(i) the first number shows to which of the six main divisions (classes) the enzyme belongs,
(ii) the second figure indicates the subclass,
(iii) the third figure gives the sub-subclass,
(iv) the fourth figure is the serial number of the enzyme in its sub-subclass.
Different Types of Enzymes
- Based on their function inside or outside the
living cells.
(a) Enzymes functional outside the living cells
are called exo-enzymes,(extracellular) e.g.,
enzymes present in digestive juices, lysozyme of
tears.
(b) Enzymes functional inside living cells are known
as endo-enzymes,(intracellular) e.g., enzymes
of Krebs cycle (inside mitochondria), enzymes of
glycolysis (inside cytoplasm). - Based on Chemical Nature of Enzymes: All
enzymes are globular proteins with the exception
of recently discovered RNA enzymes. Some
enzymes may additionally contain a non-protein
group. Accordingly there are two types of
enzymes: simple and conjugate.
(a) Simple Enzyme: It is an enzyme which is
wholly made up of protein. Active site is formed
by specific grouping of its own amino acids.
Additional substance or group is absent, e.g.,
pepsin, trypsin, urease.
(b) Conjugate Enzyme: It is an enzyme which
is formed of two parts— a protein part called
apoenzyme (e.g., flavoprotein) and a non-protein
part named cofactor.
The complete conjugate enzyme, consisting of an
apoenzyme and a cofactor, is called holoenzyme.
Active site is formed jointly by apoenzyme
and cofactor. Cofactor is small, heat stable and
dialysable part of conjugate enzyme. It may be
inorganic or organic in nature. Organic cofactors
are of two types, coenzymes and prosthetic
groups. Coenzymes are easily separable non-
protein organic cofactors. Most of the coenzymes
are made of water soluble vitamins, В and C, e.g.,
thiamine, riboflavin, nicotinamide, pyridoxine.
Inorganic cofactors include ions of a variety
of minerals e.g., calcium, iron, copper, zinc,
magnesium, manganese, potassium, nickel,
molybdenum, selenium, cobalt. Prosthetic
groups are non-protein organic cofactors firmly
(covalently) attached to apoenzymes, e.g., heme
(=haem), iron containing prosthetic group in
cytochromes, haemoglobin, myoglobin, catalase
and peroxidase.- Based on regulation:
(a)Pro-Enzyme or Zymogen: Pro-enzyme or
Zymogen is the inactive precursor of an enzyme.
Many enzymes are initially produced in the pro-
enzyme or zymogen state.They become reactive
or active enzymes only at a particular pH, in the
presence of substrate or some special treatment.
For example, pepsinogen is changed to active
enzyme pepsin in the presence of hydrochloric
acid of gastric juice. Thereafter, pepsin has
autocatalytic effect on further conversion of
pepsinogen.
(b) Allosteric Enzymes: They are enzymes which
have separate areas for different types of