ENZYMES

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ENZYMES
Key Knowledge -Cell Functioning General role of
enzymes in biochemical activities of cells

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Definitions

• Enzyme (E) protein catalyst
• Catalysts speed up reactions without being
  changed by the reaction
• Substrate (S) reactant(s) in the
  enzyme-catalyzed reaction
• Active site area of enzyme where substrate(s)
  bind(s)

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Enzymes

• Are involved in every biochemical reaction and
  thereby control metabolism
• Enzymes are globular proteins with a specific
  three-dimensional conformation
• Increase the rate of reaction without altering
  the enzyme

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Enzymes

• A single enzyme molecule can catalyse thousands
  or more reactions a second.
• Enzymes are unaffected by the reaction and are
  reusable.
• Cofactors
• Many enzymes need cofactors (or coenzymes) to
  work properly. These can be metal ions (such as
  Fe2, Mg2, Cu2) or organic molecules (such as
  haem, biotin, FAD, NAD or coenzyme A). Many of
  these are derived from dietary vitamins, which is
  why they are so important.

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Enzymes at work

• Each enzyme can usually only be the catalyst for
  a single reaction. For example, the enzyme
  maltase is the catalyst for changing maltose into
  glucose. Enzymes have the ending -ase. The action
  of maltase on this reaction is usually written
  as
• maltase Maltose Glucose
• If there is too much glucose the enzyme can help
  the reaction to go in the opposite direction
• maltase
• Glucose MaltoseBecause this reaction can
  go in either direction it is called a reversible
  reaction and it is shown with the arrows drawn in
  both directions
• maltase
• Maltose Glucose

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Enzymes at work

• Proteases and peptidases - A protease is any
  enzyme that can break down a long protein into
  smaller chains called peptides .
• Peptidases break peptides down into individual
  amino acids. Proteases and peptidases are often
  found in laundry detergents -- they help remove
  things like blood stains from cloth by breaking
  down the proteins.
• Amylases - Amylases break down starch chains into
  smaller sugar molecules. Your saliva contains
  amylase and so does your small intestine.
  Maltase, lactase, sucrase finish breaking the
  simple sugars down into individual glucose
  molecules.
• Lipases - Lipases break down fats.
• Cellulases - Cellulases break cellulose molecules
  down into simpler sugars. Bacteria in the guts of
  cows and termites excrete cellulases, and this is
  how cows and termites are able to eat things like
  grass and wood.

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Specificity

• Enzymes are highly specific with each enzyme
  acting on only one kind of substrate
• Induced Fit Model - there is some flexibility to
  the active site which can expand or contract to
  accommodate the substrate

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Enzyme-Substrate complex

• The substrate (S) binds to the active site of the
  enzyme (E) to form an enzyme-substrate complex
  (ES)
• The enzyme and the substrate are held together by
  weak bonds
• Then the substrate is converted into product
  while attached to the enzyme, and finally the
  product is released.
• The enzyme (E) is unchanged at the end of the
  reaction - it returns to its original shape after
  releasing P

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How do enzymes speed up reactions?

• Enzymes lower the activation energy (EAamount of
  energy that reactant molecules require to start a
  reaction) of the chemical reactions that they
  catalyze
• E-S binding causes a strain on the bonds in the
  substrate

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Factors affecting enzyme activity
Optimum temperature

• 1. TEMPERATURE
• Increases rate of reaction by increasing kinetic
  energy so that molecules collide more frequently
• Eventually at a certain temperature enzymes are
  denatured and will not function
• Denaturation - a structural change in a protein
  that results in a loss (usually permanent) of its
  biological properties

Enzyme denatures
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Factors affecting enzyme activity

• 2. pH
• Can change enzyme shape by changing the charge on
  amino acids
• Enzymes have an optimal pH at which they function
  best
• e.g. pepsin pH optimum is 2
• e.g. trypsin pH optimum is 8
• pH can also denature enzymes by changing state of
  ionization of R groups

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Factors affecting enzyme activity

• 3. SUBSTRATE CONCENTRATION
• Reaction rate increases with increasing substrate
  concentration
• There is a limit to this increase however - once
  all active sites are occupied (saturated), adding
  more substrate will not increase the reaction
  rate
• 4. ENZYME CONCENTRATION
• As the enzyme concentration increases the rate of
  the reaction increases linearly, because there
  are more enzyme molecules available to catalyse
  the reaction. At very high enzyme concentration
  the substrate concentration may become
  rate-limiting, so the rate stops increasing.
  Normally enzymes are present in cells in rather
  low concentrations.

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Factors affecting enzyme activity

• 5. Inhibitors
• molecules that bind to the substrate and change
  it so that the enzyme cannot bind to it
• e.g. cyanide binds to copper ions in cytochrome
  oxidase (final enzyme in respiration)
  irreversibly
• e.g. sarin (nerve gas) binds to
  acetylcholinesterase irreversibly

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Enzymes in biotechnology

• At present, cellulases and related enzymes are
  used in food, brewery and wine, animal feed,
  textile and laundry, pulp and paper industries,
  as well as in agriculture and for research
  purposes. Indeed, the demand for these enzymes is
  growing more rapidly than ever before, and this
  demand has become the driving force for research
  on cellulases and related enzymes.

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Brewing

• Brewing uses the living organism yeast. Enzymes
  within yeast catalyse anaerobic respiration,
  which converts glucose solution into ethanol
  (alcohol) and carbon dioxide.

This reaction is called fermentation. glucose ?
carbon dioxide ethanol
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Bread making

• Fermentation is also used in bread making. Flour,
  water, sugar and yeast are the main ingredients
  in bread. The ingredients are mixed together to
  make a dough. This dough is left in a warm place
  for an hour or two. During this time,
  fermentation takes place and carbon dioxide is
  produced. It is the carbon dioxide that helps to
  make the bread rise when the dough is cooked in
  an oven.

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Biological washing powders

• One common use in the home is in biological
  washing powders. The enzymes are supposed to
  digest protein stains from the clothes. Since
  enzymes work at a low temperature this saves
  electricity and makes them good for delicate
  fabrics.

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Wider uses of enzymes

• Enzymes are used in many industrial processes.
  Some of the enzymes and the products are
• Amylases- used in textile and paper production
• Ficin- used in photographic processes
• Pepsin - used in the pharmaceutical industry
• Bacterial proteases- used in making leather,
  textiles and in laundry
• Catalase- used in rubber production

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Enzyme deficiency

• Most genetic disorders are due to a deficiency in
  enzyme function
• PKUPhenylketnonuria is an inborn error of
  metabolism in which a missing enzyme causes the
  amino acid phenylalanine to build up, with
  devastating effects on the nervous system unless
  the individual follows a restrictive diet.

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Enzyme deficiency

• Lactose intolerance - The inability to digest
  lactose (the sugar in milk) is caused by a
  missing lactase gene. Without this gene, no
  lactase is produced by intestinal cells.
• Albinism - In albinos, the gene for the enzyme
  tyrosinase is missing. This enzyme is necessary
  for the production of melanin, the pigment that
  leads to suntans, hair color and eye color.
  Without tyrosinase, there is no melanin.
• Cystic fibrosis - In cystic fibrosis, the gene
  that manufactures the protein called cystic
  fibrosis transmembrane conductance regulator is
  damaged.

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Quick Quiz

• 1. What is an enzyme?
• 2. One characteristic of an enzyme is that it is
  unchanged at the end of a reaction. Give two more
• characteristics of an enzyme.
• 3. Draw a graph to show the effect of changing
  the temperature on the rate of an
  enzyme-catalysed reaction.
• 4. Draw a graph to show the effect of changing
  the pH on the rate of an enzyme-catalysed
  reaction.
• 5. What happens to an enzyme when it is
  denatured?
• 6. Give two conditions that will denature an
  enzyme.
• 7. List four industries that use enzymes.