PEP dependent Phosphotransferase system (PTS) in E.coli
- PTS is a major carbohydrate transport system seen in bacteria. Through them bacteria transport glucose, mannose, cellobiose, fructose etc into the cell.
- PTS is a multicomponent system that 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 (Glucose is phosphorylated to glucose – 6 phosphate)
- The energy for transport is provided by PEP (phosphoenol pyruvate) – phosphate group donor
- Glucose PTS transport is specific for enteric bacteria.
Components of the PTS
Cytoplasmic phosphoryl transfer proteins: Enzyme I, HPr, Enzyme IIA (IIA) and Enzyme IIB (IIB)
Integral membrane protein: Sugar permease (EIIC)
Enzyme I and HPr: Non specific components of the system
EIIA, EIIB and EIIC: Specific components of glucose PTS
EIIC- permease through which glucose is transported
EIIB – phosphorylate glucose to glucose -6 phosphates during transport through EIIC
The phosphoryl group on PEP is eventually transferred to the imported sugar via several proteins.
There are four successive phosphoryl transfers in the PTS.
PEP transfers its phosphoryl group to a histidine residue on EI – EI transfers the phosphate to HPr – From HPr the phosphoryl group is transferred to EIIA – EIIA transfers the phosphoryl group to EIIB.
With the glucose phosphotransferase system, the phosphorylation status of EIIA can have regulatory functions.
At low glucose concentrations: Accumulation of phosphorylated EIIA
Reason: If glucose is not in the medium, it is unavailable for transport. So there is no sugar to receive the phosphate group from phosphorylated EIIA.
Phosphorylated form of EIIA activates many membrane bound enzymes (Eg: Adenylyl cyclase)
At high glucose concentration: Accumulation of dephosphorylated EIIA
Reason: When high concentration of glucose is present in the medium, continous transport occurs through the EIIC permease. During transport they receive phosphate group from EIIA and EIIA gets dephosphorylated.
Dephosphorylated form of EIIA inhibits many membrane bound enzymes (Adenylyl cyclase, glycerol kinase, lactose permease, maltose permease etc)
How does EIIA control the activity of adenylyl cyclase enzyme?
- Phosphorylated EIIA activates membrane-bound adenylyl cyclase (At low glucose conc)
(Intracellular cAMP levels rise and this then activates CAP – operon turns on)
- Dephosphorylated EIIA inhibits membrane bound adenylyl cyclase (At high glucose conc)
(Intracellular cAMP level decreases and inhibition of CAP binding to promoter – operon turns off)