Enzyme Action

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Enzyme Action

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Catalysts

• Catalysts lower the energy input (activation
  energy) needed for a chemical reaction.

Catalysed reaction
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Enzymes

• Living cells contain biological catalysts which
  speed up the reactions by lowering the energy
  input required.
• Biological catalysts are called enzymes.
• Enzymes are made of protein and are found in all
  living cells.

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Enzymes, substrates products

• All the reactions which take place in cells are
  controlled by enzymes
• But each biological reaction has its own
  specific enzyme
• The substance an enzyme acts on is called its
  substrate
• The substances produced are called products

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Enzymes, Substrates and Products

• Enzyme controlled reactions by enzymes can be
  summarised in the following way.

product(s) enzyme
substrate enzyme
enzyme-substrate complex
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Enzyme Controlled Reactions

• The substrate is changed into the product and is
  no longer present at the end of the reaction.
• The enzyme is present at the start of the
  reaction and is still present at the end of the
  reaction i.e. the enzyme has not changed.

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The Specificity of Enzymes for Their Substrates

• An enzyme is specific to its substrate.
• This means that each enzyme can only act on one
  particular substrate
• e.g. the enzyme amylase will break down starch
  but not protein.

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The Lock and Key Mechanism

• The lock and key mechanism shows how enzymes work
  and why they are specific.

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The Lock Key

• The enzyme is the key - it has a specific shape
  that only fits one type of lock
• The substrate is the lock - when it reacts with
  the enzyme it is changed
• After the reaction the enzyme remains unchanged
  and can then carry out more reactions

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Lock key mechanism- enzymes

• Within the structure of an enzyme are areas
  called active sites.
• The shape of the active sites fit only one
  substrate.
• This is what makes each enzyme specific.

Complementary shapes
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Lock key mechanism- enzymes

• The enzyme and its substrate fit together to form
  a complex in the same way as a key fits a lock
• The enzyme is complementary in shape to its
  substrate

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Lock key mechanism- enzymes

• The substrate has been changed.
• Products are formed.
• The enzyme has not been changed which means that
  it can be used again.

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Enzymes

• Enzymes catalyse two types of reactions
• Degradation (Breakdown) - large molecules are
  broken down into smaller, simpler molecules
• Synthesis - small molecules are built up into
  larger, more complex molecules

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Degradation (Breakdown) reaction
Amylase
Substrate
Product
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Breakdown of hydrogen peroxide In
living cells

• Hydrogen peroxide oxygen water

Catalase
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Synthesis reaction
Phosphorylase
Glucose-1-phosphate
Starch
substrate
product
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Enzymes

• Optimum conditions are the conditions at which an
  enzymes works best
• Rate of reactions may be affected by temperature
  and pH

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Effect of temp on enzyme activity

• At low temperatures enzyme activity is slow.
• B. As temperature increases enzyme activity
  increases as enzyme substrate molecules move
  about at a faster rate collide more.

B
A
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• Enzyme activity is at a maximum. The temperature
  at which this occurs is known as the optimum
  temperature.
• At temperatures greater than the optimum, enzyme
  structure starts to change and loses the ability
  to fit with its substrate. The enzyme has been
  denatured.

C
D
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• As the temperature increases, more and more
  enzyme molecules are denatured and the reaction
  stops.

E
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Temperature

• At higher temperatures the bonds which determine
  the shape of the enzymes active site are broken
• This permanently changes the shape of the active
  site
• The enzyme no longer fits the substrate and a
  reaction cannot occur
• The enzyme has been denatured

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pH enzyme activity

• Enzymes are active over a narrow range of pH
  values.
• Within each range there will be a pH value at
  which the enzyme is most active. This is the
  optimum pH of that enzyme.

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• This enzyme will be active over the range pH 6 to
  pH 8.
• The optimum pH of this enzyme is pH 7.

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Effect of pH on enzyme activity

• Different enzymes work within different ranges.
• Most enzymes work best in a narrow range around
  neutral pH 7.
• Some enzymes work best near the extremes of pH
  e.g. pepsin works best in a narrow range around
  pH 2. in stomach.
• pH values below or above the optimum can cause
  the bonds which determine the shape of the
  enzymes active site to be broken. This causes the
  enzyme to change shape i.e to be denatured

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