Video Transcript
Which of the following is not a
characteristic of enzymes? (A) They are affected by pH and
temperature. (B) They are specific in their
action. (C) They speed up the rate of
reaction. Or (D) they cannot be reused after
the reaction is complete.
A reaction is a chemical process in
which substances called reactants are transformed into different substances called
products. For a reaction to proceed, a
certain amount of activation energy is needed. Let’s look at an example.
Starch is a relatively stable
macromolecule, which is found in a lot of the food we eat. When we eat food containing starch,
our body cannot take the starch up as it is. This is because it is too
large. So, in order for our body to take
up starch molecules, they need to be broken down into smaller sugar molecules. This happens during digestion. For the reaction which breaks down
starch into smaller molecules to happen by itself, without the help of enzymes, lots
of energy is required. Enzymes speed up such reactions by
lowering the energy needed for the change of conformation of molecules.
The question asks about the
characteristics of enzymes, so let’s repeat them. We have already repeated that
enzymes speed up the rate of reaction. As the question asks for the answer
choice which does not describe a characteristic of enzymes, we can exclude answer
option (C).
Enzymes are highly specific to the
substrate which they help to transform into a product. In our starch example, the enzyme
amylase has an active site which is highly specific to bind to starch molecules. Starch is therefore the substrate
of amylase. When the starch molecule is bound
to the enzyme in an enzyme–substrate complex, the enzyme helps to break bonds within
the starch. At the end of this process, amylase
releases maltose molecules as products, which are more simple sugars. So answer choice (B) is incorrect,
as a characteristic of enzymes is that they are specific in their action.
After having freed the product out
of its active site at the end of the reaction, the enzyme is ready to bind to the
next substrate to perform its action again. So answer choice (D) seems to be
our correct answer, as enzymes can be reused after the reaction is complete. But let’s discuss the last answer
choice just to be sure.
Enzymes are indeed affected by pH
and temperature. Every enzyme has a specific pH
range in which it works best. Our mouth has quite a neutral
pH. That’s where salivary amylase
breaking down starch into maltose is found and works best. In a more or less acidic
environment, it denatures and cannot work anymore. In terms of temperature, most
enzymes in our body have a similar preference. This is because our general body
temperature is at around 37 degrees Celsius and our enzymes are adapted to this. If they’re in a colder environment,
they are less active. If the temperature exceeds a
certain rate, the enzymes degenerate and cannot work anymore.
Now we can answer with certainty
that the statement which is not describing a characteristic of enzymes is statement
(D): they cannot be reused after the reaction is complete.
