What are different classes of enzymes? Explain any two with the type of reactions they catalyse.

Enzymes are divided into six classes each with 4-13 sub-classes and named accordingly by a four-digit number.

(i) Oxidoreductases / dehydrogenases These enzymes take part in oxidation and reduction or transfer of e- .

S (reduced) + S’ (oxidised) —> (oxidised) + S' (reduced)

(ii) Transferaes These enzymes transfer a functional group from one molecule to another (other than hydrogen). The chemical group transfer does not occur in free state.

S-G + S' $\stackrel{\mathrm{Transferese}}{\to }$ S + S'-G

(iii) Hydrolases These enzymes catalyse the hydrolysis of bonds like ester, ether, peptide, glycosidic C-C, C-halide, P-N etc.

(iv) Lyases These enzymes causes cleavage, removal of groups without hydrolysis and addition of groups to double bonds or removal of groups producing double bond.

(v) Isomerases These enzymes cause rearrangement of molecular structure to effect isomeric changes. They are of three types, i.e., isomerases, epirnerases and mutases.

Glucose- 6 - phosphate $\stackrel{\mathrm{Isomerase}}{\to }$ Fructose 6 - phosphate
(Aldose to ketose group or vice-versa)

Glucose- 6 - phosphate $\stackrel{\mathrm{Mutas}e}{\to }$ Glucose 1 - phosphate
(Shifting the position of side group)

Xylulose 5-phosphate  Ribulose-5-phosphate
(Change in position of one constituent or carbon group)

(vi) Ligases are enzymes catalysing bonding of two chemicals with the help of energy obtained from ATP resulting formation of bonds such as C - O, C - S, C - N and P - O e.g.. pyruvate carboxyl use

Ab + C $\to$ A-C + b

Pyruvric acid +CO2 + ATP + H2 Oxaloacetic +ADP + Pi