ENZYMES

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ENZYMES
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How Do Reactions Occur?

• Spontaneous reactions may occur very slowly.
• All reactions require free energy of activation
  (EA)
• Uphill portion represents the EA required to
  start the reaction.
• Downhill portion represents the loss of free
  energy by the molecules in the reaction.
• DG is the difference in free energy of products
  and reactants.

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Is this reaction exergonic or endergonic?
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How can the EA barrier be overcome?

• Temperature
• Temperatures that are too high denature organic
  molecules, so what else is there?
• ENZYMES
• Enzymes lower the EA barrier so that reactions
  can occur at lower temperatures.

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What are Enzymes?

• Catalysts change the rate of the reaction without
  being altered themselves.
• Enzymes are biological catalysts.
• Enzymes are proteins, whose three dimensional
  shape allows for their ability to react
  specifically.

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Enzyme / Substrate Relationship

• What is the substrate?
• It is the reactant upon which an enzyme reacts.
• Enzymes are substrate specific.
• Only the active site of the enzyme actually binds
  the substrate.

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The Active Site

• The active site is where the enzyme-substrate
  interaction occurs.
• Most enzyme-substrate interactions are the result
  of weak bonds.
• The active site may cause the enzyme to hold onto
  the substrate in a very specific way.
• The active site may provide a micro-environment
  (e.g. low pH) which enhances a reaction.

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Enzyme Activity

• Temperature
• pH
• Enzyme Concentration
• Substrate Concentration

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Cofactors

• Non-protein molecules that help enzymes function.
• Bind to active site to enhance enzymatic
  reactions.
• Cofactors may be inorganic metals such as zinc,
  iron, or copper.
• Coenzymes are organic cofactors (e.g. vitamins)

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Enzyme Inhibition

• Competitive inhibitors - mimic the substrate and
  compete for the active site.
• Non-competitive inhibitors bind to the enzyme
  away from the active site, and indirectly cause a
  change in the active site.

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Allosteric Regulation

• Regulatory molecules that bind to the enzymes
  allosteric site changing the shape of the enzyme.
• Allosterically regulated enzymes have a
  quaternary protein structure.
• Each subunit of the enzyme has an active site and
  an allosteric site.
• Allosteric activators stabilizes the active site
• Allosteric inhibitors deactivates the active site.

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Feedback Inhibition
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Cooperativity