Energy and Enzymes

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Energy and Enzymes

• Thursday, July 17

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Energy

• Capacity to do work
• Potential energy (concentration gradient across a
  membrane)
• Kinetic energy (movement of substance across
  membrane)
• Potential energy to kinetic energy respiration
• Types of Energy
• Mechanical height, pressure, tension
• Electrical voltage - charge
• Osmotic solute concentration
• A system may (open system) or may not (closed
  system) exchange energy with its surroundings

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Thermodynamics study of energy transformations

• First Law of Thermodynamics Conservation of
  energy
• Energy cannot be created or destroyed
• Total amount of energy in the universe remains
  constant
• ?E (change in energy) Q (heat) W (work)
• Energy can be converted into different forms
• Second Law of Thermodynamics
• Entropy (S) (disorder) in the universe increases
  with energy transfers
• Organisms create order (decrease entropy), but
  release waste product (heat, water, CO2) that
  increase the entropy of the universe

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Free Energy

• Portion of a systems energy that can perform
  work when temperature is uniform throughout the
  system
• ?G ?H - T ?S
• ?G (free energy)
• ?H (total energy content, enthalpy)
• ?S (entropy)
• ?G lt 0 spontaneous reaction
• Thermodynamically favorable, no energy required
• ?G 0 system at equilibrium

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Chemical Reactions

• Exergonic Reaction - ?G
• Net release of free energy
• Thermodynamically favorable
• Spontaneous
• Endergonic Reactions ?G
• Net absorption of free energy
• Thermodynamically unfavorable
• Nonspontaneous

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Figure 6.6  Energy changes in exergonic and
endergonic reactions
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Metabolic Disequilibrium

• Cellular metabolism is not at equilibrium
  because the cell is an open system
• Constant flow of materials into and out of the
  cell
• Metabolism exists at steady state
• Respiration is a series of reactions that occur
  near equilibrium because as products are formed
  in one reaction they become reactants in the next
• The overall sequence of reaction is driven in one
  direction by the constant intake of new
  substrates (glucose and oxygen) and the removal
  of terminal products (CO2 and water)

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Energy Coupling

• Use of an exergonic process to drive an
  endergonic process
• ATP hydrolysis (exergonic) is used to drive most
  processes in the cell
• Mechanical, Transport, and Chemical
• ATP ? ADP Pi
• ?G lt 0
• Concentration of the products are lower than the
  reactants in a cell
• Negative charges of the 3 phosphates repel

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Figure 6.8 The structure and hydrolysis of ATP
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Figure 6.9  Energy coupling by phosphate transfer
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Regeneration of ATP
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Enzymes

• Catalysts for virtually all chemical reactions in
  the cell (which would otherwise be imperceptibly
  slow)
• Accelerate the rate at which a favorable reaction
  proceeds
• Not altered irreversibly or consumed during
  reaction
• Present in small amounts
• No effect on the thermodynamics of a reaction
• Many enzymes require cofactors (nonprotein)
• Inorganic metals (Mg, Ca)
• Organic components (coenzymes)
• Enzyme work at an optimal temperature and pH
  which is usually around physiological conditions

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Figure 6.12 Energy profile of an exergonic
reaction
Exergonic reaction
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Figure 6.13 Enzymes lower the barrier of
activation energy
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Substrate Specificity

• The reactant of an enzyme is termed its substrate
• An enzyme is highly specific for its substrate
• Enzyme usually binds its substrate noncovalently
  forming an enzyme-substrate complex (ES)
• Active site of the enzyme binds the complementary
  shaped substrate (induced fit) and contains the
  catalytic activity

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Figure 6.14 The induced fit between an enzyme
and its substrate
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Figure 6.15 The catalytic cycle of an enzyme
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Mechanisms of Enzyme Catalysis

• Substrate proximity and orientation
• Changing substrate reactivity
• charge alteration
• bond formation
• Inducing strain in the substrate
• induced fit

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Enzyme Inhibitors

• Inhibitors can regulate enzymes by decreasing
  their activity
• Irreversible Inhibitors bind covalently to
  enzyme to inactivate
• Reversible Inhibitors bind weakly
• Competitive Inhibitors compete with substrate
  for the enzyme active site
• Can be overcome by increasing substrate
• Noncompetitive Inhibitors acts at a site other
  than the active site (allosteric site)

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Figure 6.17 Inhibition of enzyme activity
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Figure 6.18 Allosteric regulation of enzyme
activity
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Figure 6.19 Feedback inhibition turning off a
metabolic pathway by its end product
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Figure 6.20 Cooperativity
Cooperativity amplifies the response of enzymes
to substrates one substrate molecule primes the
enzyme to accept additional substrate molecules
Example Hemoglobin binding oxygen