Factors Affecting Enzyme Activity

Biology: AQA A Level, topic 1.4.6

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.

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.

Reaction rate is influenced by the presence of competitive and non-competitive inhibitors.

Inhibitors are chemicals that slow down the rate or stop the reaction altogether.
Enzyme-substrate complexes cannot be formed or are formed at a much lower rate.

Competitive inhibitors are similar in shape to the usual substrate and affect the active site directly, blocking access for the formation of ES complexes.
Increasing the substrate concentration can compensate for the effects of a competitive inhibitor as there is no permanent damage to the shape of the active site.
Malonate ions are similar in shape to succinate ions and act as a competitive inhibitor of succinate dehydrogenase, an important enzyme in the Krebs cycle.

Some non-competitive inhibitors have reversible effect but others are irreversible and denature the enzyme.
E.g. Lead denatures a number of enzymes required to synthesise haemoglobin.

Non-competitive inhibitors affect another part of the enzyme molecule causing a change to the shape of the active site.
The active site is no longer complementary to the substrate molecules.

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, assuming that the new temperature is still in the optimum range of the enzyme?
Which of the following statements about enzyme and substrate concentration are true?
What happens to the rate of reaction when the enzymes become saturated?