CH 5 continued

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CH 5 continued.Energy
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What Is Energy?

• Capacity to do work
• Life depends on the fact that energy can be
  converted from one form to another
• Forms of energy
• Potential energy stored energy (chemical
  positional)
• Kinetic energy energy of motion (heat, light,
  electricity)

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More on potential energy

• Potential energy - stored energy
• Chemical energy potential energy due to
• Concentration gradient
• Energy stored in a chemical bond the potential
  energy of molecules

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• Thermodynamics study of energy transformations
• The First Law of Thermodynamics
• Energy can be changed from one form to another
  but cannot be created or destroyed
• When one form of energy is converted to another
  the total energy present remains the same

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• Thermodynamics study of energy transformations
• The Second Law of Thermodynamics
• Energy transformations increase disorder
  (entropy)
• Each energy transformation in living organisms
  releases some of the energy as heat
• Heat increases the movement of the particles and
  their disorder

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Energy and Cells

• Each time a cell releases energy stored in
  chemical bonds that energy is
• some is captured in new, lower energy bonds
• some is used to do the work of the cell
• and
• some is released as heat.
• Cells cannot use heat to do the work of the cell

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Heat
Chemical reactions
Carbon dioxide
Glucose
ATP
ATP
Water
Oxygen
Energy for cellular work
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• Chemical reactions either release or store energy
• Exergonic reactions
• Release energy
• Yield products that contain less potential energy
  than their reactants
• Examples cellular respiration, burning
• Endergonic reactions
• Require an input of energy from the surroundings
• Yield products higher in potential energy than
  reactants
• Example photosynthesis

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Exergonic reaction
Reactants
Amount of energy released
Energy released
Potential energy of molecules
Products
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Examples of Exergonic Reactions

• Reactions that release energy
• Cellular respiration
• Multi-step process by which energy stored in
  glucose is released
• Some of the energy released is used to make ATP
• Some of the energy is released as heat
• Hydrolysis of ATP
• ATP ? ADP P energy

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Endergonic reaction
Products
Amount of energy required
Energy required
Potential energy of molecules
Reactants
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Examples of Endergonic Reactions

• Reactions that require an input of energy
• Photosynthesis
• Multi-step process in which the suns energy is
  used to convert CO2 and water into high energy
  sugar molecules
• Synthesis of ATP
• ADP P energy ? ATP

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Energy Coupling

• Coupled Reactions
• Energy given off by the exergonic reaction is
  used to fuel/drive the endergonic reaction
• Examples
• Active transport
• First step of glycolysis

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ATP

• ATP is the cells primary energy carrier
• ATP adenosine triphosphate
• ATP 1 adenine, 1 ribose, and 3 phosphates
  (nucleotide)
• The energy in an ATP molecule is in the bonds
  between phosphates

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• The hydrolysis of ATP releases energy.

Triphosphate
Adenosine diphosphate
Adenosine
Phosphate group
H2O
P
P
P
P
P
P


Energy
Hydrolysis
Adenine
Ribose
ADP
ATP
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ATP and Coupled Reactions

• The hydrolysis of ATP often transfers a P from
  ATP to another molecule
• Called a phosphorylation reaction
• As a result the ATP is converted to ADP and is of
  lower energy
• The phosphorylated molecule is of higher energy
  (energized)

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ATP
Chemical work
Mechanical work
Transport work
Membrane protein
Solute
P
Motor protein
P
Reactants
P
P
P
Product
P
Molecule formed
Protein moved
Solute transported
ADP ?
P
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How Enzymes Function

• Even with an abundant source of ATP chemical
  reactions in cells would not occur without the
  presence of catalysts called enzymes.

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How Enzymes Function

• Chemical reactions require an input of energy to
  get started
• Called the activation energy (EA)
• Reactions with a large EA are relatively slow.
• Enzymes lower the activation energy
• All reactions in the cell require the presence of
  a specific enzyme

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• Enzymes proteins that function as biological
  catalysts
• Increase the rate of a reaction without
  themselves being changed
• Decrease the energy of activation needed to begin
  a reaction

EA without enzyme
EA with enzyme
Reactants
Energy
Net change in energy
Products
Progress of the reaction
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Enzymes and Substrates

• Enzymes are substrate specific
• Each enzyme has a unique three-dimensional shape
  that determines which chemical reaction it
  catalyzes
• What determines the enzymes 3-D shape?

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Enzymes and Substrates

• Terms
• Substrate the reactant that fits into the active
  site of an enzyme
• Active site a pocket on the enzymes surface
  that the substrate attaches to
• Each active site binds only specific substrates

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Induced Fit Model

• Binding of the substrate causes the enzyme to
  change shape and hold the substrates more
  tightly.
• This creates conditions that are ideal for the
  reaction by
• Holding reactants close together
• Orienting reactants properly
• Straining bonds
• Shutting out / bringing in water
• Creating an acid or base environment

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Enzyme available with empty active site
Active site
Substrate (sucrose)
Substrate binds to enzyme with induced fit
Enzyme (sucrase)
Glucose
Fructose
H2O
Products are released
Substrate is converted to products
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Enzyme Summary

• Most enzymes are proteins.
• Speed up reactions by lowering the EA
• Enzymes are substrate specific
• Enzymes are not permanently changed in the
  reaction.
• Enzymes can be used over and over again.
• A single enzyme may act on thousands or millions
  of substrate molecules per second!

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Cellular Environment and Enzymes

• Enzymes must be in their correct 3-D shape to
  function
• Each enzyme functions at its best under a
  specific set of conditions.
• Temperature
• pH
• Salinity
• Why do these conditions matter?

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Temperature and Enzyme Activity
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Activity Data for 3 Enzymes
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Co-factors

• Some enzymes require cofactors to function
• Metal ions
• Such as ions of iron, zinc, and calcium
• Organic molecules called coenzymes
• The B vitamins are all coenzymes

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Enzyme Inhibitors

• Inhibitors are substances that interfere with an
  enzymes ability to function
• Many toxins/poisons are enzyme inhibitors
• For example Mercury binds to sulfur groups on
  enzymes and cause the enzyme to change shape and
  lose function

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Enzyme Inhibitors

• Inhibitors may bind to the enzyme with covalent
  bonds or H bonds
• Covalent bonding inhibitors ? Irreversible
  inhibition
• H bonding inhibitors ? Reversible inhibition

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More on Enzyme Inhibitors

• Some enzyme inhibitors are deadly
• Cyanide inhibits an enzyme needed to make ATP
• Sarin inhibits an enzyme needed for nerve
  transmission
• Pesticides and herbicides bind to key enzymes
  in insects and plants

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More on Enzyme Inhibitors

• Some inhibitors are beneficial
• Antibiotics bind to essential enzymes found in
  bacteria
• Enzymes not found in eukaryotic cells
• Kills the bacteria
• Protease inhibitors are HIV drugs that bind to
  and inhibit an essential viral enzyme
• Ibuprofen inhibits the enzyme needed to make
  pain/inflammation messengers

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Types of Inhibitors

• Competitive inhibitors compete with the
  substrate for binding at the active site
• Competitive inhibitors are similar in structure
  to the real substrate

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Types of Inhibitors

• Noncompetitive inhibitors bind to the enzyme at
  a location other than the active site
• Binding changes the shape of the active site so
  that the substrate cannot bind
• Called allosteric control
• Release of the inhibitor returns the active site
  to its proper shape

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Substrate
Active site
Enzyme
Normal binding of substrate
Competitive inhibitor
Noncompetitive Inhibitor -- also called an
allosteric inhibitor
Enzyme inhibition
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• SUMMARY
• A competitive inhibitor takes the place of a
  substrate in the active site
• Inhibitor blocks entry of the true substrate
• A noncompetitive inhibitor alters an enzyme's
  function by binding at a location other than the
  active site and the binding changes the shape of
  the active site
• Also called - allosteric control

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Feedback Inhibition

• The end-product of the pathway is an inhibitor of
  the first enzyme in the pathway
• When the product is no longer needed, it builds
  up, attaches to the 1st enzyme in the pathway ?
  turns the enzyme and pathway OFF
• When the cell needs to make the product again the
  product/inhibitor detaches from the enzyme
• the enzyme resumes its original shape and bind
  substrate 1 again
• .

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Feedback Inhibition of Tryptophan Synthesis
enzyme 2
enzyme 3
enzyme 4
enzyme 5

• enzyme activity of the first enzyme is blocked by
  the end-product of the pathway

enzyme 1
Tryptophan builds up in the cell, binds to the
first enzyme in the pathway, the active site
changes shape so that it cannot bind the first
substrate, and tryptophan synthesis stops
END PRODUCT (tryptophan)
SUBSTRATE