Video Transcript
The graph shows the activity of the
enzyme pepsin at different pH levels. At what pH range is pepsin most
likely to be denatured? (A) Zero to two, (B) zero to six,
(C) two to five, or (D) nine to 14.
Before we answer the question, we
must first understand the term “denatured.” An enzyme is denatured when it’s
undergone a permanent structural change, which stops it from functioning. This means that the enzyme can no
longer catalyze chemical reactions. If we now review the key facts
about how enzymes work, we can then look at how changes in pH may bring about
denaturation.
Enzymes are biological catalysts,
made of protein. Each one has a specific shape based
on their amino acid sequence. The most important part of the
shape is its active site. The active site has a complementary
shape to one particular molecule, the substrate. And so each enzyme will only
catalyze one specific reaction. The substrate and active site fit
together like a lock and key, to form the enzyme–substrate complex. The enzyme helps to reduce the
energy requirement for the reaction to occur, allowing the formation of products,
which are then released. The enzyme is not changed or used
up, so it can be used again and again.
Now let’s look at what happens when
an enzyme is denatured. The structure of an enzyme is held
together by bonds and interactions between the amino acids and their
surroundings. These bonds and interactions can be
affected by changes in conditions, such as temperature and pH. Enzymes have optimum conditions
where they function at their best. If conditions such as pH change too
much from the optimum, then the bonds are affected and the structure starts to break
down. This means that the shape of the
active site will change and, as a result, the substrate molecule will no longer be
able to bind. This causes the rate of reaction to
slow. When the conditions are extreme,
the structure is damaged irreversibly, the enzyme is said to be denatured, and the
reaction will stop.
Having reviewed the key facts about
enzyme function and how they can be denatured, let’s return to our question. When we look at the graph, we can
see that the rate of reaction for pepsin is highest at pH two. This must be around its optimum
pH. From pH nine to 14, the rate of
reaction is zero. This must be when the substrate can
no longer bind to the active site. We can therefore make the
conclusion that the correct answer is (D). Pepsin is most likely to be
denatured between pH nine and 14.
