Chapter 8 Introduction to Metabolism The Energy of
Chapter 8 Introduction to Metabolism 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 matter and energy An organisms metabolism transforms matter and energy, subject to the laws of thermodynamics Metabolism is the totality of an organisms chemical reactions Metabolism is a property of life that arises from interactions between molecules within the cell
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 A Enzyme 2 B Reaction 1 Starting
molecul e Enzyme 3 C Reaction 2 Reaction 3 D Product Metabolic Pathways 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 Metabolic Pathways 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 Forms of Energy Energy Energy is the capacity to cause change
exists in various forms, some of which can perform work Forms of Energy 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 Forms of Energy A diver has more potential energy on the platform than in the water. Climbing up converts the kinetic energy of muscle movement to potential energy.
Diving converts potential energy to kinetic energy. A diver has less potential energy in the water than on the platform. 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 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 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 The Second Law of Thermodynamics Heat Chemical energy First law of thermodynamics Second law of thermodynamics 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 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 Free-Energy Change, G A living systems free energy is energy that can do work when temperature and pressure are uniform, as in a living cell Free-Energy Change, G 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 TS Only processes with a negative G are spontaneous Spontaneous processes can be harnessed to perform work Free Energy, Stability, and Equilibrium Free energy is a measure of a systems 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 Free Energy, Stability, and
Equilibrium 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(lower work G) Less free energy More stable Less work capacity Gravitational motion Diffusion Chemical reaction Free Energy and Metabolism The
concept of free energy can be applied to the chemistry of lifes processes 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
Exergonic Exergonic reaction: energy released, spontaneous Free energy Reactants Amount of energy released (G 0)) Energy Products Progress of the reaction
Endergonic Free energy Endergonic reaction: energy required, nonspontaneous Products Amount of energy required (G 0)) Reactants Energy
Progress of the reaction 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 Closed and open hydroelectric systems can serve as analogies Equilibrium and Metabolism G 0) An isolated hydroelectric system
G 0) Equilibrium and Metabolism An open hydroelectric system 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 The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) is the cells energy shuttle
ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups The structure of ATP Adenine Phosphate groups Ribose The hydrolysis of ATP Adenosine triphosphate (ATP)
Energy Inorganic phosphate Adenosine diphosphate (ADP) ATP The bonds between the phosphate groups of ATPs 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 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 How the Hydrolysis of ATP Performs Work 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 How the Hydrolysis of ATP Performs Work Solute Transport protein ATP ADP
P i Solute transported Transport work: ATP phosphorylates transport proteins. Vesicle ATP ATP Motor protein
ADP P i Protein and vesicle moved Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed. 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 ATP Cycle ATP
Energy from catabolism (exergonic, energy-releasing processes) ADP H2 O P i Energy for cellular work (endergonic,
energy-consuming processes) 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
Enzymes Sucrase Sucrose (C12H22O11) Glucose (C6H12O6) Fructose (C6H12O6) 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
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 How Enzymes Lower the EA Barrier Free energy
Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Course of reaction
with enzyme Progress of the reaction G is unaffected by enzyme Products Substrate Specificity of Enzymes The reactant that an enzyme acts on is called the enzymes 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 Substrate Specificity of Enzymes Substrate
Active site Enzyme Enzyme-substrate complex Catalysis in the Enzymes Active Site In an enzymatic reaction, the substrate binds to the active site of the enzyme The active site can lower an E barrier by A
Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate Catalysis in the Enzymes Active Site Substrates enter active site. Substrates are held in active site by weak interactions. Substrates Enzyme-substrate complex
Active site is available for two new substrate molecules. Active site can lower EA and speed up a reaction. Enzyme Products are released. Products
Substrates are converted to products. Effects of Local Conditions on Enzyme Activity An enzymes 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 Optimal conditions favor the most active shape for the enzyme molecule Optimal pH for pepsin (stomach enzyme Rate of reaction 0)
Optimal pH for trypsin (intestinal enzyme 4 pH Optimal pH for two enzymes 10) 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 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 Normal binding Competitive inhibition Noncompetitive inhibition
Competitive inhibitor Noncompetitive inhibitor 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 Regulation of enzyme activity helps control metabolism Chemical chaos would result if a cells 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 Allosteric Regulation of Enzymes Allosteric regulation may either inhibit or stimulate an enzymes activity Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and
affects the proteins function at another site 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 Allosteric Activation and Inhibition Allosteric activators and inhibitors Active site (one of four) Allosteric enzyme with four subunits Active form Activator Stabilized active form Oscillation
Nonfunctional active site Inactive form Inhibitor Stabilized inactive form Cooperativity Cooperativity: another type of allosteric activation Substrate Inactive form
Stabilized active form Cooperativity 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 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 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 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
Mitochondria 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.
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