Energy and Metabolism

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Energy and Metabolism

• Chapter 7

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Energy

• Energy is the capacity to do work
• expressed in kilojoules, kJ
• Heat energy
• thermal energy flows from higher temperature to
  lower temperature
• kilocalorie (kcal)
• unit of heat energy 4.184 kJ
• Heat energy cant do cell work

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Potential and Kinetic

• Potential energy
• stored energy
• Kinetic energy
• energy of motion

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POTENTIAL
Energy of position
Fig. 7-1a, p. 153
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KINETIC
Energy of motion
Fig. 7-1b, p. 153
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• Example in biological systems
• Fatty acids store chemical energy in their C-H
  bonds and C-C bonds
• That energy can be released to do biological work

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Energy Conversion

• All forms of energy are interconvertible
• Photosynthesis converts radiant energy to
  chemical energy

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Metabolism

• Sum of all the chemical activities taking place
  in an organism
• Anabolism
• Complex molecules synthesized from simpler
  substances
• Catabolism
• Larger molecules broken down into smaller ones

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2 types of activities

• Anabolic reactions
• Link simple molecules together to make complex
  ones
• Energy storing process
• Reactions consume energy
• Catabolic reactions
• Break down complex molecules into simpler ones
• Some reactions provide energy for anabolic
  reactions
• Reactions release energy

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• Cellular activities require energy
• Activities would not proceed without source of
  energy

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KEY CONCEPTS

• Energy is the capacity to do work
• kinetic energy (energy of motion)
• potential energy (energy due to position or
  state)

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Closed and Open Systems

• Closed system
• no energy exchange with surroundings
• Organisms are open systems
• exchange energy with surroundings

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• First and Second Law of Thermodynamics

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The First Law of Thermodynamics

• Energy cannot be created or destroyed but can be
  transferred and changed in form
• Organisms capture energy from their surroundings

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The Second Law of Thermodynamics

• Disorder (entropy) in the universe, a closed
  system, is continuously increasing
• No energy transfer is 100 efficient
• Some energy dissipates as heat, random motion
  that contributes to entropy (S)

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Energy is neither created or destroyed.
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When energy is converted from one form to
another, some of that energy becomes unavailable
to do work.
Total energy
Usable energy unusable energy
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In Biological Systems

• Total energy
• Enthalpy (H)
• Usable energy that can do work
• Free energy (G)
• Unusable energy (S X T)
• Entropy (S) measure of disorder of system
• Absolute Temperature (T)

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Free Energy

• As entropy increases, free energy decreases
• G H -TS
• G (free energy)
• H (enthalpy, potential energy of system)
• T (absolute temperature in Kelvin units)
• S (entropy)

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Energy of Chemical Reactions

• ?G ?H - T?S
• Change in free energy (?G) during a chemical
  reaction
• change in enthalpy (?H)
• absolute temperature (T)
• change in entropy (?S)

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• Important concept
• Change in free energy
• ? G of reaction ? G of products ? G of
  reactants
• If ?G is negative
• Free energy is released
• If ?G is positive
• Free energy is required (consumed)

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• If free energy is not available, reaction does
  not occur.
• As a result of energy conversions, disorder tends
  to increase.
• Life requires a constant input of energy to
  maintain order.

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KEY CONCEPTS

• Energy cant be created or destroyed
• - first law of thermodynamics
• Total energy available to do work in a closed
  system decreases over time
• - second law of thermodynamics
• Organisms follow laws of thermodynamics
• - are open systems
• - use energy from surroundings to do work

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Exergonic Reactions

• have a negative ?G value
• free energy decreases
• are spontaneous
• release free energy that can perform work

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Endergonic Reactions

• have a positive ?G value
• free energy increases
• are not spontaneous

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Reactants
Free energy decreases
Free energy (G)
Products
Course of reaction
(a) In an exergonic reaction, there is a net loss
of free energy. The products have less free
energy than was present in the reactants, and the
reaction proceeds spontaneously.
Fig. 7-3a, p. 156
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Products
Free energy (G)
Free energy increases
Reactants
Course of reaction
(b) In an endergonic reaction, there is a net
gain of free energy. The products have more free
energy than was present in the reactants.
Fig. 7-3b, p. 156
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• Exergonic reaction
• Releases energy that can perform work
• Endergonic reaction increases free energy

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• Exergonic reaction
• Releases energy that can perform work
• Endergonic reaction increases free energy

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Coupled Reaction

• Input of free energy required to drive an
  endergonic reaction is supplied by an exergonic
  reaction
• A?B ?G 20.9 kJ/mol
• C?D ?G -33.5 kJ/mol
• Overall ?G -12.6 kJ/mol

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KEY CONCEPTS

• In cells, energy-releasing (exergonic) processes
  drive energy-requiring (endergonic) processes

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Dynamic Equilibrium

• Dynamic equilibrium
• in a chemical reaction
• Rate of change is exactly the same in both
  directions
• No work is done
• zero free-energy difference between reactants and
  products

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Shifting Equilibrium

• Increase reactant concentration
• reaction shifts to the right
• more product molecules are formed
• equilibrium is re-established

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Concentration gradient
Exergonic (process occurs spontaneously)
(b) When molecules are evenly distributed, they
have high entropy.
(a) A concentration gradient is a form of
potential energy.
Fig. 7-4, p. 156
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ATP

• Adenosine triphosphate (ATP)
• immediate energy currency of cells
• donates energy of 3rd phosphate group
• Formed by phosphorylation of adenosine
  diphosphate (ADP)
• endergonic process

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Adenine
ATP
Phosphate groups
Ribose
Adenosine triphosphate (ATP)
Hydrolysis of ATP
Fig. 7-5a, p. 158
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ADP
Inorganic phosphate (Pi)
Adenosine diphosphate (ADP)
Fig. 7-5b, p. 158
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Catabolism and Anabolism

• Catabolism
• degradation of large complex molecules into
  smaller, simpler molecules
• exergonic
• Anabolism
• synthesis of complex molecules from simpler
  molecules
• endergonic

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ATP Links Exergonic and Endergonic Reactions
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Stepped Art
Fig. 7-6, p. 159
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Coupling ATP Hydrolysis to an Endergonic Reaction

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KEY CONCEPTS

• ATP plays a central role in cell energy
  metabolism by linking exergonic and endergonic
  reactions
• ATP transfers energy by transferring a phosphate
  group

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Enzymes

• An enzyme is a biological catalyst
• increases speed of a chemical reaction without
  being consumed

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Activation Energy

• Enzymes lower activation energy (EA)
• energy used to start a reaction

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Activation energy (EA) without enzyme
Activation energy (EA) with enzyme
Free energy (G)
Energy of reactants
Change in free energy (?G)
Energy of products
Progress of reaction
Fig. 7-10, p. 162
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Over the Energy Barrier
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Animation Activation Energy
CLICKTO PLAY
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Enzyme-Substrate Complex

• Substrate binds to enzymes active site
• forming enzymesubstrate complex
• changes shapes of enzyme and substrate
• induced fit helps break and form bonds

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• Enzymes
• Biological catalysts
• Cells regulate the rate of chemical reactions
  with enzymes
• Lower activation energy (energy required to break
  existing bonds)
• Although most enzymes are proteins, some types of
  RNA molecules have catalytic activity as well

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• Substrate
• Reactants acted on by enzymes
• Active site
• Particular site on enzyme where catalysis takes
  place.
• E S ? ES ? E P
• Enzyme names often end in
• -ase

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Enzyme and Substrate
E S
ES
E P
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Enzyme structure and size

• The active site of an enzyme is usually small.
• 6-12 amino acids
• The whole enzyme is usually composed of hundreds
  of amino acids.
• The active site is the site where the specific
  substrate binds.

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Induced fit

• Many enzymes change their structure when they
  bind their substrates
• Induced fit brings reactive side chains together
  from the active site into alignment with the
  substrate.

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Enzymes

• Heating the reactants may increase their kinetic
  energy and thus lower the activation energy.
• Not efficient or specific, would speed up all
  reactions
• Could denature proteins
• Enzymes can lower required energy of activation.
• but they do not initiate reactions that could not
  eventually take place on their own.

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Optimal conditions for enzyme activity

• Work best at specific temperature and pH
  conditions

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Most human enzymes
Enzymes of heat-tolerant bacteria
Rate of reaction
Temperature (C)
(a) Generalized curves for the effect of
temperature on enzyme activity. As temperature
increases, enzyme activity increases until it
reaches an optimal temperature. Enzyme activity
abruptly falls after it exceeds the optimal
temperature because the enzyme, being a protein,
denatures.
Fig. 7-12a, p. 164
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Trypsin
Pepsin
Rate of reaction
pH
(b) Enzyme activity is very sensitive to pH.
Pepsin is a protein-digesting enzyme in the very
acidic stomach juice. Trypsin, secreted by the
pancreas into the slightly basic small intestine,
digests polypeptides.
Fig. 7-12b, p. 164
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Rates of reactions

• Reaction rates may be influenced by the
• 1. Concentration of enzyme
• 2. Concentration of substrate

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Rate of reaction
Enzyme concentration
(a) In this example, the rate of reaction is
measured at different enzyme concentrations, with
an excess of substrate present. (Temperature and
pH are constant.) The rate of the reaction is
directly proportional to the enzyme concentration.
Fig. 7-14a, p. 165
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Rate of reaction
Substrate concentration
(b) In this example, the rate of the reaction is
measured at different substrate concentrations,
and enzyme concentration, temperature, and pH are
constant. If the substrate concentration is
relatively low, the reaction rate is directly
proportional to substrate concentration. However,
higher substrate concentrations do not increase
the reaction rate, because the enzymes become
saturated with substrate.
Fig. 7-14b, p. 165
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Feedback Inhibition

• Another form of regulation of reactions by
  enzymes is called Feedback Inhibition

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Feedback Inhibition

• End product inhibits earlier reaction in
  metabolic pathway

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How can reactions be inhibited?
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Inhibition

• Reversible inhibition
• competitive (inhibitor competes with substrate
  for active site)
• noncompetitive (inhibitor binds at a different
  site)
• Irreversible inhibition
• inhibitor combines with enzyme and permanently
  inactivates it (not common in living systems)

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Competitive inhibition

• Inhibitor binds to the active site of the enzyme
• Structurally similar to substrate
• Occupies active site temporarily

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Substrate
Inhibitor
Enzyme
Inhibitor binds to active site
Substrate
(a) Competitive inhibition. The inhibitor
competes with the normal substrate for the active
site of the enzyme. A competitive inhibitor
occupies the active site only temporarily.
Fig. 7-17a, p. 167
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Non-competitive inhibitors

• Inhibitor binds at a site distinct from the
  active site
• Binding causes as conformational change in the
  active site so that substrate cannot bind (or
  cannot bind as well).
• Reversible

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Allosteric Enzymes

• Allosteric regulators
• bind to allosteric sites (noncatalytic sites)
• change enzymes activity

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Cyclic AMP
Allosteric site
Active site
Substrates
Regulator (inhibitor)
(a) Inactive form of the enzyme. The enzyme
protein kinase is inhibited by a regulatory
protein that binds reversibly to its allosteric
site. When the enzyme is in this inactive form,
the shape of the active site is modified so that
the substrate cannot combine with it.
Fig. 7-16a, p. 166
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Substrates
(b) Active form of the enzyme. Cyclic AMP
removes the allosteric inhibitor and activates
the enzyme.
Fig. 7-16b, p. 166
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(c) Enzymesubstrate complex. The substrate can
then combine with the active site.
Fig. 7-16c, p. 166
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Animation Allosteric Activation
Positive regulation of enzyme (activates enzyme)
CLICKTO PLAY
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Figure 6.19 Allosteric Regulation of Enzymes
Conformational change
Inactive form
Active form
The enzyme switches back and forth between the
two forms. They are in equilibrium.
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Figure 6.19 Allosteric Regulation of Enzymes
Conformational change
Inactive form
Active form
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Figure 6.19 Allosteric Regulation of Enzymes
Allosteric regulation
Inactive form
When the enzyme is in its inactive form, the
allosteric sites on the regulatory subunits can
accept inhibitor.
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Figure 6.19 Allosteric Regulation of Enzymes
Allosteric regulation
Inactive form
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Figure 6.19 Allosteric Regulation of Enzymes
Allosteric regulation
Active form
When the enzyme is in its active form, the active
sites on the catalytic subunits can accept
substrate.
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Figure 6.19 Allosteric Regulation of Enzymes
Cooperativity
Once a site is filled with a substrate or
inhibitor, binding at a second site of the same
type is favored.
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Figure 6.19 Allosteric Regulation of Enzymes
Cooperativity