Originally enzymes were defined as “catalytic substances produced by living cells”. Pavlov defined them as “activators and keys to life” being involved in the processes of metabolism.
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Chittenden in 1894, expressed the following to the enzymes : (1) Enzymes are proteins. (2) Catalytic activity is in some way related to protein structure. (3) Enzymes are not passive catalysts but function by forming an intermediary complex with the substrate.
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Modern definition states that “enzymes are catalysts of biological origin which accelerate the various cellular reactions, without themselves undergoing any apparent change during the course of action and that are not dependent on the intact cell for their activity
Nomenclature of enzymes:
A systematic method of naming individual enzyme is very essential because a large number of enzymes have been recognized so far.
In the modern system for naming a particular enzyme the suffix “ase” is added to the root word referring to the substrate of the enzyme. By substrate is meant the substance that is acted upon by the enzyme.
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Under this system the name sucrase obviously refers to an enzyme that causes the breakdown of the sugar, sucrose.
Another system for naming enzymes is that the suffix “lytic” is added to the substrate such as proteolytic (protein-splitting) and amylolytic (starch-splitting) enzymes.
The enzymes may also be named according to the type of reactions they cause, for example, oxidation (oxidase), dehydrogenation (dehydrogenase), hydrolysis (hydrolase), etc. Some enzymes named before any formal system was adopted retained their old fashioned names, such as pepsin, rennin, trypsin and ptyalin.