The human enzymes. The figure shows the major families of membrane proteins that primarily function as enzymes, proteins that catalyze a chemical reaction. The enzymes were divided into six major classes, depending of the character of the chemical reaction they are involved in. This was performed according to the EC system. Each of the six classes is represented by a tree structure, showing some of the hierarchical order of subclasses within each class. The number of genes for each class and subclass, and function, have been determined by combining results from clustering, using the ISODATA algorithm, with data from the literature and public databases. Primarily the BRENDA database was used. Consensus TM helix numbers have been set through evaluation of data from external resources among the literature and databases together with prediction results from Phobius, SOSUI, and TMHMM. The box contains proteins that belong to more than one class.