Q.Describe the classification and nomenclature of enzymes.

[!TLDR]

The question asks for a full account of how enzymes are classified and named.

[!ANSWER]

Enzymes are classified by the International Union of Biochemistry into six major classes based on reaction type - oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases - and each enzyme also carries a unique four-digit EC number (class.sub-class.sub-subclass.serial number) plus an everyday '-ase' working name.

The sheer number of distinct enzymes in living systems calls for a systematic way of organising them, and the International Union of Biochemistry addressed this by sorting every enzyme into one of six major classes, purely on the basis of the type of chemical reaction it catalyses. Oxidoreductases catalyse oxidation-reduction reactions, transferring electrons or hydrogen atoms from one substrate to another. Transferases catalyse the transfer of a chemical group - such as a phosphate, amino or methyl group - from one molecule to another. Hydrolases catalyse the breaking of a chemical bond through the addition of a water molecule, the reaction underlying most digestive enzymes. Lyases remove a group from a substrate without using hydrolysis, often generating a double bond in the process, or add a group across an existing double bond. Isomerases rearrange the atoms within a single molecule, converting it into one of its isomers without altering its overall molecular formula. Ligases join two separate molecules into one, a reaction that generally needs energy supplied by ATP.

Beyond this six-class scheme, every individual enzyme is assigned a unique Enzyme Commission (EC) number consisting of four digits separated by full stops: the first digit gives the major class, the second the sub-class, the third the sub-subclass, and the fourth is the enzyme's own serial number within that sub-subclass - together forming an internationally unambiguous code for that one enzyme. Alongside this formal numbering, enzymes are also given a working common name for everyday use, generally formed by adding the suffix '-ase' either to the substrate the enzyme acts on (for instance, urease acts on urea) or to the reaction type it performs (as with the oxidoreductases, named directly after oxidation and reduction). A further refinement of this picture is that a single reaction may be catalysed in the body by several distinct molecular forms of what is functionally the same enzyme; these are called isoenzymes, and although they catalyse the same overall reaction, they can differ in amino-acid composition, optimum conditions, or the tissue in which they occur - lactate dehydrogenase, which exists as several tissue-specific isoenzyme forms, is a well-known example.

[!ANSWER]

In summary, enzymes are classified into six reaction-based classes (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases), each enzyme is given a unique four-digit EC number (class.sub-class.sub-subclass.serial-number) plus a common '-ase' name, and where several molecular forms of the same enzyme exist across tissues, these are called isoenzymes.

• Listing only some of the six classes, or describing them by example rather than by the reaction type that actually defines each class.
• Treating the EC number and the common '-ase' name as the same thing, when they are two separate, complementary naming systems.
• Omitting isoenzymes when describing the fuller classification picture, despite them being an important nomenclature detail.