A 7th EC Class for Translocases

Six enzyme classes have been recognized since the first Enzyme classification and nomenclature list was first approved by the International Union of Biochemistry in 1961. These were based on the type of reaction catalyzed:

Oxidoreductases (EC 1)
Transferases (EC 2)
Hydrolases (EC 3)
Lyases (EC 4)
Isomerases (EC 5)
Ligases (EC 6)

However, it has become apparent that none of these could describe the important group of enzymes that catalyse the movement of ions or molecules across membranes or their separation within membranes. Several of these involve the hydrolysis of ATP and had been previously classified as ATPases (EC 3.6.3.-), although the hydrolytic reaction is not their primary function.

These enzymes have now been classified under a new EC class of Translocases (EC 7). The reactions catalyzed are designated as transfers from ‘side 1’ to ‘side 2’ because the designations ‘in’ and ‘out’ (or ‘cis’ and ‘trans’), which had been used previously, lack clarity and can be ambiguous. The comments associated with each entry then describe the specific translocations catalysed.

The subclasses designate the types of ion or molecule translocated:

• EC 7.1 contains enzymes catalysing the translocation of hydrons (hydron being the general name for H⁺ in its natural abundance),
• EC 7.2 contains those catalysing the translocation of inorganic cations and their chelates,
• EC 7.3 contains those catalysing the translocation of inorganic anions,
• EC 7.4 contains those catalysing the translocation of amino acids and peptides,
• EC 7.5 contains those catalysing the translocation of carbohydrates and their derivatives
• EC 7.6 contains those catalysing the translocation of other compounds.

The sub-subclasses concern the reaction that provides the driving force for the translocation, where these are relevant:

• EC 7.x.1 translocations linked to oxidoreductase reactions
• EC 7.x.2 translocations linked to the hydrolysis of a nucleoside triphosphate
• EC 7.x.3 translocations linked to the hydrolysis of a diphosphate
• EC 7.x.4 translocations linked to a decarboxylation reaction

Exchange transporters that are not dependent on enzyme-catalyzed reactions, such as the exchange of ions across membranes, are not included. Pores that change conformation between open and closed states in response to phosphorylation or some other catalyzed reaction are classified under EC 5.6 (Macromolecular conformational isomerases).

(prepared by Keith Tipton, originally published at the ExplorEnz enzyme database)