converts potential energy to kinetic energy. 6 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.

7 The Laws of Energy Transformation Thermodynamics is the study of energy transformations 7 A closed 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8 The First Law of Thermodynamics 8 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9 The Second Law of Thermodynamics 9 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10 10 The two laws of thermodynamics Chemical energy Heat CO 2 + H 2 O (a) First law of thermodynamics (b) Second law of thermodynamics

11 11 Energy flows into an ecosystem in the form of light and exits in the form of heat Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12 Free energy Free energy Exergonic and Endergonic Reactions in Metabolism 12 An exergonic reaction proceeds with a net release of energy Reactants Energy Products Amount of energy released ( G < 0) and is spontaneous Progress of the reaction (a) Exergonic reaction: energy released Free energy changes in exergonic and endergonic reactions An endergonic Products reaction absorbs free energy from its surroundings and is Reactants Energy Amount of energy required ( G > 0) nonspontaneous Progress of the reaction (b) Endergonic reaction: energy required

13 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 13 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14 G < 0 G = 0 14 (a) An isolated hydroelectric system (b) An open hydroelectric system Equilibrium and work in isolated and open systems G < 0 G < 0 G < 0 G < 0 (c) A multistep open hydroelectric system

15 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16 The Structure and Hydrolysis of ATP 16 Adenine Phosphate groups Ribose ATP (adenosine triphosphate) is the cell s energy shuttle and currency ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups

17 The bonds between the phosphate groups of ATP s tail can be broken by hydrolysis P P P The hydrolysis of ATP Adenosine triphosphate (ATP) H 2 O 17 Energy is released from ATP when the terminal phosphate bond is broken P + P P + i Energy Inorganic phosphate Adenosine diphosphate (ADP)

18 How ATP Performs Work The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP 18 ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant The recipient molecule is now phosphorylated Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19 The Regeneration of ATP 19 ATP is a renewable resource that is regenerated by addition of a phosphate group (i.e., phosphorylation) to adenosine diphosphate (ADP) The energy to phosphorylate ADP comes from catabolic reactions in the cell The chemical potential energy temporarily stored in ATP drives most cellular work Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

21 The ATP cycle 21 ATP + H 2 O Energy from catabolism (exergonic, energy-releasing processes) ADP + P i Energy for cellular work (endergonic, energy-consuming processes)

22 Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers 22 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23 23 Sucrose (C 12 H 22 O 11 ) Sucrase Example of an enzyme-catalyzed reaction: hydrolysis of sucrose by sucrase Glucose (C 6 H 12 O 6 ) Fructose (C 6 H 12 O 6 )

24 The Activation Energy Barrier 24 Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the energy of activation, or activation energy (E A ) Activation energy is often supplied in the form of heat from the surroundings Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

25 Animation: How Enzymes Work A B C D Transition state 25 Enzymes catalyze reactions by lowering the E A barrier A C B D E A Reactants Energy profile of an exergonic reaction Progress of the reaction A B C D Products G < O

26 The effect of an enzyme on activation energy 26 Course of reaction without enzyme Reactants Course of reaction with enzyme E A without enzyme E A with enzyme is lower G is unaffected by enzyme Progress of the reaction Products

27 Substrate Specificity of Enzymes 27 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28 Substrate Specificity of Enzymes 28 Substrate Active site Enzyme Enzyme-substrate complex (a) (b)

29 1 Substrates enter active site; enzyme changes shape such that its active site enfolds the substrates (induced fit). 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 29 Substrates Enzyme-substrate complex 3 Active site can lower E A and speed up a reaction. 6 Active site is available for two new substrate molecules. Enzyme The active site and catalytic cycle of an enzyme 5 Products are released. 4 Substrates are converted to products. Products

30 Effects of Local Conditions on Enzyme Activity 30 An enzyme s activity can be affected by General environmental factors, such as temperature and ph Chemicals that specifically influence the enzyme 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

31 Rate of reaction Rate of reaction Figure 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 Optimal ph for pepsin (stomach enzyme) Optimal ph for trypsin (intestinal enzyme) ph (b) Optimal ph for two enzymes

32 Cofactors 32 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

33 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 33 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

34 34 Substrate Active site Competitive inhibitor Enzyme (a) Normal binding (b) Competitive inhibition Noncompetitive inhibitor (c) Noncompetitive inhibition

35 Figure 8.19 (a) Allosteric activators and inhibitors 35 (b) Cooperativity: another type of allosteric activation Allosteric enzyme with four subunits Active site (one of four) Substrate Regulatory site (one Activator of four) Active form Stabilized active form Inactive form Stabilized active form Oscillation Allosteric regulation of enzyme activity Non- Inhibitor functional active site Stabilized inactive Inactive form form

36 Feedback Inhibition 36 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 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

37 Figure 8.21 Feedback inhibition in isoleucine synthesis Isoleucine used up by cell Active site of Feedback enzyme 1 is inhibition no longer able to catalyze the conversion of threonine to intermediate A; pathway is switched off. Isoleucine binds to allosteric site. Active site available Intermediate A Intermediate B Intermediate C Intermediate D Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 37 End product (isoleucine)

38 ??? 38????