The first number in the Enzyme Commission (EC) number indicates the class of the enzyme based on its catalytic activity. A first number of 4 identifies the enzyme as a lyase. Lyases are enzymes that catalyze the breaking of chemical bonds by means other than hydrolysis or oxidation, often facilitating the addition of a group to a double bond or the formation of a double bond by the removal of a group.
For example, lyases can break bonds by forming a double bond or can add groups to double bonds, which is essential in various metabolic pathways. Understanding the function of lyases is crucial in biochemistry and can help in comprehending how these enzymes contribute to processes such as the decarboxylation of acids or the synthesis of new compounds.
In contrast, the other classes represented by different first numbers include transferases (which are involved in transferring functional groups between molecules), ligases (which catalyze the joining of two molecules, often with the concomitant hydrolysis of ATP), and hydrolases (which catalyze the hydrolysis of various bonds). Each class has distinct characteristics and functions, which are critical in biochemistry and molecular biology but do not pertain to the classification of lyases.