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An Introduction to Metabolism

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1 An Introduction to Metabolism
Chapter 8: An Introduction to Metabolism

2 Marie Maynard Daly – American biochemist who did extensive research into the metabolism of the arterial wall in the 1960s. Her worked helped to uncover several processes related to aging.

3 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.

4 Bioluminescence by a fungus

5 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 © 2011 Pearson Education, Inc.

6 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 © 2011 Pearson Education, Inc.

7 Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3
Figure 8.UN01 Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product Figure 8.UN01 In-text figure, p. 142 7

8 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 © 2011 Pearson Education, Inc.

9 The synthesis of protein from amino acids is an example of anabolism
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.

10 Forms of Energy Energy is the capacity to cause change
Energy exists in various forms, some of which can perform work 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.

11 Figure 8.2 Transformations between kinetic and potential energy
On the platform, a diver has more potential energy. Diving converts potential energy to kinetic energy. Climbing up converts kinetic energy of muscle movement to potential energy. In the water, a diver has less potential energy.

12 The two laws of thermodynamics
Figure 8.3 The two laws of thermodynamics Heat Chemical energy Figure 8.3 The two laws of thermodynamics. (a) First law of thermodynamics (b) Second law of thermodynamics 12

13 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.

14 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.

15 Exergonic and Endergonic Reactions in Metabolism
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.

16 Figure 8.6 Free energy changes (G) in exergonic and endergonic reactions
(a) Exergonic reaction: energy released Reactants Products Energy Progress of the reaction Amount of energy released (∆G<0) Free energy released (∆G>0) (b) Endergonic reaction: energy required

17 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.

18 Figure 8.8 The structure of adenosine triphosphate (ATP)
CH –O O CH2 H OH N C HC NH2 Adenine Ribose O– P Phosphate groups

19 Figure 8.9 The hydrolysis of ATP
Adenosine triphosphate (ATP) H2O + Energy Inorganic phosphate Adenosine diphosphate (ADP) P i

20 Figure 8.11 How ATP drives cellular work
Motor protein P i Protein moved (a) Mechanical work: ATP phosphorylates motor proteins Membrane protein ATP Solute P i ADP + Solute transported (b) Transport work: ATP phosphorylates transport proteins Glu NH3 NH2 (c) Chemical work: ATP phosphorylates key reactants Reactants: Glutamic acid and ammonia Product (glutamine) made +

21 Figure 8.12 The ATP cycle ATP synthesis from ADP + P i requires energy
Energy for cellular work (endergonic, energy- consuming processes) Energy from catabolism (exergonic, energy yielding processes) ATP hydrolysis to ADP + P i yields energy

22 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.

23 Figure 8.13 The effect of enzymes on reaction rate.
Progress of the reaction Products Course of reaction without enzyme Reactants with enzyme EA EA with is lower ∆G is unaffected by enzyme Free energy

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

25 Figure 8.15 The active site and catalytic cycle of an enzyme
1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates Products Enzyme Enzyme-substrate complex 5 Products are Released. 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 3 Active site (and R groups of its amino acids) can lower EA 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. 4 Substrates are Converted into Products. 6 Active site is available for two new substrate molecules.

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

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

28 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.

29 Figure 8.17 Inhibition of enzyme activity
Normal binding (b) Competitive inhibition A substrate can bind normally to the active site of an enzyme. A competitive inhibitor mimics the substrate, competing for the active site. Substrate Active site Enzyme Noncompetitive inhibitor (c) Noncompetitive inhibition Competitive inhibitor 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.

30 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.

31 Figure 8.19 Allosteric regulation of enzyme activity
Stabilized inactive form Allosteric activater stabilizes active from Allosteric enyzme with four subunits Active site (one of four) Regulatory site (one of four) Active form Activator Stabilized active form Allosteric activater stabilizes active form Inhibitor Inactive form Non- functional active site (a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again. Oscillation

32 Stabilized active form
Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate Inactive form Stabilized active form (b) Cooperativity: another type of allosteric activation. Note that the inactive form shown on the left oscillates back and forth with the active form when the active form is not stabilized by substrate.

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