Introduction to Metabolism

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Introduction to Metabolism

• Nancy G. Morris
• Volunteer State Community College

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I. Metabolism is the sum of all chemical
processes within an organism.

• CATABOLIC PATHWAYS
• release energy by breaking down complex molecules
  to simpler molecules
• ANABOLIC PATHWAYS
• consume energy to build complicated molecules
  from simpler ones

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Metabolic reactions are often coupled, so that...

• ...energy released from a catabolic reaction can
  be used to drive an anabolic reaction.

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II. Energy Transformation...

• Energy is the capacity to do work.
• Kinetic Energy is the energy of motion.
• Potential Energy is stored energy that matter has
  because of location or structure.
• Chemical Energy is potential energy stored in
  molecular structure.

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Bioenergetics is the study of how organisms
manage their energy resources.

• Organisms transform energy.
• These energy transformations of life are subject
  to the laws of thermodynamics.

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III. Laws of Thermodynamics

• First Law of Thermodynamics
• Energy can be transferred and transformed, but it
  cannot be created or destroyed.
• or the energy of the universe is constant.
• Second Law of Thermodynamics
• Every energy transfer or transformation makes the
  universe more disordered.
• or every process increases the entropy of the
  universe.

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• The entropy of a system may decrease, but
  the entropy of the system plus its surroundings
  must always increase.
• Living things are open systems. That means that
  energy is transferred between the system and the
  surroundings.

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For example,

• Animals return to the surroundings simpler low
  energy molecules (CO2 and water) and heat.
• Animals take in complex high energy food
  molecules and extract chemical energy to create
  and maintain order.

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IV. What about Free Energy ?

• Free Energy is the part of a systems energy that
  can perform work (when temperature is uniform
  throughout the system).
• Its called free energy because it is available
  to do work, not because it can be spent without
  cost to the universe.

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Unstable systems ...

• have more free energy.
• will move toward a more stable state.
• have greater capacity to do work.

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Free Energy and Metabolism

• Exergonic reactions (energy outward) proceed
  with a net release of free energy. They occur
  spontaneously.
• Endergonic reactions (energy inward) absorb free
  energy from the surroundings. They are
  nonspontaneous.

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Free energy can be defined mathematically
G H - TS

• G is the systems quantity of free energy. H
  is the systems total energy. S is the systems
  entropy (disorder). T is absolute temperature
  in Kelvin units.

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Think of it this way...

• G is a measure of a systems instability --
• its tendency to change to a more stable state.
• Systems that change spontaneously to a more
  stable state have --
• high energy, and/or
• low entropy.

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If a process is spontaneous, then the system must
either

• give up energy
• (a decrease in H)
• give up order
• (a decrease in S)
• or both.
• ( a decrease in both H and S)

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Think about the following spontaneous
processes

• water flows downhill
• objects of opposite charge attract
• ice cubes melt at room temperature

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So...

• A process that cannot occur on its own is
  nonspontaneous
• it will happen only if an external energy source
  is added.

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Metabolic disequilibrium is necessary a cell at
equilibrium is dead.

• For example, during cellular respiration a steady
  supply of high energy reactants (glucose) coupled
  with the removal of low energy products (CO2 and
  H2O), maintain the disequilibrium necessary for
  respiration to proceed.

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• EXERGONIC RXNS
• Products have less free energy than reactants
• Reaction is energetically downhill
• Spontaneous reaction
• ?G is negative
• ? G is the maximum amount of work the reaction
  can perform

• ENDERGONIC RXNS
• Products store more free energy than reactants
• Reaction is energetically uphill
• Nonspontaneous reaction
• ? G is positive
• ? G is the minimum amount of work required to
  drive the reaction

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V. ATP powers cellular work

• by coupling exergonic reactions with endergonic
  reactions.

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• Mechanical work --
• such as beating of cilia, muscle contraction,
  cytoplasmic flow, and chromosome movement
• Transport work --
• such as pumping substances across membranes
• Chemical work --
• such as the endergonic process of polymerization

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The ATP Cycle. Energy released by catabolism is
used to phosphorylate ADP regenerating ATP. ATP
couples the cells energy yielding processes to
the energy-consuming ones.
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VI. Enzymes in review

• Enzymes are biological catalysts that
• end in -ase.
• affect the rate of a reaction but remain
  unchanged by it.
• are complementary to the substrate on which they
  act.

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The catalytic cycle of an enzyme
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Enzymes have 2 important sites

• The first is the active site. The specific enzyme
  is complementary to the substrate on which it
  acts. The place where this match occurs is the
  enzymes active site.
• An enzyme may also have an allosteric site. An
  allosteric effector exactly fits this site
  thereby altering the shape of the active site by
  either activating or deactivating the enzyme.

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Enzymatic Control

• Feedback Inhibition - one of the products acts as
  an allosteric effector, inactivating the enzyme
  when amounts of it are high.
• Competitive Inhibition - occurs when the active
  site is occupied by some other substance. The
  regulatory compound and the substance compete for
  the active site of the compound. Ex sulfa drugs
• Non-competitive Inhibition - chemicals combine
  with enzyme prevent its function. Ex cyanides,
  mercury, nerve gas, lead poisoning

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Feedback Inhibition - one of the products acts as
an allosteric effector, inactivating the enzyme
when amounts of it are high.
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Competitive Inhibition - occurs when the active
site is occupied by some other substance. Both
the regulatory compound and the substance compete
for the active site of the compound. Ex sulfa
drugs
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Non-competitive Inhibition - chemicals combine
with enzyme prevent its function. Ex mercury
nerve gas, lead poisoning
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Factors affecting Enzyme Activity

• 1. Amount of enzyme present
• 2. Amount of substrate present
• 3. Temperature
• 4. pH
• 5. Chemicals

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Affect of temperature and pH on enzyme activity.

• Each enzyme has an optimal (a) temperature
• and (b) pH that favor the active conformation of
  the protein molecule.

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Enzyme Nomenclature

• Least specific
• dehydrogenase
• hydrolase
• dehydrolase
• Moderately specific
• carbohydrase
• lypase
• Very specific
• sucrase
• carbonic anhydrase

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• End of EXAM I material.