2.5 Enzymes


Lesson one: Altering the rate at which enzymes work


  • Substrate: the substance being acted on by an enzyme
  • Active site: the part of the enzyme molecule that binds with an enzyme
  • Enzyme activity: the measure of how fast an enzyme is working
  • Enzyme-substrate complex: the combination of an enzyme and a substrate, bound by the active site
  • Inhibition: when a substance interferes with the formation of an enzyme substrate complex (eg. cyanide as a poison inhibits key enzymes in respiration)
  • Concentration: number of particles (Mols) of a substance per litre of solute.


Enzymes are globular proteins that work as catalysts.

Globular means, they are literally shaped like a ball. The molecule will have hydrophilic groups around the outside of the molecule, in order to be soluble – as water is the medium for metabolic reactions. A catalyst is a substance that speeds up the rate of a chemical reaction, without being altered itself. An enzyme molecule can therefore be reused as often as is needed.

As enzymes are proteins, they can also be denatured by heat of extremes of pH.

image credit: sally buy


Common enzymes:

1) Catalase – Occurs in a huge variety of cells, breaks down toxic by-product of respiration hydrogen peroxide into harmless water and oxygen.

2) Amylase – Breaks down starch (amylose), into maltose (a disaccharide) in the mouth and in the pancreas.

3) Rubisco (ribulose biphosphate carboxylase) – an enzyme involved in photosynthesis, and probably the most common protein on the planet.

The ‘lock and key’ mechanism

Each enzyme has a part of its molecule called an ‘active site’, which binds to the molecule it is working on, the ‘substrate’.

image credit: wikipedia.org

image credit: wikipedia.org


Enzymes are ‘specific’, in that only one particular substrate ‘fit’ into the active site.


 Factors affecting enzyme activity

The rate at which an enzyme works is called ‘enzyme activity’.

Enzyme activity is affected by:

  • temperature
  • pH
  • substrate concentration

Temperature: As temperature increases, there are more collisions between faster moving enzyme and substrate molecules – making enzyme activity faster. Denaturation starts to occur, and after an optimum temperature (for human enzymes usually 40 C), the activity decreases sharply as the enzyme is denatured.



image credit: revisionworld

image credit: revisionworld


pH: At a low or a high pH, enzyme activity is low. Enzymes can even be denatured by extreme pH values. Enzyme activity rises to an optimum pH ( for human enzymes usually 7) in the middle. The digestive system enzymes have varying optimum pH values.




image credit: study.com

image credit: study.com


Substrate concentration: Enzyme activity increases when there is more substrate available, until a point when all the active sites are occupied (saturation point). After this further increases in substrate concentration will have no effect on enzyme activity.

sites: sachina.org

sites: sachina.org


Watch the video, and make a list of all the commercial uses of enzymes that you remember:

Q) What is meant by the term ‘immobilised enzymes?’


Immobilised enzymes are enzymes that have been ‘fixed’ to a solid media eg. glass beads. This means that they are not added to the solutions, but the reaction mix is passed through the media containing the enzyme.

Immobilised enzymes being used to create lactose-free milk

Discuss: What are the advantages of using immobilised enzymes?

image credit bpi schools

image credit bpi schools











  • Enzyme can be separated easily from the products of the reaction, stopping the reaction at an ideal time and preventing contamination of the final product with left-over enzyme
  • Enzyme can be used again and again (recycled)
  • Enzymes are less likely to be denatured if they are immobilised.
  • It is possible to use higher enzyme concentrations, speeding up reaction rates.


Leave a Reply

  • twelve + 2 =