1. Amin W., Zara A., Melissa L., Mesret , Urwa, Carolina B.
IB Biology: Topic 3.6 Enzymes
Introduction to Enzymes
Enzymes are a biological catalyst or protein that catalyses chemical reactions in living organisms.
During enzyme activity, substrates are the reactant in the biochemical reaction, enzymes act as the
catalyst for the reaction, and the active site is the entire region where the enzyme and substrate bind,
which catalyses the reaction. During the reaction, the substrate locks into the active site, is catalyzed,
then the products of the biochemical reaction are released and the enzyme is then reused.
Enzyme Activity Diagram
Types of models on Enzyme Activity
There are different ideas on models of the cause of enzyme activity. One theory states how
enzymes and substrates are only able to react with one another if they are chemically and structurally
compatible; this is known as The Lock-and-Key Hypothesis. But this model is not a viable theory. If this
model were to be true, only one enzyme could catalyze one reaction when in reality an enzyme can
catalyze multiple reactions.

2. Amin W., Zara A., Melissa L., Mesret , Urwa, Carolina B.
The Lock-and-Key Model
The induced-fit model is known to be a better theory of enzyme activity as it states that when a
substrate approaches an enzyme, it induces a sort of change in the active site, then changing shape in
order to fit the substrate. The activation energy is reduced because of the stressing of the substrate in
this reaction.
Induced-Fit Model
Factors that affect enzyme based reactions
Effect of temperature
Both enzymes and substrate motion is dependent on temperature of the fluid. Fluids with higher
temperature have faster moving molecules. The faster the molecules move the more they collide with
great energy. The reaction rate increase with the increase in temperature. The temperature increase can
affect the shape of the enzyme which can lead to its denaturation.

3. Amin W., Zara A., Melissa L., Mesret , Urwa, Carolina B.
Effects of pH
The pH of the solution is dependent on the relative number of hydrogen ions (H+) compared to
hydroxide ions (OH-) in the same solution. Substrate giving off hydrogen ions is an acid with pH below 7.
Substrate giving off hydroxide ions is a base with pH more than 7. Water is neutral with equal number of
both ions. Amino acids are charged positive and negative, the opposite charge must match the when
substrate is in active site for a catalytic action. With too much acid or too little the charge does not bond
correctly and can lead to reaction being inefficient or inactive.
Effects of substrate concentration
With increase of substrate concentration there is an increase of molecular collisions leading to
the rate of reaction increase. The more molecules there are, the more they collide but enzymes have a
maximum rate in which they work. Increase in substrate solution when the enzymes are working as fast
as possible will not increase the reaction rate.