1. B3. Enzymes Pg

2. What are enzymes?
enzymes are proteins that function as biological catalysts
a catalysts is a substance that usually speeds up the rate of a chemical reaction and is not changed by the reaction
thus enzymes speed up the rate of metabolic reactions in the cells

3. Enzymes work
enzymes lower the minimum amount of energy required for chemical reaction to occur (activation energy)
this allows biochemical reactions to take place at a faster rate in the cells of living organisms at body temperature which is usually low
enzymes catalyse metabolic reactions (i.e. chemical reactions that sustain life) in a cells
metabolic reactions are either breakdown reactions - catabolism or build up reactions - anabolism

4. Naming of Enzymes
Enzyme are named according to the reaction they catalyse
A suffix – ase is usually added to the name of the substrate of the reaction catalysed by the enzyme
Sometimes they are given special names such as catalase & amylase
Catalase is the fastest acting enzyme

5. How Enzymes Work

6. “Lock and Key” Model
Each enzyme has a shape that allows it to catalyse one reaction
Reactant(s) in an enzyme – catalysed reaction is/are called substrate(s)
Enzyme’s molecule has a specific shape which is complementary to that of a substrate –this is called active site
The substrate fits into the active site of the enzyme the same way a ‘key fits into a lock’ - this is called “ lock and key” model
The substrate (the ‘key’) fits exactly into the active site (the ‘key hole’) of the enzyme (the ‘lock’)
An enzyme-substrate complex is formed.
The substrate undergoes a chemical change – a new substance, the product, is formed
The product is released from the active site.
The free unaltered active site is ready to receive fresh substrate.

8. General Properties of Enzymes
enzymes are all Proteins
enzymes are Specific in reaction they catalyse
enzymes are not changed by the reaction they catalyse i.e. they can be used again and again
enzymes are influenced by change in temperature
enzymes are influenced by change in pH
enzymes work best at particular temperature (37 oC in humans)and pH (pH 2 for pepsin & pH 7 for most enzymes)

9. Factors Affecting Enzyme Activity
Enzyme activity is determined by measuring the rate of reaction that the enzyme catalyses
This is usually measured by measuring the products formed per unit time
Enzyme activity is affected by;
temperature,
pH &
substrate concentration

10. Effect of Temperature on Enzymes
at low temperature (e.g. 0°C) enzyme activity is low because the movement of molecules is slow due to low kinetic energy
the collision frequency between enzyme and substrate is therefore low
increasing the temperature, increases kinetic energy of molecules thus speeding up their movement, collision frequency between the substrates and the enzymes increases therefore enzyme activity increases
maximum enzyme activity is at 40°C – due to maximum collision frequency between enzymes and substrates molecules – this is called optimum temperature (i.e. the temperature at which the enzyme works fastest)
enzyme action decreases above 40°C because the enzymes are denatured by excess temperature (i.e. the shape of active site is altered & substrates molecules can no longer fit into it)
when all the enzymes are denatured, at 60°C, enzyme activity stops

11. Effect of pH on Enzymes
enzyme activity is greatest within a narrow range of pH, since all the enzymes are functioning normally because their active sites have the correct shape
this is called the optimum pH
change in pH from the optimum, alters the shape of active site thus affecting the ability of the substrates to bind to the active site & so enzyme activity decreases
a major change in pH from the optimum denatures the enzymes so enzyme action stops
What is the optimum pH for;
enzyme X?
enzyme Y?

12. Revision activity
After reading pages of your biology textbook and your lesson notes, attempt summary Questions 1, 4, 5, 7, & 6 on page 42.