Q.Why are enzymes that catalyse oxidation-reduction reactions called oxidoreductases?

Because so many distinct enzymes exist, the International Union of Biochemistry sorts every enzyme into one of six major classes purely according to the type of reaction it catalyses: oxidoreductases (oxidation-reduction reactions, transferring electrons or hydrogen), transferases (transfer of a chemical group from one molecule to another), hydrolases (breaking a bond by adding water), lyases (removing a group without hydrolysis, often forming a double bond, or the reverse addition across a double bond), isomerases (rearranging the atoms of a single molecule into an isomeric form), and ligases (joining two molecules together, generally using energy from ATP).

Every enzyme is further given a unique four-digit Enzyme Commission (EC) number. The first digit identifies which of the six major classes the enzyme falls into; the second digit specifies its sub-class; the third digit specifies its sub-subclass, narrowing the reaction type still further; and the fourth digit is simply that enzyme's serial number within its sub-subclass, so that no two enzymes share the same four-digit code. Alongside this formal numbering, most enzymes also carry an everyday working name, commonly formed by adding the suffix '-ase' to the substrate they act on or to the reaction they perform - which is also why enzymes that catalyse oxidation-reduction are named oxidoreductases, directly after the type of reaction (oxidation and reduction) they carry out.

A related idea is that of isoenzymes (or isozymes): multiple, slightly different molecular forms of what is functionally the same enzyme, catalysing the same overall reaction but differing in details such as amino-acid composition, optimum conditions or the tissue in which they are produced. Lactate dehydrogenase, which exists as several distinct isoenzyme forms distributed differently across body tissues, is a commonly cited example.