Factors Affecting Enzyme Activity

Biology: Cambridge International A Level, topic 3.1.2

Investigating Rates of Enzyme-Controlled Reactions

Changes to the tertiary structure of an enzyme through changing the pH or temperature will affect how fast reactions are catalysed.

Increasing the temperature will increase the kinetic energy of the molecules.
This increases the chance of a collision between the enzyme and substrate and so more collisions are likely in a set period of time. In other words, the rate of reaction is faster.
Increasing the temperature by 10^oC will approximately double the rate of reaction for most enzyme-controlled reactions.
The temperature coefficient (Q₁₀) measures the change in the rate of reaction when temperature increases by 10^oC. Q₁₀ is usually around 2.

Changing the pH changes the number of hydroxide ions and hydrogen ions (OH⁻ and H⁺) surrounding the enzyme.
These interact with the charges on the enzyme’s amino acids, affecting hydrogen bonding and ionic bonding, so resulting in changes to the tertiary structure.

Increasing or decreasing the temperature or pH outside of an optimal range can affect chemical bonds within the active site and the enzyme will not work as well.
At extreme temperatures and pH values, the enzyme's structure may be changed. This is called a denatured enzyme.

Reaction rate is influenced by the relative enzyme and substrate concentrations.

Increasing the concentration of enzyme in a solution means there are more enzyme molecules available to catalyse the substrate in a given amount of time

Increasing the concentration of the substrate increases the numbers of substrate molecules that can form enzyme-substrate (ES) complexes at any one time.
This increases the initial rate of reaction but when all the enzyme molecules are engaged in ES complexes the rate cannot increase any further.
The rate will then plateau because the enzyme is said to be saturated.
This is known as V_max.

K_M is known as the Michaelis-Menten constant.
We can calculate K_M by looking at the concentration of substrate at V_max/2.
So, we can think of K_m as the substrate concentration at which the rate is half of the maximum rate of that enzyme.
We use K_m as a measure of enzyme-substrate affinity.
The application of enzyme activity in this way is known as Michaelis-Menten kinetics.

A smaller K_M means an enzyme has a high affinity for a substrate.
This is because a smaller concentration of substrate is required to reach V_max/2.

Which of the following statements about temperature and pH are true?
What is the effect of increasing temperature on enzyme function?
What is the impact of a 10°C increase in temperature on the rate of reaction?
Which of the following statements about enzyme and substrate concentration are true?
What happens to the rate of reaction when the enzymes become saturated?