Enzymes: The Catalyst of Life

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Enzymes The Catalyst of Life

• The World of the Cell Chapter 6

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Energy of Activation

• EA minimum amount of energy reactants must have
  before collisions between them will occur
• Transition State intermediate chemical stage
• DG difference of free energy between reactants
  and products
• Metastable State stable molecules that could
  react

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Energy of Activation

• Two ways of overcoming a metastable state
• Increase the proportion of molecules with
  sufficient energy (input of heat)
• Lowering EA (catalyst)

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Enzymes as Biological Catalyst

• Catalyst
• Increase rate of reaction
• Forms transient reversible complexes
• Doesnt change DG
• Organic/Inorganic molecules called enzymes
  (exception ribozymes)

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Active Site of Enzymes

• Grove or pocket within enzymes
• Place for substrates to bind
• Very specific for only one substrate

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Active Site of Enzymes

• Not all amino acids of enzymes are involved
• Most common enzymes are cystiene, histidine,
  serine, aspartate, glutamate and lysine
• Prosthetic groups or coenzymes are located within
  enzymes
• Catalase has porphyrin ring for iron
• Iron, copper, molybdenum, lithium

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Enzyme Specificity

• Inorganic Enzymes
• Not as specific as organic
• Example Platinum or Nickel
• Organic Enzymes
• Extremely specific
• Example Succinate Dehydrogenase
• Group Specificity
• Degradation of enzymes at C-terminus

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Enzymes are Named Based on Function

• Oxidoreductase redox reaction
• Transferase transfer of functional groups
• Hydrolase cleavage of one molecule into 2
  molecules
• Ligase removal or addition of a group
• Isomerase movement of a group
• Ligase joining of two molecules

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Enzyme Sensitivity

• Enzyme-catalyzed reactions increases with
  temperature causing more frequent collisions then
  declines at a certain temperature that will
  causes denaturation
• pH sensitive within 3-4 pH units. Excess
  hydrogen ions interfere with certain amino acids
  and causes them to lose their charged (ionized)
  form. Extreme pH changes will disrupt ionic and
  hydrogen bonds of maintaining the 3o shape.

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Substrate Binding

• Involves forming hydrogen bonds and ionic bonds
  with charged amino acids
• Weak bonds that are easily broken
• Lock and Key Model (Emil Fisher)
• Induced Fit Model (Daniel Koshland)

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Substrate Activation

• Substrate binding to active site causes
  conformational change that causes the bonds of
  substrate to weaken
• Proton transfer enzyme accepts or donates
  protons to substrate to increase chemical
  reactivity
• Electron transfer enzyme accept or donates
  electrons to make temporary bonds

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Catalytic Cycle of an Enzyme

• Random collision of enzyme and substrate
• Amino acids are important for binding
• Enzyme/substrate binds tightly
• lowers free energy of transition state
• Products released and temporary bonds are broken
• Active site restored

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Enzyme Inhibition

• Enzymes are affected by products, alternative
  substrates, substrates analogs, drugs, toxins and
  allosteric effectors
• Substrate analogs may be useful tools in fighting
  diseases
• HIV/AZT
• Toxins may covalently bind to enzyme and render
  it inactive permanently
• Penicillin inhibits bacterial cell wall
  production
• Reversible inhibitors binds non-covalently to
  active site

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