Download Enzymes and Temperature

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 where the rate of
Each specific enzyme can only work reaction is maximum
over a particular range of pH
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
Hydrogen ions (provided by acids) are attracted
towards negatively charged ions, molecules or parts
of molecules.
 Hydrogen bonds and ionic forces mold 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.
2.
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.
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.
1.
(a)
(i)Energy put in to get reaction started (Look for idea of getting started); 1
(ii)
Curve showing energy levels at start and finish the same;
and lowered activation energy;
2
(b)
Benedict’s / Fehling’s reagent and heat;
orange / red / brown / yellow / green;
2
(c)(i) Acid hydrolyses starch / breaks glycosidic bond;
1
(ii)
Not specific / forms by–products / alters pH / corrosive;
1
(d)(i)
Molecules would have less (kinetic) energy;
move slower;
fewer collisions / fewer E–S complexes form;
max 2
(ii)
Change in pH alters charge / shape;
distorts active site / tertiary structure of enzyme / denatures enzyme;
substrate will no longer fit active site;
3

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