Unit 3 - Enzymes
3.1 – Enzyme Action, 3.2 – Effects of temperature and pH.
SUFEATIN SURHAN  BIOLOGY 5090 2012
SYLLABUS CHECKLIST
Candidates should be able to:
a) define enzymes as proteins that function as biological catalysts;
b) explain enzyme action in terms of the ‘lock and key’ hypothesis;
c) investigate
activity.
and
describe
the
effect
of
temperature
and
pH
on
enzyme
Enzymes and us
 Enzymes play a big role in our lives. When we eat a piece of bread, enzymes in our digestive system
work to break down the complex carbohydrates found in the bread into simpler sugars for absorption
into the blood stream.
 Digestive enzymes are also used in washing powders. They help to break down and remove stains caused
by organic matter such as sweat, blood, curry, etc.
What are enzymes?
 DEFINITION: Enzymes are biological catalysts made up of proteins which speed up the chemical
reactions within an organism without themselves being chemically changed at the end of the
process.
 Catalysts are chemicals that can speed up the rate of a reaction. There are two types of catalysts i.e.
organic (biological) and inorganic catalysts.
 The substance acted upon by enzymes in an enzyme-catalysed reaction is called substrate.
substance(s) produced in the reaction are known as product(s).
The
General Characteristics of enzymes
1. Enzymes are all proteins.
2. Enzymes are biological catalysts.
3. Enzymes reactions are reversible.
4. Each enzyme works best at a particular temperature and pH.
5. Enzymes are specific – they work on one kind of substrate only.
6. Only small amounts of enzymes are needed to do their job.
7. All enzymes are water soluble.
8. Rate of enzyme activity is affected by temperature and pH.
Intracellular and Extracellular enzymes
 Intracellular enzymes are produced in living cell and work inside the same cell. They are found in the
nucleus, cytoplasm, mitochondrion and chloroplasts. For example, catalase which breaks down harmful
hydrogen peroxide in the liver cells.
 Extracellular enzymes are produced inside cells but are released from the cells to carry out their job.
For example, amylase is produced in the salivary gland but is transported to the mouth to breakdown
starch into maltose.
Enzyme Action: ‘LOCK AND KEY’ HYPOTHESIS
 Each enzyme molecule has a very precise three dimensional shape. On part of its surface is a precisely
shaped ‘dent’ known as the active site where the substrate molecule(s) will fit in exactly. The active
site has to have a complementary (mirror image) to the shape of the substrate molecule(s).
 The enzyme is the ‘lock’ and the substrate is the ‘key’ as they work in the same manner as a lock and
key i.e. key has to go towards the lock, which has a shape specific for that particular lock (=specific
shape of active site).
 In catabolic (breaking down) reactions, large molecules are broken down into simpler ones:
1. When the substrate molecule is in position in the active site, the enzyme slightly stresses or bends the
substrate, splitting it into 2 product molecules.
2. The product molecules move out of the active site leaving the active site free to combine with new substrate
molecules.
3. It if involves water to split the substrates into products, this is known as hydrolysis.
4. Examples of catabolic reactions are digestion of food and respiration.
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 In anabolic (building up) reactions, large, complex substances are formed from small simple molecules. Examples
of anabolic reactions are cell division, photosynthesis, synthesis of proteins and fats and repair of cells.
Factors affecting enzyme activity
1. Temperature
 All enzymes will have an optimum working temperature, usually close to that at which they normally
function. It is approximately 40 – 450C for most enzymes.
 In humans, the normal body temperature 370C is the optimum temperature where enzymes are most
effective and rate of enzyme reaction is maximum.
 A rise in temperature increases the rate of activity of an enzyme. In many cases, a 10OC rise, will
double the rate of reaction in a cell.
 The figure shown is a graph of enzyme activity against temperature.
1. Enzymes are inactive at very low temperature as the enzymes do not have enough kinetic energy and
become inactivated.
2. As temperature rises, so does the rate of reaction.
3. The increase in enzyme activity is due to the increase in the kinetic energy causing the enzyme and
substrate molecules to move faster.
4. This results in more successful collisions between enzyme and substrate molecules i.e. more
substrates can enter the active site of an enzyme within a unit time to form products.
5. Also, the product molecules leave the active site of the enzyme more quickly.
6. At a certain temperature, a peak rate of activity is reached and this is known as the optimum
temperature (temperature at which the enzyme works best).
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7. When the temperature exceeds the optimum temperature (more than 500C), the rate of activity
decreases rapidly to 0.
a. This is because at these temperatures, the very high heat energy causes atoms of the enzyme
molecule to change position relative to one another breaking the bonds holding them together.
b. As a result, there is a change in the shape of the active site of the enzyme.
c. The enzymes can no longer do their job since substrate molecules can no longer fit into their
active site.
d. At this stage, the enzymes are described as denatured (NOT killed as they were never alive).
e. This damage is usually irreversible (permanent).
2. pH
 Most enzyme activities are affected by the pH of the solutions in which enzymes act.
 The optimum pH is the pH at which the rate of the enzyme activity is fastest.
 If the pH level falls on either side of the optimum, the rate of enzyme activity gradually decreases also
due to denaturation of the enzyme molecules (unsuitable pH changes the shape of the enzyme molecule,
hence changes the shape of the active site).
 Damage caused by pH is reversible, provided that the normal pH is restored as soon as possible.
 For most enzymes in a mammal’s body, the optimum pH is 7 or slightly above (slightly alkaline).
 Each enzyme molecule has a very precise three dimensional shape. On part of its surface is a precisely
shaped ‘dent’ known as the active site where the substrate molecule(s) will fit in exactly. The active
site has to have a complementary (mirror image) to the shape of the substrate molecule(s).
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Uses of enzymes in daily life and industry
 Enzymes are produced abundantly from bacteria, fungi, plants and animals. They are used extensively
at home as well as in industries.
APPLICATION
Meat tenderizer
ENZYMES
Protease
USES
Tenderize meat
Protease
Laundry detergent
Lipase
Breakdown protein molecules such as blood and dirt.
Breakdown and removes oil and fatty stains.
-
Food processing:
Production of chocolates,
syrups, fruit juices and bread
Meat and fish products in the
food canning industry
Amylase
Protease
Lactase
Dairy products
Removes starch in cocoa during chocolate
production.
- Converts starch to sugar in the making of syrup
and fruit juices.
- Changes starch flour to sugar during the making of
bread.
- Tenderize meat.
- Removes the skin of fish before canning it.
Changes lactose to glucose and galactose in the
making of ice cream.
Lipase
Removes the fat part of milk to make cheese
Leather industry
Trypsin
Removes hairs from animal skin
Medical products
Trypsin
-
Textile industry
Amylase
Removes starch which is used as stiffeners in fabrics.
Treats swelling of wounds.
Pre-digests baby food.
Removes blood clots.
 A summary of enzymes involved in digestion in the human alimentary canal:
REGION OF
DIGESTION
SECRETION
SOURCE
ENZYME
ACTION
MOUTH
Saliva
Salivary
Glands
Salivary Amylase
Starch  maltose
STOMACH
Gastric
juice
Gastric
glands
Pepsin
Proteins  polypeptides
Rennin
Soluble caseinogens  insoluble casein
Amylase
Starch  maltose
Trypsin
Proteins  polypeptides
Lipase
Fats  fatty acids and glycerol
Sucrase
Sucrose  glucose + fructose
Maltase
Maltose  glucose
Erepsin
Polypeptides  amino acids
Lactase
Lactose  glucose + galactose
Lipase
Fats  fatty acids and glycerol
Pancreatic
juice
Pancreas
SMALL
INTESTINES
Intestinal
juice
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Intestinal
glands
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