Фосфотрансферазная система бактерий: механизм транспорта и фосфорилирования сахаров.

Bacterial metabolic pathway
PEP (phosphoenol pyruvate) 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 (PEP). It is known to be a multicomponent system that always involves enzymes of the plasma membrane and those in the cytoplasm. The PTS system uses active transport. After the translocation across the membrane, the metabolites transported are modified. The PTS system was discovered by Saul Roseman in 1964. The bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) transports and phosphorylates its sugar substrates in a single energy coupled step. This transport process is dependent on several cytoplasmic phosphoryl transfer proteins Enzyme I (I), HPr, Enzyme IIA (IIA), and Enzyme IIB (IIB)) as well as the integral membrane sugar permease (IIC). The PTS Enzyme II complexes are derived from independently evolving 4 PTS Enzyme II complex superfamilies, that include the (1) Glucose (Glc), (2) Mannose (Man), (3) Ascorbate Galactitol (Asc Gat) and (4) Dihydroxyacetone (DHA) superfamilies.

The phosphotransferase system is involved in transporting many sugars into bacteria, including glucose, mannose, fructose and cellobiose. PTS sugars can differ between bacterial groups, mirroring the most suitable carbon sources available in the environment every group evolved. In Escherichia coli, there are 21 different transporters (i. e. IIC proteins, sometimes fused to IIA and/or IIB proteins, see figure) which determine import specificity. Of these, 7 belong to the fructose (Fru) family, 7 belong to the glucose (Glc) family, and 7 belong to the other PTS permease families.

The phosphoryl group on PEP is eventually transferred to the imported sugar via several proteins. The phosphoryl group is transferred to the Enzyme E I (EI), Histidine Protein (HPr, Heat stable Protein) and Enzyme E II (EII) to a conserved histidine residue, whereas in the Enzyme E II B (EIIB) the phosphoryl group is usually transferred to a cysteine residue and rarely to a histidine. In the process of glucose PTS transport specific of enteric bacteria, PEP transfers its phosphoryl to a histidine residue on EI. EI in turn transfers the phosphate to HPr. From HPr the phosphoryl is transferred to EIIA. EIIA is specific for glucose and it further transfers the phosphoryl group to a juxtamembrane EIIB. Finally, EIIB phosphorylates glucose as it crosses the plasma membrane through the transmembrane enzyme II C (EIIC), forming glucose 6 phosphate. 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 recognizing the substrate again, thus maintaining a concentration gradient that favours further import of the substrate through the transporter.

In many bacteria, there are four different sets of IIA, IIB, and IIC proteins, each specific for a particular sugar (glucose, mannitol, mannose, and lactose/chitobiose). To make things more complicated, IIA may be fused to IIB to form a single protein with 2 domains, or IIB may be fused to IIC (the transporter), also with 2 domains.