Name the class of enzyme that usually catalyze the following reaction:

S – G + S^# → S + S^# – G

Where, G → a group other than hydrogen

S → a substrate

S# → another substrate

The correct option/answer is: Transferase.

To understand why this is the correct classification, let us analyze the chemical reaction given in the question:

$S-G+S^{\#}\to S+S^{\#}-G$

In this reaction, the functional group $G$ is transferred from substrate $S$ to substrate $S^{\#}$. The chemical bonds are rearranged such that the group $G$ leaves $S$ and binds to $S^{\#}$.

Enzymes are classified into six major classes by the Enzyme Commission (EC number scheme) based on the type of reaction they catalyze:
1. Oxidoreductases: Catalyze oxidation-reduction reactions.
2. Transferases: Catalyze the transfer of a functional group (e.g., a methyl, ethyl, phosphate, or amino group) from one molecule (the donor) to another (the acceptor).
3. Hydrolases: Catalyze the cleavage of bonds by the addition of water (hydrolysis).
4. Lyases: Catalyze the cleavage of C-C, C-O, C-N, and other bonds by elimination, leaving double bonds, or conversely adding groups to double bonds.
5. Isomerases: Catalyze geometric or structural changes within a single molecule.
6. Ligases: Catalyze the joining of two molecules coupled with the hydrolysis of a pyrophosphate bond in ATP or a similar triphosphate.

Since the reaction explicitly depicts the transfer of group $G$ from substrate $S$ to $S^{\#}$, the class of enzyme catalyzing this reaction is a transferase.