Chapter%206:%20Metabolism%20

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Chapter 6 Metabolism Energy and Enzymes
(Outline)

• Forms of Energy
• Two Laws of Thermodynamics
• Cells and Entropy
• Metabolic Reactions
• ATP Energy for Cells
• Metabolic Pathways and Enzymes
• Energy of Activation
• Enzyme-Substrate Complex
• Redox reactions

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Forms of Energy

• The capability to do work or produce an effect
  and is divided into two types
• Kinetic- the energy of motion, such as waves,
  electrons, atoms, molecules, substances and
  objects
• Mechanical energy of motion
• Electrical (e.g. lightning)
• Thermal (i.e. geothermal energy)
• Radiant (e.g. visible light, x-rays, gamma rays
  and radio waves)

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Forms of Energy

• Potential
• Stored energy or energy that is "waiting to
  happen"
• Chemical - energy stored in the bonds of atoms
  and molecules
• Nuclear energy stored in the nucleus of an atom
  (the energy that holds the nucleus together)
• Stored mechanical energy (e.g. compressed springs)

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Laws of Thermodynamics

• First law
• Law of conservation of energy
• Energy cannot be created or destroyed, but
• Energy CAN be changed from one form to another

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Laws of Thermodynamics

• Second law
• Energy cannot be changed from one form to another
  without a loss of usable energy
• Therefore, no process requiring a conversion of
  energy is ever 100 efficient

Carbohydrate Metabolism
Carbohydrate Synthesis
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Cells and Entropy

• Law of entropy
• The spontaneous movement from order to disorder
  (randomness)
• Waste energy goes to increase disorder
• Entropy a measure of the degree of a systems
  disorder which increases over time
• Cars rust, dead trees
  decay, buildings
  collapse all these
  are examples of
  entropy
  in action

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Cells and Entropy
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Energy Flow in Living Things
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Metabolic Reactions andEnergy Transformations

• Metabolism
• Sum of cellular chemical reactions in a cell
• Reactants participate in a reaction
• Products form as result of a reaction
• Free energy the amount of energy available to
  perform work
• Concept of free energy was developed by Gibbs
• For a reaction to occur spontaneously free energy
  must decrease - the products must have less free
  energy than the reactants

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Metabolic Reactions Energy Transformations

• Energy Changes in Metabolic Reactions
• Exergonic Reactions - products have less free
  energy than reactants (i.e. spontaneous and
  releases energy)
• Example ATP breakdown
• Endergonic Reactions - products have more free
  energy than reactants (i.e. not spontaneous and
  requires energy)
• Example
  Muscle contraction

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ATP Energy for Cells

• Adenosine triphosphate (ATP)
• High energy compound used to drive metabolic
  reactions
• Constantly being generated from adenosine
  diphosphate (ADP) and a molecule of inorganic
  phosphate
• Composed of
• Adenine, ribose (C5 sugar), and 3 phosphate
  groups
• Biological advantages to the use of ATP
• It provides a common energy currency used in many
  types of reactions
• When ATP becomes ADP ? the amount of energy
  released is sufficient for biological function
  with little waste of energy ( 7.3 kcal per
  molecule)
• ATP breakdown can be coupled to endergonic
  reactions that prevents energy waste

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The ATP Cycle
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ATP and Coupled Reactions

• Coupled reactions
• Energy released by an exergonic reaction is
  captured in ATP
• That ATP is used to drive an endergonic reaction
• Occur in the same place, at the same time, why?
• Because the energy released by the hydrolysis of
  ATP is higher than the energy consumed by the
  endergonic reaction

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Coupled Reactions

• A cell has two ways to couple ATP hydrolysis
• ATP is used to energize a reactant
• ATP is used change the shape of a reactant
• Both can be achieved by transferring ? to the
  reactant so that the product is phosphorylated
• Example
• Ion movement across the plasma membrane of a cell
  through carrier proteins
• Attachment of amino acids to a growing
  polypeptide chain

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Coupled Reactions Muscle Contraction
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Enzymes

• Protein molecules that function as catalysts,
  however ribozymes are made of RNA not proteins
• The reactants of an enzymatically accelerated
  reaction are called substrates
• Each enzyme accelerates a specific reaction
• Each reaction in a metabolic pathway requires a
  unique and specific enzyme
• End product will not appear unless ALL enzymes
  are present and functional

E1 E2 E3 E4 E5 E6 A ? B ? C ? D ? E
? F ? G
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Metabolic Pathways

• Reactions usually occur in a sequence
• Products of an earlier reaction become reactants
  of a later reaction
• Such linked reactions form a metabolic pathway
• Begins with a particular reactant,
• Proceeds through several intermediates, and
• Terminates with a particular end product

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Energy of Activation

• Reactants are often reluctant to participate in
  the reaction
• Energy must be added to at least one reactant to
  initiate the reaction
• Energy of activation - minimum amount of energy
  required to trigger a chemical reaction
• Enzyme Operation
• Enzymes operate by lowering the energy of
  activation
• Accomplished by bringing the substrates into
  contact with one another under mild conditions
• Does not get consumed by the reaction nor does it
  alter the equilibrium of the reaction, thus it
  remains intact

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Energy of activation (Ea)
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Enzyme-Substrate Complex

• The lock and key model of enzyme activity
• The active site is made up of amino acids and has
  a very specific shape
• The enzyme and substrate slot together to form a
  complex, as a key slots into a lock
• This complex reduces the activation energy for
  the reaction
• Then the products no longer fit into the active
  site and are released allowing another substrate
  in

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Enzyme-Substrate Complex

• Induced fit model
• The active site complexes with the substrates by
  weak interactions, such as hydrogen bonds, etc
• Causes active site to change shape
• Shape change forces substrates together,
  initiating bond
• Some enzymes participate in the reaction (e.g.
  Trypsin active site contains 3 amino acids with
  R groups interacting with the peptide bond (to
  break the bond and introduce H2O

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Synthesis vs. Degradation

• Synthesis
• Enzyme complexes with two substrate molecules
• Substrates are joined together and released as
  single product molecule
• Degradation
• Enzyme complexes with a single substrate molecule
• Substrate is broken apart into two product
  molecules

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Enzymatic action
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Naming enzymes

• Enzyme names usually end in ase
• Many enzyme names have 3 parts (substrate
  name) (type of reaction) (ase)

Substrate Enzyme
Lipid Lipase
Urea Urease
Maltose Maltase
Cellulose Cellulase
Lactose Lactase
Sucrose Sucrase
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Factors Affecting Enzyme Activity

• Substrate concentration
• Enzyme activity increases with substrate
  concentration
• More collisions between substrate molecules and
  the enzyme
• When the active sites of the enzyme are filled,
  with increasing substrate,
  the enzymes rate of activity
  cannot
  increase any more
• Amount of active enzyme
  can also increase or limit the
  rate of an
  enzymatic reaction

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Factors Affecting Enzyme Activity

• Temperature
• Enzyme activity increases with temperature
• Warmer temperatures cause more effective
  collisions between enzyme and substrate
• However, hot temperatures destroy enzymes, how?
• Denaturation enzymes shape changes and it can
  no longer bind its substrate efficiently
• However, there are exceptions such as
• Some prokaryotes living in hot springs
• Coat color pattern in Siamese cats

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Factors Affecting Enzyme Activity

• Optimal pH
• Most enzymes are optimized for a particular pH
• Changes in pH can make and break intra- and
  intermolecular bonds, and/or hydrogen bonds thus
  changing the globular shape of the enzyme
• pH change can alter the ionization of R side
  chains
• Under extreme conditions of pH, the enzyme
  becomes inactive (due to altered shape)
• Example Pepsin (stomach) and trypsin (small
  intestine)

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Effect of Temperature and pH
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Enzyme Cofactors

• Many enzymes require additional help in
  catalyzing their reaction from a coenzyme or
  cofactor
• Cofactor is used to refer to inorganic metallic
  ions such as zinc, copper and iron, which is
  required by an enzyme
• Coenzyme is a non-protein organic molecule
• Many of the coenzymes are derived from vitamins
• Enzyme activity decreases if vitamin is not
  available (i.e. vitamin-deficient disorder)
• Niacin deficiency results in skin disease
  (pellagra), while riboflavin deficiency results
  in cracks at the corner of mouth

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Enzyme Inhibition

• Competitive inhibition
• Substrate and the inhibitor are both able to bind
  to the active site
• If a similar molecule is present, it will compete
  with the real substrate for the active sites
• Product will form only when the substrate, not
  the inhibitor, is at the active site
• This will regulate the amount of product

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Enzyme Inhibition

• Noncompetitive inhibition
• Noncompetitive inhibitors are considered to be
  substances which when added to the enzyme change
  the enzyme in a way that it cannot accept the
  substrate
• The inhibitor binds to another location
  (allosteric site) on the enzyme and inactivates
  the enzyme molecule
• Both competitive and noncompetitive inhibition
  are examples of feedback inhibition

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Competitive and Noncompetitive Inhibitors
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Feedback Inhibition

• The end product of a pathway inhibits the
  pathways first enzyme
• The metabolic pathway is shut down when the end
  product of the pathway is bound to an allosteric
  site on the first enzyme of the pathway
• Normally, enzyme inhibition is reversible and the
  enzyme is not damaged

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Oxidation and Reduction

• Oxidation
• The loss of one or more electrons
• Reduction
• The gain of one or more electrons
• Reduction-oxidation (redox) reactions
• Simultaneous reaction in which one molecule is
  oxidized and another is reduced
• Redox reactions occur during photosynthesis and
  cellular respiration

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Electron Transfer in Redox Reactions