FACT
The optimal pH and
temperature for an
enzyme will be
determined by the
kind of living thing
it is found in. The
enzymes in the
human body have
an optimum
temperature of 37
◦C. Bacteria that
live in compost
heaps have
enzymes with an
optimal range in the
40’s, and bacteria
called
hyperthermophiles
(lovers of very high
temperatures) that
live in hot springs
have enzymes with
optimum
temperatures above
80 ◦C.Lactose is pre-digested by adding lactase to the milk.Factors affecting enzyme action
- Temperature
In humans, enzymes function best at 37◦C (Figure 1.19). This is the optimum temperature.
At very high temperatures proteinsdenature; this means that the hydrogen, hydrophobic and
electrostatic interactions that result in the protein’s three-dimensional shape break down, un-
ravelling the protein into its primary structure, a long chain of amino acids. When an enzyme
is denatured, the shape of its active site, as well as the rest of the protein shape is altered.
The substrate can no longer fit in the active site of the enzyme and chemical reactions cannot
take place. Low temperatures can slow down or even inactivate enzymes, as low tempera-
ture means less available kinetic energy, so that even the lower energy of activation that the
enzyme allows is not available. The first graph shows the effect of temperature on enzyme
activity.
- pH
Enzyme activity is sensitive to pH. Enzymes have an optimum pH as shown on the graph,
but they can function effectively within a pH range. The effectiveness of the enzyme falls
sharply when the pH is outside its optimum range. An enzyme can become denatured
when exposed to a pH outside its pH range, as pH affects the charge on some amino acids,
and therefore affects the electrostatic interactions holding the tertiary structure together. The
second graph shows the effect of pH on enzyme activity.
Figure 2.21: Graphs showing the effect of temperature and pH on enzyme activity respectively.In the investigation that follows, the effect of temperature on catalase enzyme activity will be
investigated. Hydrogen peroxide is potentially toxic and so living tissues contain an enzyme
named catalase to break it down into non-toxic compounds, namely water and oxygen. You
Chapter 2. The chemistry of life 45