PEP group translocation, also known as the phosphotransferase system or PTS, is a distinct method used by
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
for sugar uptake where the source of energy is from
phosphoenolpyruvate
Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester derived from the enol of pyruvate and phosphate. It exists as an anion. PEP is an important intermediate in biochemistry. It has the highest-energy phosphate bond found (−61.9 kJ/ ...
(PEP). It is known to be a multicomponent system that always involves enzymes of the
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
and those in the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
.
The PTS system uses active transport. After the translocation across the membrane, the metabolites transported are modified. The 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.
Specificity
The phosphotransferase system is involved in transporting many sugars into bacteria, including
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
,
mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...
,
fructose
Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...
and
cellobiose
Cellobiose is a disaccharide with the formula (C6H7(OH)4O)2O. It is classified as a reducing sugar. In terms of its chemical structure, it is derived from the condensation of a pair of β-glucose molecules forming a β(1→4) bond. It can be hyd ...
. PTS sugars can differ between bacterial groups, mirroring the most suitable carbon sources available in the environment every group evolved. In ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'', 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.
Mechanism
The
phosphoryl {{unreferenced, date=May 2015
A phosphoryl group is the chemical ion or radical: P+O32−, containing phosphorus and oxygen. (The correct chemical name for this −PO32− group is phosphonato, and phosphono for −PO3H2; as ''phosphoryl'' in ch ...
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 (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...
residue, whereas in the Enzyme E II B (EIIB) the phosphoryl group is usually transferred to a
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
residue and rarely to a histidine.
![Phosphotransferase system](https://upload.wikimedia.org/wikipedia/commons/7/7d/Phosphotransferase_system.svg)
In the process of glucose PTS transport specific of
enteric bacteria
Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mic ...
, 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
juxtamembraneEIIB. Finally, EIIB phosphorylates glucose as it crosses the plasma membrane through the
transmembrane Enzyme II C (EIIC), forming
glucose-6-phosphate
Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose sugar phosphorylated at the hydroxy group on carbon 6. This dianion is very common in cells as the majority of glucose entering a cell will become phosphorylated in this wa ...
.
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.
Specificity. 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.
Regulation. 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 (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
. Intracellular
cyclic AMP
Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal transd ...
levels rise and this then activates CAP (
catabolite activator protein
Catabolite activator protein (CAP; also known as cAMP receptor protein, CRP) is a trans-acting transcriptional activator that exists as a homodimer in solution. Each subunit of CAP is composed of a ligand-binding domain at the N-terminus (CAPN, re ...
), which is involved in the
catabolite repression Carbon catabolite repression, or simply catabolite repression, is an important part of global control system of various bacteria and other microorganisms. Catabolite repression allows microorganisms to adapt quickly to a preferred (rapidly metaboli ...
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 (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
,
glycerol kinase
Glycerol kinase, encoded by the gene ''GK'', 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 3-phosph ...
,
lactose permease
Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as lactose in t ...
, and
maltose permease
}
Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two ...
. 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.
Structural analysis
Three-dimensional structures of examples of all the soluble, cytoplasmic complexes of the PTS were solved by
G. Marius Clore
G. Marius Clore MAE, FRSC, FRS is a British-born, Anglo-American molecular biophysicist and structural biologist. He was born in London, U.K. and is a dual US/U.K. Citizen. He is a Member of the National Academy of Sciences, a Fellow of the ...
using multidimensional
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
spectroscopy, and led to significant insights into how
signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...
proteins recognize multiple, structurally dissimilar partners by generating similar binding surfaces from completely different structural elements, making use of large binding surfaces with intrinsic redundancy, and exploiting side chain conformational plasticity.
References
External links
*
{{Multienzyme complexes
Enzymes