An Introduction to Metabolism

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• An Introduction to Metabolism

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Metabolism/Bioenergetics

• Metabolism The totality of an organisms
  chemical processes managing the material and
  energy resources of the cell
• Catabolic pathways degradative process such as
  cellular respiration releases energy
• Exergonic
• Anabolic pathways building process such as
  protein synthesis photosynthesis consumes
  energy
• Endergonic

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Thermodynamics

• Energy (E) capacity to do work
• Kinetic energy energy of motion
• Potential energy stored energy
• Thermodynamics study of E transformations
• 1st Law conservation of energy E
  transferred/transformed, not created/destroyed
• 2nd Law transformations increase entropy
  (disorder, randomness)
  
• Combo quantity of E is constant, quality is not

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Free energy (G)

• Free energy portion of systems E that can
  perform work (at a constant T)
• Exergonic reaction net release of free E to
  surroundings (?Glt0) - spontaneous
• Endergonic reaction absorbs free E from
  surroundings (?Ggt0) NOT spontaneous

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Gibbs Free Energy Formula

• ?G ?H - T ?S
• ?G Gibbs Free Energy, the amount of free
  energy available to a system
• Unit kJ (kiloJoules)
• ?H heat of reaction (or Enthalpy), the amount
  of heat energy in a system
• Unit kJ (kiloJoules)
• T temperature
• Unit (Kelvin ? C 273)
• ?S entropy, the amount of order/disorder in a
  system
• Unit J/K (Joules per Kelvin)

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What it means

• ?G positive vs. negative
• ?G reaction is endergonic (energy must be
  supplied to make the reaction occur)
• - ?G reaction is exergonic (generally, this
  reaction is spontaneous and releases energy)
• ?H positive vs. negative
• ?H reaction is endothermic (heat energy is
  absorbed from the surrounding area)
• - ?H reaction is exothermic (heat energy is
  released into the surrounding area)

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What it means

• ?S positive vs. negative
• ?S the entropy of the system in increasing
  (you have more free moving molecules, things are
  breaking apart, catabolism)
• - ?S the entropy of the system is decreasing
  (you have more structured arrangement of
  molecules, tends to be fewer, more complex,
  molecules, anabolism)
• EXAMPLE HC2H3O2 has less entropy than 2 H
  (more number of molecules as opposed to just more
  atoms in molecule)

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EXAMPLE

• A biological reaction has an exothermic reaction
  (?H) that produces -55.8kJ of energy. If the
  reaction is decreasing entropy (?S) by -0.35kJ/K
  at a temperature of 25C, what would be the free
  energy (?G)? Is this reaction exergonic
  (releasing energy) or endergonic (absorbing
  energy)?

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SOLUTION

• ?G ?
• ?H -55.8kJ
• ?S -0.35kJ/K
• T 25C
• 298K

• ?G ?H T?S
• ?G -55.8kJ (298K)(-0.35kJ/K)
• ?G -55.8kJ 104.3kJ
• ?G 48.5kJ
• NOTE The reaction is endergonic

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QUESTIONS

• How do you think the following affects Free
  Energy? Does it make it more likely to happen or
  less likely?
• Increasing the temperature of a system?
• Decreasing the entropy?
• Decreasing the enthalpy (heat of reaction)?

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ENERGETICS AND REACTIONS

• All reactions need energy to start (Energy of
  activation)
• Many times this energy cost is insurmountable
• Strategies to deal with energy cost
• Couple reaction with another exergonic reaction
• Enzymes to reduce energy of activation

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Energy Coupling ATP

• E coupling use of exergonic process to drive an
  endergonic one (ATP)
• Adenosine triphosphate
• ATP tail high negative charge
• ATP hydrolysis release of free E
• Phosphorylation (phosphorylated intermediate)
  enzymes
• KINASES

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Enzymes LOCK AND KEY MODEL

• Catalytic proteins change the rate of reactions
  w/o being consumed
• Free E of activation (activation E) the E
  required to break bonds
• Substrate enzyme reactant
• Active site pocket or groove on enzyme that
  binds to substrate
• Induced fit model

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Effects on Enzyme Activity

• Temperature
• pH
• Cofactors
• inorganic, nonprotein helpers ex. zinc, iron,
  copper
• Coenzymes
• organic helpers ex. vitamins

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Rate of enzyme activity

• There are many actions that can speed up the rate
  of enzyme activity (already discussed)
• Optimal pH, increased temp, increased amount of
  enzyme, increased amount of substrate
• Regardless of optimal concentrations, thing
  always limits the enzyme rates
• Amount of enzyme
• Once you have maxed out the use of enzymes, the
  rate of reaction is constant

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Enzyme Inhibitors

• Irreversible (covalent) reversible (weak bonds)
• Competitive competes for active site
  (reversible) mimics the substrate
• Noncompetitive bind to another part of enzyme
  (allosteric site) altering its conformation
  (shape) poisons, antibiotics