Lesson 4
 Describe
and explain the effect of pH on
enzyme activity
 Think,

pair, share
Effect of pH on enzymes
pH
The acidity of a solution is measured by the concentration of
hydrogen ions (H+) and is expressed in terms of pH
The pH scale ranges from 0 to 14
Pure water has a pH of 7.0, which is the pH
of a neutral solution with equal numbers of H+ and OH- ions
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
INCREASING ACIDITY
INCREASING ALKALINITY
If an acid is added NEUTRAL If a base is added to
to pure water, the
pure water, the hydrogen
hydrogen
ion concentration
ion concentration
decreases and the
increases, causing
hydroxyl ion (OH-)
the solution
concentration increases.
to become acidic,
The solution becomes
which is measured
more basic (alkaline) and
as a lower pH
is measured as a higher
pH
Enzymes and pH
Each enzyme has its own
optimum pH
Each specific enzyme can only work where the rate of reaction is
over a particular range of pH
maximum
The effects of pH on the rate of enzyme controlled reactions display
characteristically bell shaped curves
B
A
C
Enzyme A = amylase
optimum pH = 7.2
Enzyme B = pepsin
optimum pH = 2.0
Enzyme C = lipase
optimum pH = 9.0
Changes in pH can affect the ionic and hydrogen
bonds responsible for the specific tertiary shape of enzymes.
Extremes of pH break these bonds and denature the enzyme
 Investigating
the effect of pH on enzyme
activity.
 Instructions/sheet
in folder.
Hydrogen ions (provided by acids) are attracted
towards negatively charged ions, molecules or parts
of molecules.
 Hydrogen bonds and ionic forces old the shape of the
active site.
 Excess hydrogen ions interfere with these bonds,
changing the shape of active site.

What will this do to the rate of reaction and why?

Enzymes only work within a narrow pH range.

Small changed in pH either side of the optimum will only slow
the rate of reaction.
This is because the shape of the active site is disrupted, but
not completely altered.



Extreme changes in pH cause the active site’s shape to change
permanently.
The enzyme is denatured and can no longer catalyse the
reaction.
1.
Explain why enzymes only work within narrow ranges of pH.
Changes in pH result in excess H+ or OH- ions. These disrupt the
bonds in the enzymes structure. This changes the shape of the active
site. This means that an enzyme-substrate complex cannot form.
2.
The proteases pepsin and trypsin are both produced by cells
in an inactive form. The acid in the stomach changes the
enzymes into their active form. Suggest why these enzymes
are first secreted in their inactive form.
If the enzymes are active as soon as they are produced, they will
digest proteins within the cells themselves.

Have a go at the exam question

In folder.

A buffer is something that resists changes in pH.

In laboratory experiments, buffer solutions maintain the
desired pH for investigating enzyme action at a particular pH.
They can also help to keep the pH at a constant level as you
investigate another factor.


There are chemicals in your blood that resist changes to pH by
donating or accepting hydrogen ions.
Explain how this stops the pH from changing.
 Low
pH = lots of H+ ions
 H+ ions have a positive charge
 Either extreme of H+ ion concentration can
interfere with the hydrogen and ionic bonds
holding the tertiary structure together.
 If the pH change affects the charge on the
amino acids at the active site, then the
properties of the active site change and the
substrate can no longer bind
 At high pH values, the –COOH group will
dissociate to become a charged –COO- group
 Discuss
5 things you have learnt this lesson
with the person sitting next to you.
 Describe
and explain the effect of pH on
enzyme activity