What does the first number in an Enzyme Commission number indicate?

The first number in an Enzyme Commission number identifies the class of enzyme to which the enzyme belongs. This classification is based on the type of reaction that the enzyme catalyzes. Enzyme Commission numbers are structured as a series of four numbers, with the first number categorizing the enzyme into one of six major classes, such as oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.

For instance, if an enzyme's commission number begins with a '1', it indicates that the enzyme is an oxidoreductase, which catalyzes oxidation-reduction reactions. This classification helps in understanding the fundamental role of the enzyme within biological systems, including the types of substrates it may act on and the specific biochemical processes it influences.