Enzymes are made up of proteins, they are biological catalysts.

They increase the rate of metabolic reactions. Nearly all of the reactions in the body use an enzyme, when a reaction involving an Enzyme occurs, a Substrate is turned into a Product. The Substrate can be one or more molecules. The Active Site of an Enzyme is Complementary to the Substrate it catalyses.

They are soluble in water to hydrophilic side group

They very large molecules, but only small parts of the molecules act as the catalyst

Enzymes are specific are specific to one type of reaction Enzymes. All enzymes are Globular Proteins with a specific Tertiary Shape. The rest of the Enzyme is much larger and is involved in maintaining the specific shape of the Enzyme.

Enzymes in respiration

Hydrogen peroxide is a product of respiration, enzymes are required to break down this as it is harmful

Hydrogen Peroxide         ->             Water                                +             Oxygen

                         4H2O2                                                                      4H2O                                     2O2

The use of catalase is used to break down Hydrogen peroxide down into water and oxygen

a) Measure 25 cm3 of hydrogen peroxide solution into each of three conical flasks.

b) At the same time, add a small piece of liver to the first flask, a small piece of potato to the second flask, and a small piece of celery to the third flask.

c) Hold a glowing splint in the neck of each flask.

d) Note the time taken before each glowing splint is re-lit by the evolved oxygen.

e) Dispose of all mixtures into the bucket or bin provided.


Extracellularly and intracellularly

Extracellularly are enzymes that is secreted by a cell and functions outside of that cell. Many enzymes secreted in digestion are extracellularly such as amylase or pepsin.

Intracellularly are enzymes that functions within the cell in which it was produced

DNA replication is intracellularly

All reactions require energy before they can start.  It is shown as activation energy.  This starts in breaking bonds of the reactants: enzymes lower this activation energy. This creates a transition state between enzymes and substances that are more stable.

Lock and Key Theory

Active sites are a small area with a specific shape to the substrate of the substance. The shape of the Active Sites of Enzymes are exactly complementary to the shape of the Substrate.

Enzymes that bonds with the correct substrate form enzyme substrate complex. The enzyme will catalyse the reaction, and the products, together with the enzyme, will form an Enzyme-Product Complex. According to this model, it is possible for an enzyme to catalyse a reverse reaction.





Induced Fit Theory

The active site is not the exact size of the substrate, but change shape in the presence of a specific substrate to become Complementary.

As the substrate moves closer to the enzyme it is ‘moulded’ around the substrate. This tight envelope of the substrate forms enzyme substrate complexes. When a substrate molecule collides with an enzyme, if its composition is specifically correct, the shape of the enzyme’s Active Site will change so that the substrate fits into it and an Enzyme-Substrate Complex can form. The reaction is then catalysed and an Enzyme-Product Complex forms.



Enzyme actions

For enzymes to work they require:

  • Physical contact with substrate
  • Must have correct shaped active sites

Measuring Enzyme action

  • Rate of the formation of products





  • Rate of Disappearance


 As the enzyme reacts with the substrate of the molecule, the mass of the substrate decreases as the amount of product increases over time

Graph 1 shows that at the beginning there is lots of substrate, when these substrates bond to the active sites the reaction begins. This process occurs rapidly causing amount of product to rise quickly and the amount of reactant to fall too, as shown in graph 2. The curve levels of nearer the end as a result of the amount of substrates was reducing. The decreased concentration of substrates causes the drop off of amount of product produced, this is the result of the enzyme finding it harder to find a substrate molecule that haven’t been reacted yet as well as products of previous reactions being in the way.

Rate of enzyme actions

  • Temperature

Lower temperatures means there is less kinetic energy in the molecules, so fewer successful collisions occur.

Optimum temperature for enzymes is the maximum amount of kinetic energy, thus successful collisions, before the enzyme begins to denature to excessive heat.

At high temperature there is an increase in kinetic energy and heat which causes the breakdown of hydrogen bonds, denaturing the tertiary structure of the active site and enzyme shape. At first enzymes fit less easily, therefore there is a slower rate of reaction.



Only extreme pH causes denaturing

pH affects the amino acids of the enzyme as it changes the charge of them. This causes different molecules to bond to it, as a result the active site is different. This alters the tertiary structure.

Fluctuations in internal pH of small amounts does little however


Concentration of Enzymes

Low enzyme concentration means there are fewer active sites occupied. This means more active sites are available.

Increased enzymes concentration means more active sites available and the reaction can produced at a faster rate due to more successful collisions

Eventually increasing the enzyme concentration beyond a point means that there are too many enzymes to substrate.





  • Substrate Concentration

Low substrate concentration means there are fewer substrate sites occupied. This means more substrates are available.

Increased substrate concentration means more substrates available and the reaction can produced at a faster rate due to more successful collisions

Eventually increasing the substrate concentration beyond a point means that there are too many substrate to enzymes