Which class of enzyme is indicated by a first number of 6 in the Enzyme Commission number?

The correct class of enzyme indicated by a first number of 6 in the Enzyme Commission number is ligases. This classification is based on the systematic nomenclature established by the Enzyme Commission, where the first digit in the Enzyme Commission (EC) number specifies the main class of the enzyme.

Ligases are enzymes that catalyze the joining of two molecules with the concomitant hydrolysis of a diphosphate bond in ATP or a similar triphosphate. This activity is critical for many biological processes, including the synthesis of nucleic acids and proteins, where the formation of covalent bonds between substrates is necessary.

In contrast, transferases, which are indicated by a first number of 2, transfer functional groups from one molecule to another. Hydrolases, starting with a 3, catalyze the hydrolysis of various bonds, and isomerases, marked with a 5, facilitate the rearrangement of atoms within a molecule. Each of these classes has distinct enzymatic functions and characteristics that clearly differentiate them from ligases.