(A) $\rightarrow$ Thousands of enzymes have been discovered,isolated,and studied.
$\rightarrow$ Most of these enzymes have been classified into different groups based on the type of reactions they catalyse.
$\rightarrow$ Enzymes are divided into $6$ classes,each with $4-13$ subclasses,and named accordingly by a four-digit number.
$(1)$ Oxidoreductases/dehydrogenases: Enzymes which catalyse oxidoreduction between two substrates $S$ and $S'$ e.g.,$S$ reduced $+ S'$ oxidised $\longrightarrow S$ oxidised $+ S'$ reduced.
$(2)$ Transferases: Enzymes catalysing a transfer of a group $G$ (other than hydrogen) between a pair of substrate $S$ and $S'$ e.g.,$S-G + S' \longrightarrow S + S'-G$.
$(3)$ Hydrolases: Enzymes catalysing the hydrolysis of ester,ether,peptide,glycosidic,$C-C$,$C$-halide,or $P-N$ bonds.
$(4)$ Lyases: Enzymes that catalyse the removal of groups from substrates by mechanisms other than hydrolysis,leaving double bonds,e.g.,$X-Y$ removal from $C-C$ to form $C=C$.
$(5)$ Isomerases: Includes all enzymes catalysing the inter-conversion of optical,geometric,or positional isomers.
$(6)$ Ligases: Enzymes catalysing the linking together of $2$ compounds,e.g.,enzymes which catalyse the joining of $C-O, C-S, C-N, P-O$ etc. bonds.