Video Transcript
Which of the following factors has no influence on the reaction rate of enzymes? (A) pH, (B) enzyme concentration, (C) enzyme color, or (D) substrate
concentration.
Let’s briefly review how enzymes work and then go through the answer options to find
the one which does not influence enzyme activity.
Enzymes are biological catalysts, which means they speed up chemical reactions
without being changed or used up themselves. They may work on just one substrate, such as when salivary amylase breaks starch down
into maltose molecules. Or they may work on more than one substrate, for example, DNA ligase, an important
enzyme in DNA repair and replication. For simplicity, we will just refer to one substrate for this question. But how do enzymes work?
Enzymes are made up of one or more polypeptide chains. Each type of enzyme has its own specific shape due to the interactions between the
different amino acids along the chain and with the environment. An area called the active site is where the substrate binds. The shape of the active site is complementary to the substrate, which means they fit
together like jigsaw pieces. Or you can say the substrate fits into the active site like a key fits into a
lock. Just like one key can only open one lock, each type of enzyme can only catalyze one
reaction.
When an enzyme and a substrate collide, they form an enzyme–substrate complex. The reaction then occurs, and the products are released. Enzymes have specific conditions, such as temperature or pH, at which they work
best. These are known as the optimum conditions. If these conditions change too far from the optimum, then the bonds holding the
structure together begin to break. The shape of the active site is permanently changed, and the enzyme is said to be
denatured. This means that looking at option (A), enzymes are indeed influenced by pH.
Every enzyme has a specific pH range at which the rate of reaction is fastest. Salivary amylase is produced in our mouth to start starch digestion. The pH of the mouth is quite neutral, ideal for amylase. In a more or less acidic environment, it denatures and cannot work anymore. The same is the case for pepsin, an enzyme which breaks down proteins into small
peptides. As pepsin is found in the stomach and the stomach is highly acidic, pepsin works best
at a pH of about two and denatures when in a less acidic environment. As the question asks for a factor that does not influence enzyme reaction rate, we
can exclude answer option (A).
Once enzymes have released the products at the end of the reaction, they are ready to
bind to the next substrate to perform its action all over again. Imagine there are only a few enzyme molecules in solution; the enzyme concentration
is low. Then, the reaction rate is quite slow because all enzymes are constantly
occupied. When there are more enzyme molecules, the reaction rate increases as more enzyme
molecules are available to perform the reaction. The greater the enzyme concentration, the more enzyme molecules are available and the
faster the rate of reaction, until there are no spare substrate molecules to fit
into the active sites. So enzyme concentration does influence the rate of reaction of enzymes, and we can
therefore cross out option (B).
We can observe similar events when researching reaction rates compared to substrate
concentration. Imagine there is little substrate around. Then, the enzymes encounter their matching substrate seldomly, and the rate of
reaction is low. However, if there is a higher concentration of substrate, then there are more
substrate molecules available to collide with the enzyme to form enzyme–substrate
complexes, and therefore rate of reaction increases.
Reaction rate continues to increase until the enzymes are saturated and no more
substrate molecules can fit at any one time as there are no free active sites. This means answer choice (D) describes a factor which influences enzyme reaction
rates and is therefore incorrect with respect to the question.
This leaves us with the answer choice (C), enzyme color. Enzymes are proteins. And even though we tend to represent them in a specific color, many enzymes are not
colored. And there is no known case in which color influences enzyme activity.
The correct answer to the question is therefore (C). The factor that does not influence enzyme activity is enzyme color.
