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Cytology

and substrate concentration. They are very carbondioxide and water to form carbonic
efficient, thus, they are needed in a very acid in tissues where the concentration of
small amount. This means that a small carbon dioxide is high. In the lungs, where
amount of enzyme acts on large quantities the concentration of carbon dioxide is low,
of substrates. Enzymes work reversibly; the same carbonic anhydrase, catalyses the
meaning that they catalyse reversible dissociation of carbonic acid into carbon
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reactions. For example, an enzyme carbonic dioxide and water.
anhydrase catalyses the combination of

Classification of enzymes
In 1964, the International Union of Biochemistry (IUB) introduced a system of classifying
enzymes based on the type of reactions they catalyse. This system recognised six major
functional classes of enzymes.

(a) Oxidoreductases: Catalyse redox reactions (biological oxidation and reduction
reactions) by the transfer of hydrogen, oxygen, or electrons from one molecule to
another. Example;
Ethanal + NADH + H dehydrogenase Ethanol + NAD +
+
Hydrogen is simultaneously lost from NADH and gained by ethanal. NADH is
+
oxidised to NAD and ethanal is reduced to ethanol.

(b) Transferases: Catalyse the transfer of a group from one compound to another.
Example;

Glutamic acid + Pyruvic aminotransferase α-ketoglutaric acid + alanine

The R-group on the amino acid, glutamic acid, is exchanged with the R-group on a
keto acid, pyruvic acid. A new amino acid, alanine is formed along with a new keto
acid, α-ketoglutaric acid.

(c) Hydrolases: Catalyse the splitting of a large substrate molecule into two
smaller products in the presence of water (hydrolysis).
Example;
Lactose + Water Lactase Glucose + Galactose

The disaccharide, lactose, is broken down into two monosaccharide residues by
the lactase enzyme in the presence of water. All digestive enzymes fall under this
category.

(d) Lyases: Catalyse the removal of a chemical group by the process other than
hydrolysis.
Example;
Pyruvic acid decarboxylase Ethanal + Carbon dioxide

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