PEP group translocation

PEP group translocation, also known as the phosphotransferase system or PTS, is a distinct method used by bacteria for sugar uptake where the source of energy is from phosphoenolpyruvate. It is known as multicomponent system that always involves enzymes of the plasma membrane and those in the cytoplasm. An example of this transport is found in E. coli cells. The system was discovered by Saul Roseman in 1964.

The phosphotransferase system is involved in transporting many sugars into bacteria, including glucose, mannose, fructose and cellobiose. The phosphate group on phosphoenolpyruvate (PEP) is eventually transferred to the imported sugar via several proteins. All the proteins have the phosphate group transferred to a conserved histidine residue.

In glucose transport, PEP transfers its phosphate to a histidine residue on Enzyme I. Enzyme I in turn transfers the phosphate to histidine protein (HPr). From HPr the phosphate is transferred to IIA protein. This IIA protein is specific for glucose and it transfers the phosphate to glucose, forming glucose-6-phosphate. The HPr is common to the phosphotransferase systems of the other substrates mentioned earlier, as is the upstream Enzyme I.

Proteins downstream of HPr tend to vary between the different sugars. The transfer of a phosphate group to the substrate once it has been imported through the membrane transporter prevents the transporter from recognising the substrate again, and it maintains a concentration gradient that favours further import of the substrate through the transporter.

With the glucose phosphotransferase system, the phosphorylation status of IIA can have regulatory functions. For example, at low glucose concentrations phosphorylated IIA accumulates and this activates membrane-bound adenylyl cyclase. Intracellular cyclic AMP levels rise and this then activate CAP (catabolite activator protein), which is involved in the catabolite repression system. When the glucose concentration is high, IIA is mostly dephosphorylated and this allows it to inhibit glycerol kinase, lactose permease, and maltose permease. Thus, as well as the PEP group translocation system being an efficient way to import substrates into the bacterium, it also links this transport to regulation of other relevant proteins.