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Enzymes
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ENZYMESA catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.
Enzymes are proteins that function as biological catalysts (biological because they are made in living
cells). They alter the rate of chemical reactions without themselves being chemically changed at the end of
the reaction.
Enzymes are necessary to all living organisms as they maintain reaction speeds of all metabolic
reactions (all the reactions that keep an organism alive) at a rate that can sustain life.
For example, if we did not produce digestive enzymes, it would take around 2 – 3 weeks to digest one
meal; with enzymes, it takes around 4 hours.
Examples of enzyme-catalysed processes:
1. Digestion: It is the process by which large, insoluble food molecules are broken down into small,
soluble food molecules.
2. Anabolic reactions or Building-up reactions: synthesis of molecules.
Example: -special enzymes are needed to build up cell proteins from amino acids inside cells,
-during photosynthesis carbon dioxide and water combine to form glucose and oxygen in the
presence of enzymes.
3. Catabolic reactions or Breaking-down reactions: hydrolysis
Example: - the breakdown of glucose to release energy, carbon dioxide and water during
respiration involves a series of enzymes, - breakdown of large insoluble molecules into simpler
soluble ones during digestion.
4. Hydrogen peroxide is produced during chemical reactions in cells. It can be poisonous to tissues if
it accumulates in cells. Thus, the enzyme catalase is used to breakdown hydrogen peroxide to
water and oxygen.
Examples of enzymes in the digestive system:
Enzyme Substrate( substance broken down) Products
Carbohydrases: Salivary Starch Maltose
amylase, pancreatic amylase
Proteases: pepsin in stomach Protein Amino acids
Lipase in pancreatic juice and Fats(lipids) Fatty acids and Glycerol
intestinal juice
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, Characteristics of enzymes
1. Enzymes speed up the rate of chemical reactions that occur in a cell.
2. A small quantity of enzyme can catalyse a huge chemical reaction.
3. Enzymes remain unchanged at the end of a reaction; therefore, the same enzyme can be used over
and over again in a reaction.
4. Enzymes are specific. Each chemical reaction that occurs in a cell is catalysed by a unique
enzyme. Enzymes possess active sites where reaction takes place. These active sites have specific
shapes which are complementary to a specific substrate. E.g: Amylase acts only on starch.
5. Enzyme activity is affected by pH, temperature, substrate concentration, and enzyme
concentration.
6. Enzymes work best at an optimum temperature and pH.
7. Enzymes are denatured by high temperatures and extremes of pH.
Enzyme action (Lock and key hypothesis)
The substances on which enzymes act are called substrates (molecules that get broken down or
joined together in the reaction) e.g. starch, proteins.
Enzymes have a structure that is called an active site. It is a depression on the surface of an
enzyme into which a specific substrate can fit/attach. The shape of the active site is complementary
to the shape of a specific substrate.
Enzyme action can be described according to the ‘lock and key’ hypothesis, whereby the enzyme
is the lock and the substrate is the key.
Enzymes and substrates randomly move about in solution.
When an enzyme and its complementary substrate randomly collide – with the substrate fitting into
the active site of the enzyme – an enzyme-substrate complex forms, and the reaction occurs.
The substrate is converted to products.
The products are then released from the active site.
The enzyme is unchanged and will go on to catalyse further reactions.
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