When the first number is 5 in the Enzyme Commission number, what class of enzyme is it?

Enzymes are categorized into different classes based on the type of reaction they catalyze, and this is reflected in their Enzyme Commission (EC) number. The first digit of the EC number indicates the major class of the enzyme.

In the case where the first digit is 5, this corresponds to isomerases. Isomerases are enzymes that catalyze the rearrangement of molecular structure, allowing one isomer to be transformed into another. This class includes enzymes that facilitate various types of structural changes, such as converting aldoses to ketoses or altering geometric configurations.

The other classes of enzymes identified by the first digit in their EC numbers include oxidoreductases (1), transferases (2), hydrolases (3), lyases (4), ligases (6), and so forth. Therefore, a specific designation related to isomerases is justified when the first number in the EC number is 5.