Enzymes

Discovery

'Diastase' which is a mixture of amylases was first to be discovered by French chemist Anselme Payen in 1833.
The term 'enzyme' was first used by German physiologist Wilhelm Kühne in 1877. Enzymes are also called as 'biocatalyst'.

General features

• Enzymes are biocatalyst that increase the rate of chemical reaction without itself being changed in the overall process.
• All the vital reactions in a cell are catalyzed by enzymes.
• All enzyme have following unique features.
  • All (except for ribozyme) enzymes are protein.
  • Enzymes are highly specific with the reactions.
  • They exhibit tremendous catalytic power.
  • Enzymes increase the rate of reaction by lowering the activation energy.

Types of enzymes

Depending upon the components, enzymes are of two types.

1. Simple enzymes
2. Conjugated enzymes

1. Simple enzymes

Simple enzymes are composed of polypeptides (proteins) only.
Example: Pepsin, trypsin, lysozyme

2. Conjugated enzymes

Conjugated enzymes consist of proteinaceous 'apoenzyme' and a non-protein components 'coenzyme'.

A. Apoenzyme

• It is proteinaceous part of conjugated enzymes.
• It is know as 'apoenzyme'.
• Apoenzymes can not catalyze a reaction.
• For catalysis, it requires coenzyme.
• Apoenzyme with its coenzyme makes functional 'holoenzyme' or 'conjugated enzyme'.

B. Coenzyme or cofactor

• If the non-protenaceous component required for enzyme activity is a metallic ion, it is known as 'cofactor'.
Example: Cu+ is a cofactor of alcohol dehydrogenase.
• If the non-protenaceous component required for enzyme activity is a small organic molecule, it is known as 'coenzyme.
• Coenzyme are vitamins or their derivatives.
Example: Thiamin pyrophosphate is coenzyme of transketolase and enzymes of oxidative decarboxylation.

Nomenclature of enzymes

There are many enzymes known by their common names, e.g., pepsin, trypsin, enterokinin, etc. There are more than one enzymes known by a common name, while sometimes single enzyme is known for different names. Still some enzymes are there, which are known for the reaction they catalyze with suffix '-ase', e.g., ATPase (synthesize ATP). Common names of enzyme provide little information and creates confusions.
In order to overcome these problems, International Congress of Biochemistry in Brussels set up an 'Enzyme Commission', which gave nomenclature and classification system for enzymes in 1961. International Union of Biochemistry and Molecular Biology (IUBMB) published its sixth and current version in 1992 and the list was updated up to 1999.
Accordingly, each enzymes is written with suffix '-ase' and assigned and Enzyme Commission (EC) number. The EC number is four-digits separated by dots, where each digit provide specific information about the reaction that it catalyzes.
An enzyme with n1.n2.n3.n4 EC number contains following information.

• n1 represents Major class
• n2 represents Sub-class
• n3 represents Sub sub-class, and
• n4 represents Order in which enzyme is added to the list

EC number provides more about the reaction that it catalyze than the nature of enzyme. Old names, common names and EC number of over 5500 enzymes are maintained at The Enzyme database accessed at https://www.enzyme-database.org/. The list is dynamic and being updated from time to time.

Classes of enzymes

According to the Enzyme Commission, there 6 major classes of enzymes. Recently, one more major class 'EC7' has been added by IUBMB to the list in August, 2018. These classes are:

EC 1: Oxidoreductases:

Catalyze oxidation-reduction reaction.
Ared + Box → Aox + Bred
E.g. Oxidases, peroxidases, dehydrogenases.

EC 2: Transferases:

Catalyze reactions that involve the transfer of functional groups from one molecule to other.
A-B + C → A + B-C
E.g. Transaminase, Kinase

EC 3: Hydrolases:

Catalyze reaction in which cleavage of bonds is accomplished by adding water.
A-B + H2O → A-H + B-OH
E.g. Phosphatase, Peptidase

EC 4: Lyases:

Catalyze reaction in which C-C, C-O, C-N bonds is broken by means other than hydrolysis or oxidation.
A=B + HX → A-X + B-H
E.g. Decarboxylase, Aldolase

EC 5: Isomerases:

Catalyze intramolecular rearrangement and yield isomeric forms.
AB → BA
E.g. Mutase, isomerase, epimerase

EC 6: Ligases:

Catalyze formation of C-C, C-O, C-N bonds. The energy is always supplied by the hydrolysis of ATP.
A+B + ATP → A-B + ADP
E.g. Carboxylase, DNA ligase

EC 7: Translocases:

Catalyze the translocation of ions or molecules within membrane or across membrane or their separation within membranes.
E.g., Transporters

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First published on 13-04-2021
Last updated on 02-02-2022