You can also find the Exam Thread list for A-levels here and GCSE here.
Just quoting in Puddles the Monkey so she can move the thread if needed
Limiting factors are factors which affect the rate of activity of the enzyme and therefore the rate of reaction.
For example, temperature:
- at really low temperatures, neither the enzyme nor the substrate (make sure you refer to both) have enough kinetic energy to collide successfully and therefore fewer enzyme-substrate complexes form. This means the rate of reaction is low.
- as the temperature increases, the enzyme and substrate have more kinetic energy and so collide with energy more often so form more enzyme substrate complexes which increases the rate of reaction.
- the optimum temperature of the enzyme is the temperature at which the enzyme works at its best which is why the rate of reaction is the fastest at this point.
- after this point, the increasing vibration of the enzymes caused by the high temperature results in hydrogen bonds breaking and so means the tertiary structure of the active site is broken. This means the shape of the active site will change and the enzyme will be denatured which means the active site will no longer be complementary to its substrate and so the rate of reaction will eventually reach 0.
Other examples of limiting factors would be pH, substrate concentration and enzyme concentration.
Hope that helped!