The enzyme classification and nomenclature list was first approved by the International Union of Biochemistry in 1961. Six enzyme classes have been recognized based on the type of chemical reaction catalyzed, including oxidoreductases (EC 1), transferases (EC 2), hydrolases (EC 3), lyases (EC 4), isomerases (EC 5) and ligases (EC 6). However, it was not noticed back at the time that none of these classes clearly describe an important group of enzymes, which move ions or molecules across membranes or their separation within membranes. Although the hydrolytic reaction is not the primary function, several of these enzymes are involved the hydrolysis of ATP had been previously classified as ATPases (EC 3.6.3.-). Nowadays, these enzymes have been classified under a new EC class of translocases (EC 7).

Definition

Translocase is a general term for a protein that assists in moving another molecule, usually across a cell membrane. These enzymes catalyze the movement of ions or molecules across membranes or their separation within membranes. The reaction is designated as a transfer from “side 1” to “side 2” because the designations “in” and “out”, which had previously been used, can be ambiguous. Translocases are the most common secretion system in Gram positive bacteria.

Classification

The subclasses designate the types of ion or molecule translocated:

The further classification concerns the reaction that provided the driving force for the translocation:

Functions of Several Common Translocases

Translocases play an important role in biological activities because of their critical functions. For example, most of the proteins needed for mitochondrial function are encoded by the nucleus of the cell. Translocase of the outer membrane (TOM) can work in conjunction with translocase of the inner membrane (TIM) to transport proteins into the mitochondrion. In the case of preproteins, transportation into the mitochondrial matrix requires the presequence translocase of the inner membrane (TIM23 complex) and the presequence translocase-associated motor (PAM). Carnitine-acylcarnitine translocase is responsible for transporting both carnitine-fatty acid complexes and carnitine across the inner mitochondrial membrane, which is required since fatty acids cannot cross the mitochondrial membranes without assistance. ADP/ATP translocases are transporter proteins that enable the exchange of cytosolic adenosine diphosphate (ADP) and mitochondrial adenosine triphosphate (ATP) across the inner mitochondrial membrane. Free ADP is transported from the cytoplasm to the mitochondrial matrix, while ATP produced from oxidative phosphorylation is transported from the mitochondrial matrix to the cytoplasm, thus providing the cells with its main energy currency. DNA translocases are motor proteins that can convert the chemical energy of ATP hydrolysis into directional movement along DNA. Different transport enzymes play different functions in organisms, and maintain the stability of the internal environment, which is of great significance to the life activities.