B3. Enzymes

Pg 36 - 39

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

Enzymes workenzymes 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

Naming of EnzymesEnzyme 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

How Enzymes Work

“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.

General Properties of Enzymesenzymes are all Proteins enzymes are Specific in

reaction they catalyseenzymes 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)

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

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

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?

Revision activity After reading pages

36 -39 of your biology textbook and your lesson notes, attempt summary Questions 1, 4, 5, 7, & 6 on page 42.