Enzymes are biological catalysts made of protein. They alter the rate of chemical reactions without themselves being chemically changed at the end of the reaction.

They are described as biological catalysts because they are produced by living cell.

Note: Catalysts are substances that can change the speed of a chemical reaction.


Properties of enzymes

  • They are made up of proteins, which is one reason why we need protein in our diet.
  • They are all biological catalysts. They speed up a reaction without being used up; this means they can be used over and over again.
  • A small amount of enzyme can effect the change of a large amount of chemical.
  • The way enzymes work is affected by temperature, pH and pressure. They can be denatured (destroyed) by excessive heat.
  • Enzymes are specificthat is they control only one reaction. So maltase only acts on maltose, sucrase on sucrose etc.
  • All metabolic reactions are controlled by enzymes.
    g. anabolic reaction and catabolic reaction.

Structure of an enzyme molecule


  • Enzymes have a part of a molecule where it just has the shape where only certain kind of substrate can bind to it; this site of activity is known as the active site. 
  • The molecules that react and bind to the enzyme is known as the substrate.


Lock and hypothesis


  • The molecules on which the enzymes act on are called substrate.
  • The molecules that are formed in an enzyme controlled reaction are called products.
  • The mechanism in which enzymes work are refered as lock and key hypothesis.
  • The enzymes and substrate molecule react in a similar way as the lock and key works.
  • Every enzyme has an active site of a particular shape, just like every lock has a particular shape.
  • The substrate molecule which is of the same shape as the active site can only fit into the active site as a particular key can open a particular lock.
  • When the substrate enters into the active site it forms enzyme-substrate complex.
  • In the active site the substrate molecule will be broken down and released as products.


Effect of temperature and pH on enzyme activity

Effect of temperature and pH

  • Temperature Enzymes have an optimum temperature at which they work fastest.
  • Increasing the temperature increases the kinetic energy of the enzyme and substrate molecules so that they move faster and are more likely to collide. So increasing the temperature increases the rate of the reaction up to a certain temperature. This temperature is known as the enzyme’s optimum temperature.
  • Different enzymes have different optimum temperatures. The enzymes in animal bodies work best at 37˚C.
  • Enzymes are inactive at very low temperature.
  • If the temperature is increased beyond the optimum the enzyme has so much kinetic energy that the bonds holding the enzyme molecule together start to vibrate and eventually break.
  • The enzyme loses its specific shape so that the substrate no longer fits in to the active site.  At this point enzyme is said to be denatured.
  • Denaturation is a permanent process.


Effect of pH

  • pH Enzymes have an optimum pH at which they work fastest.
  • pH is a measure of the degree of acidity or alkalinity.
  • Each enzyme requires a specific pH level for optimum efficiency. This is the optimum pH of the enzyme, at which the enzyme work at their best.
  • Small changes in pH above or below the Optimum do not cause a permanent change to the enzyme, since the bonds can be reformed. However, extreme changes in pH can cause enzymes to Denature and permanently lose their function.