PEP group translocation
Encyclopedia
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
(PEP). 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. PTS sugars can differ between bacterial groups, mirroring the most suitable carbon sources available in the environment every group evolved. 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
. The benefit of transforming glucose into glucose-6-phosphate is that it will not leak out of the cell, therefore providing a one-way concentration gradient of glucose. The HPr is common to the phosphotransferase systems of the other substrates mentioned earlier, as is the upstream EI.
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.
With the glucose phosphotransferase system, the phosphorylation status of EIIA can have regulatory functions. For example, at low glucose concentrations phosphorylated EIIA accumulates and this activates membrane-bound adenylate cyclase
. Intracellular cyclic AMP levels rise and this then activate CAP (catabolite activator protein
), which is involved in the catabolite repression
system, also known as glucose effect. When the glucose concentration is high, EIIA is mostly dephosphorylated and this allows it to inhibit adenylate cyclase
, 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.
It is an active transport. After the translocation, the metabolites transported are modified.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
for sugar uptake where the source of energy is from phosphoenolpyruvate
Phosphoenolpyruvate
Phosphoenolpyruvic acid , or phosphoenolpyruvate as the anion, is an important chemical compound in biochemistry. It has the high-energy phosphate bond found in living organisms, and is involved in glycolysis and gluconeogenesis...
(PEP). It is known as multicomponent system that always involves enzymes of the plasma membrane and those in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within 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. PTS sugars can differ between bacterial groups, mirroring the most suitable carbon sources available in the environment every group evolved. 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
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
residue, whereas in the Enzyme E II B (EIIB) the phosphoryl group is usually transferred to a cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
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
Glucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
. The benefit of transforming glucose into glucose-6-phosphate is that it will not leak out of the cell, therefore providing a one-way concentration gradient of glucose. The HPr is common to the phosphotransferase systems of the other substrates mentioned earlier, as is the upstream EI.
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.
With the glucose phosphotransferase system, the phosphorylation status of EIIA can have regulatory functions. For example, at low glucose concentrations phosphorylated EIIA accumulates and this activates membrane-bound adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
. Intracellular cyclic AMP levels rise and this then activate CAP (catabolite activator protein
Catabolite Activator Protein
Catabolite Activator Protein or CAP is a transcriptional activator that exists as a homodimer in solution, with each subunit comprising a ligand-binding domain at the N-terminus , which is also responsible for the dimerization of the protein, and a DNA-binding domain at the C-terminus...
), which is involved in the catabolite repression
Catabolite repression
Carbon catabolite repression, or simply catabolite repression, is an important part of global control system of various bacteria and other micro-organisms. Catabolite repression allows bacteria to adapt quickly to a preferred carbon and energy source first...
system, also known as glucose effect. When the glucose concentration is high, EIIA is mostly dephosphorylated and this allows it to inhibit adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
, glycerol kinase
Glycerol kinase
Glycerol kinase is a phosphotransferase enzyme involved in triglycerides and glycerophospholipids synthesis.Glycerol kinase catalyzes the transfer of a phosphate from ATP to glycerol thus forming glycerol phosphate . Adipocytes lack glycerol kinase so they cannot metabolize the glycerol produced...
, lactose permease
Lactose permease
Lactose permease has a structure that consists of two halves, each of which comprises six membrane-spanning alpha helices.The two halves are well separated and are joined by a single stretch of polypeptide....
, 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.
It is an active transport. After the translocation, the metabolites transported are modified.