Enzymes are protein molecules that are able to catalyse a biological reaction.

Each type of enzyme is highly specific for only one type of a reaction

With the use of an enzyme the rate of a particular reaction can increase by a factor greater than 108

They work by providing an alternative pathway with a lower activation energy

More reactant molecules then have the necessary minimum activation energy

The specificity of the enzyme depends on their tertiary and quaternary structure.

The substrate (reactant molecule) binds to a part of the enzyme known as the active site

The active site is able to change shape allowing the substrate to fit effectively and this is known as the induced fit theory

Inhibitors are substances that slow down the rate of an enzyme catalysed reaction

Competitive inhibitors resemble the substrate in shape but they are unable to react

They occupy the active site making it less accessible to the substrate

As the concentration of the substrate molecules increases the effect of the competitive inhibitors decreases

Competitive inhibition

Non-competitive inhibitors bind to the enzyme but not to the active site

The enzyme then changes shape and the substrates are no longer able to bind

With this type of inhibitor increasing the concentration of the substrate molecules will have no effect as the non-competitive inhibitors don’t bind to the active site

Non-competitive inhibition

At low substrate concentrations the rate of the reaction is proportional to the concentration of the substrate

At high substrate concentrations the rate of the reaction reaches a maximum point known as Vmax

This means that that at low substrate concentrations there are enough active sites for the substrates to bind and react

When all the active sites are used the enzyme is not able to work any faster

This constant Km is the concentration of the substrate at half of the Vmax

It is always the same for a particular enzyme with a particular substrate

It indicates whether the enzyme functions appropriately at low substrate concentrations or whether high substrate concentrations are needed for the efficient catalysis

Lower Km → more efficient enzyme

Competitive vs Non-competitive

Competitive inhibitor Blocks active sites Same Vmax Larger Km

Non-competitive Enzyme changes shape Smaller Vmax Same Km

An increase in temperature at first increases the rate of the reaction until the optimum temperature is reached (usually 40°C)

Until then more reactants posses the necessary activation energy to react

After the optimum temperature is reached the enzyme starts to denaturate as the bonds holding the structure together break

At certain pH values there is a change in the charge of the amino acid which affects the bond and alters the structure of the enzyme making it inactive

Heavy metals “poison” the enzyme They react with the –SH group and

replace the hydrogen atom with a heavy metal atom, or ion altering once more the structure of the enzyme

Effect of temperature and pH on rate of enzyme activity