Enzymes are biological catalysts, protein in nature which speed up and control the rate of chemical reaction in the body.

Classification of enzymes
There are two forms of enzymes,
1. Intracellular enzymes.
2. Extracellular enzymes.

Intracellular enzymes are those which work within the protoplasm of the cell in which they are made.eg respiratory enzymes (decarboxylase)
Extracellular enzymes are the enzymes which are secreted outside the cells in which they work. The majority of the digestive enzymes are extracellular.

The Conventional Way of naming enzymes is,
1. By adding the suffix “ase” at the end of the food acted upon.
Maltose is acted upon by maltase.
Sucrose is acted upon by…………sucrase
A lipid is acted upon by…………lipase
Cellulose is acted upon by……….cellulase
2. They may also be named according to the type of the reaction they catalyze
Dehydrogenase enzyme removes hydrogen from a compound
Transferase catalyses transfer of a group from one compound to another.
Hydrolases- splitting of large molecules into smaller molecules.


1. Enzymes are specific in the reactions they catalyze. A given enzyme catalyses and
Controls a particular reaction e.g. sucrose acts on controls not maltose.
2. They are produced in the cells of living organisms.
3. They are proteins in nature.
4. Only a small amount of enzyme is needed to produce a large amount of chemical change.
5. Enzymes are not used up in the reactions they catalyse. They remain the same after the reaction.
6. Enzymes can cause reactions to go in both directions. The direction to which the direction proceeds normally depends on concentration of reactants and products. An example Starch broken to maltose, maltose can form starch as well
7. Enzymes work but at specific (PH) degree of activity or alkalinity. Those working best in acidic conditions may not work in alkaline condition.
8. They are inactivated by chemical reactions eg cyanide (poison) such chemicals are called inhibitors.
9. They are denatured by heat.


1. Substrate concentration.
If the concentration of the substrate is increased while that of the enzyme remains constant, the rate of the reaction will increase for some time and then becomes constant. Any further increase in substrate concentration will result in a corresponding increase in the rate of the reaction.

2. Enzyme concentration.
The rate of an enzyme controlled reactions increases as the enzyme concentration increases as long as no other factors are limiting as shown above,

3. Temperature of the medium.
The rate of an enzyme- catalyzed reaction increases with temperature up to a maximum, called the optimum temperature.
Most enzymes work best (optimum temperature) between 30c-40c. Like all proteins enzymes are denatured when heated over 60c. They are in activated by low temperatures (0c and below).

4. PH.
Every enzyme has a particular PH range over which it works best. Some enzymes work best in acidic medium while others function best in alkaline medium.
Pepsin has its optimum PH at 2.2 while trypsin has its optimum PH of around 7-8 as shown below.

5. The presence of inhibitors. (These are small molecules).
They reduce the enzyme activity. They make the enzyme deformed rendering it useless hence lowering the rate of enzyme action.
6. Presence of activators.
These are mainly;
• Cofactors
• Coenzymes.
Cofactors are non protein compound, which promote efficiency activity e.g. Zn, Fe, Cu, etc.
Coenzymes are organic non- protein molecules which promote efficiency action of enzymes.

The substance which is acted upon by an enzyme called a substrate and the resulting substance is called a product.
Where A- substrate

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