Video Transcript
Pepsin is an enzyme that works best
at a pH of 1.5. Which graph shows how the activity
of pepsin varies with pH?
To answer this question, let’s
first review how enzymes work. An enzyme is a biological catalyst
that speeds up the rate of reactions without being used up.
So, how do enzymes work? Enzymes are globular proteins, with
a specific region called an active site, shown here in pink. To catalyze a reaction, the
substrate binds to the active site on the enzyme forming an enzyme–substrate
complex. The reaction can then take place,
following which the product, or products, are released. The enzyme remains unchanged. So it can then bind with another
substrate molecule and catalyze further reactions.
You may have noticed that in our
example, the substrate molecule fits perfectly into the active site. Enzymes are highly specific and
will only catalyze reactions when their particular substrate molecule binds to their
active site. We can say that the enzyme has a
complementary shape to a specific substrate molecule. Though enzymes are reusable,
they’re not indestructible. Enzymes all have an optimum
temperature and optimum pH, at which they will catalyze a reaction fastest.
If the conditions stray too far
from the optimum, the rate of reaction will decrease. This is because if the temperature
rises too high, or the pH shifts too far from the optimum, the enzyme will start to
denature. This means the enzyme’s active site
changes shape so that it no longer has a complementary shape to its specific
substrate molecule. The substrate can no longer bind to
the active site. So the enzyme can no longer
catalyze the reaction. Different enzymes have different
optimum conditions.
In our question, we are told that
pepsin is an enzyme that works best at a pH of 1.5. Pepsin is found in the stomach,
where it digests proteins in our food. Cells within the stomach lining
secrete hydrochloric acid, lowering the pH of the stomach, providing ideal
conditions for pepsin.
Let’s have a look at our graphs and
decide which shows how the activity of pepsin varies with pH.
Since pepsin works best at a pH of
1.5, this is the pH where rate of reaction will be highest. We are looking for a graph that
reflects this, with the highest rate of reaction and therefore the highest point on
the curve at pH 1.5. We can eliminate graph (A) as the
highest rate of reaction is at pH seven. In graph (B), the highest rate of
reaction is at an even higher pH, around pH 11. So we can eliminate this graph
too. Graph (D) shows the highest rate of
reaction around pH 12. Graph (C) shows the highest rate of
reaction at a low pH, around 1.5. So the graph that shows how the
activity of pepsin varies with pH is (C).
