What is an enzyme?

An enzyme is a protein catalyst. 

A catalyst is a substance which alters the rate of a chemical reaction. (So it can speed up a chemical reaction). 

The key thing about an enzyme is that it is not itself changed into another compound when it does this. 

Therefore a catalyst is a substance which alters the rate of a chemical reaction without itself being permanently changed into another compound. 

An enzyme is a protein catalyst that works in the  body.

How does it do this?

It decreases the free energy of activation. Each chemical reaction has an activation energy that needs to be reached in order for the chemical reaction to take place. The enzyme lowers this energy requirement, so that the chemical reaction can occur at lower energy levels. 

Enzyme Specificity

Enzymes are specific to different chemical reactions. Each individual enzyme will have a different role and different chemical reactions in the body to speed up. 

This is known as enzyme specificity.

Reason for enzyme specificity:

Lock and Key Theory- This is the old theory of how enzymes exhibitied specificity. The idea that each enzyme has an active site (cluster of amino acids), that will only fit a specific type of substrate (the start substance in the chemical reaction). In the same way that a lock will only have one key that fits it. 

This has given way to the:

Induced Fit Theory- It is now understood that enzymes are more flexible than the lock and key theory suggests. The induced fit theory suggests that there is more flexibility, which means that one enzyme can catalyse different reactions. 

What factors influence the activity of an enzyme?

-PH

-Temperature

-Concentration of enzyme and substrate

At extremes of temperature and PH the enzyme can become denatured which means it is no longer able to perform its role.