CHAPTER 5
ENZYMES
Catalysts are the substances that alter the rate of chemical reaction and are not changed
by the reaction.
Enzymes are proteins that function as biological catalysts.
The substances on which the enzymes act are called as Substrate.
The part of the enzymes where the metabolic activities takes place is called as Active site.
Properties of enzymes:
 All enzymes are proteins: All enzymes have 3-D shape containing an active site.
 Enzymes are catalysts: They are unchanged by the reaction they catalyst and can be
used again and again.
 Enzymes are specific: Each enzyme can only convert one kind of substrate molecule
into product. It is because of specific shape of active site.
For example the enzyme amylase acts only on starch but not on protein.
Enzyme
Pepsin Protease
Amylase
Lipase
Catalase
Substrate
Proteins
Carbohydrates
Fats
hydrogen peroxide
Products
Amino acid
Glucose
fatty acid and glycerol
hydrogen and oxygen
 Enzymes work best at a particular pH: Changes in pH may not only affect the shape of
an enzyme but it may also change the shape or charge properties of the substrate so
that either the substrate cannot bind to the active site or it cannot undergo catalysis.
In general enzyme has a pH optimum. However the optimum is not the same for each
enzyme.
For example, the enzyme pepsin (a protease enzyme) is most active at an acidic pH[pH
2], whereas the enzyme trypsin (another protease enzyme) performs best at a slightly
alkaline pH[pH7.5].
 Enzymes are affected by temperature: Enzymes are made of proteins and proteins are
affected by temperature. Rise in temperature increases the kinetic energy of the
substrate molecule, which increases the collision of substrate and the enzyme and thus
increases the rate of reaction.
At low temperature the enzymes are inactive and at high temperature, enzymes are
denatured.
The temperature at which the enzymes can work effectively is called optimum
temperature. Optimum temperature of enzymes catalysing human metabolic reaction is
370C -40 0C.
Factors affecting Enzymatic action:
1. Substrate
2. Temperature
3. pH
Lock and key hypothesis of enzymes
The specific action of an enzyme with a single substrate can be explained using a Lock
and Key analogy first postulated in 1894 by Emil Fischer.
Enzymes break down a large molecule (the substrate molecule) into smaller molecules
(the product). The lock and key hypothesis suggests how this may be achieved.
Each enzyme is a molecule with a specific shape. One part of its surface is the active site
(the lock) a section where its substrate molecule (the key) fits exactly. The substrate
molecule is then split into products.
Role of enzymes in plants and animals:
Enzymes play an important role in digestion of both plants and animals. In green
plants, enzymes are required in the process of photosynthesis and to break food
substance to release energy for respiration and growth.
In animals, enzymes are used during every stage of digestion, from the time the food
enters the mouth till it is broken down into substances that can be absorbed by the body.
Enzymes are also needed to convert ADP to ATP and in the conversion of amino acids to
protein.
Role of enzymes in germinating seeds
Seeds contain stored food in the cotyledons to provide energy and materials for the
growth. This is in the form of insoluble molecule called starch. The enzyme amylase,
break this starch and convert it into soluble molecule called glucose which can be used by
the seeds.
Enzymes in baking, brewing cheese making and in making baby foods
Baking:
In baking, the enzymes in the yeast convert sugar to ethanol and carbon dioxide.
The carbon dioxide makes the bread dough to rise
Brewing:
In brewing, the enzymes in the yeast convert sugar to ethanol and carbon dioxide.
Ethanol makes the drink alcoholic and the carbon dioxide gives the drink its fizz.
Cheese making:
In cheese making, the enzyme rennin is used to clot milk.
Baby foods:
In making baby foods, the enzyme trypsin is used to pre digest baby food.