Synthesis of phosphoenol pyruvate (PEP) analogues and evaluation as inhibitors of PEP-utilizing enzymes

The synthesis of 10 new phosphoenolpyruvate (PEP) analogues with modifications in the phosphate and the carboxylate function is described. Included are two potential irreversible inhibitors of PEP-utilizing enzymes. One incorporates a reactive chloromethylphosphonate function replacing the phosphate group of PEP. The second contains a chloromethyl group substituting for the carboxylate function of PEP. An improved procedure for the preparation of the known (Z)- and (E)-3-chloro-PEP is also given. The isomers were obtained as a 4 : 1 mixture, resolved by anion-exchange chromatography after the last reaction step. The stereochemistry of the two isomers was unequivocally assigned from the (3)J(H-C) coupling constants between the carboxylate carbons and the vinyl protons. All of these and other known PEP-analogues were tested as reversible and irreversible inhibitors of Mg2+- and Mn2+- activated PEP-utilizing enzymes: enzyme I of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), pyruvate kinase, PEP carboxylase and enolase. Without exception, the most potent inhibitors were those with substitution of a vinyl proton. Modification of the phosphate and the carboxylate groups resulted in less effective compounds. Enzyme I was the least tolerant to such modifications. Among the carboxylate-modified analogues, only those replaced by a negatively charged group inhibited pyruvate kinase and enolase. Remarkably, the activity of PEP carboxylase was stimulated by derivatives with neutral groups at this position in the presence of Mg2+, but not with Mn2+. For the irreversible inhibition of these enzymes, (Z)-3-Cl-PEP was found to be a very fast-acting and efficient suicide inhibitor of enzyme I (t(1/2) = 0.7 min).

Mechanism-based inhibition of enzyme I of the Escherichia coli phosphotransferase system. Cysteine 502 is an essential residue

Four phosphoenolpyruvate (PEP) derivatives, carrying reactive or activable chemical functions in each of the three chemical regions of PEP, were assayed as alternative substrates of enzyme I (EI) of the Escherichia coli PEP:glucose phosphotransferase system. The Z- and E-isomers of 3-chlorophosphoenolpyruvate (3-Cl-PEP) were substrates, presenting K(m) values of 0.08 and 0.12 mm, respectively, very similar to the K(m) of 0.14 mm measured for PEP, and k(cat) of 40 and 4 min(-1), compared with 2,200 min(-1), for PEP. The low catalytic efficiency of these substrates permits the study of activity at in vivo EI concentrations. Z-Cl-PEP was a competitive inhibitor of PEP with a K(I) of 0.4 mm. E-Cl-PEP was not an inhibitor. Compounds 3 and 4, obtained by modification of the carboxylic and phosphate groups of PEP, were neither substrates nor inhibitors of EI, highlighting the importance of these functionalities for recognition by EI. Z-Cl-PEP is a suicide inhibitor. About 10-50 turnovers sufficed to inactivate EI completely. Such a property can be exploited to reveal and quantitate phosphoryl transfer from EI to other proteins at in vivo concentrations. Inactivation was saturatable in Z-Cl-PEP, with an apparent K(m)(inact) of 0.2-0.4 mm. The rate of inactivation increased with the concentration of EI, indicating a preferential or exclusive reaction with the dimeric form of EI. E-Cl-PEP inactivates EI much more slowly, and unlike PEP, it did not protect against inactivation by Z-Cl-PEP. This and the ineffectiveness of E-Cl-PEP as a competitive inhibitor have been related to the presence of two EI active species. Cys-502 of EI was identified by mass spectrometry as the reacting residue. The C502A EI mutant showed less than 0.06% wild-type activity. Sequence alignments and comparisons of x-ray structures of different PEP-utilizing enzymes indicate that Cys-502 might serve as a proton donor during catalysis.

A reporter gene assay for inhibitors of the bacterial phosphoenolpyruvate: sugar phosphotransferase system

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) plays a key role in sugar uptake and metabolic regulation in bacteria. PTS proteins form a divergent phosphorylation cascade. Enzyme I (EI) is at the top of the cascade and mediates phosphoryl-transfer from phosphoenolpyruvate to the phosphoryl-carrier protein HPr, which then distributes the phosphoryl-groups to the different carbohydrate transporters. In addition, some PTS proteins have a regulatory function in catabolite repression, inducer exclusion and chemotaxis which is modulated by their degree of phosphorylation in response to the availability of substrates. Using as a reporter the IacZ gene under control of the bgl t2 transcriptional terminator and as an effector the transcriptional antiterminator LicT from B. subtilis, a two-plasmid reporter gene system was constructed in order to monitor PTS activity. LicT, when present at low concentration in E. coli, is inactivated by EI/HPr-dependent phosphorylation and conversely is active in a ptsl- mutant lacking El. Active LicT allows for transcriptional readthrough at bgl t2, resulting in a full-length lacZ transcript. Beta-galactosidase activities are increased 4-8-fold in a ptsl+ strain growing on PTS substrates relative to growth on non-PTS substrates and approximately 30-fold in a ptsl- mutant. This gain-of-function in response to dephosphorylation of El or lack of active El can be used to monitor changes of El activity caused by mutations and environmental factors and for screening and validation of inhibitors of the PTS as potentially novel antibacterial compounds.

Enzyme I(Ntr) from Escherichia coli. A novel enzyme of the phosphoenolpyruvate-dependent phosphotransferase system exhibiting strict specificity for its phosphoryl acceptor, NPr

The phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) phosphorylates sugars and regulates cellular metabolic processes using a phosphoryl transfer chain including the general energy coupling proteins, Enzyme I (EI) and HPr as well as the sugar-specific Enzyme II complexes. Analysis of the Escherichia coli genome has revealed the presence of 5 paralogues of EI and 5 paralogues of HPr, most of unknown function. The ptsP gene encodes an EI paralogue designated Enzyme I(nitrogen) (EI(Ntr)), and two genes located in the rpoN operon encode PTS protein paralogues, NPr and IIA(Ntr), both implicated in the regulation of sigma(54) activity. The ptsP gene was polymerase chain reaction amplified from the E. coli chromosome and cloned into an overexpression vector allowing the overproduction and purification of EI(Ntr). EI(Ntr) was shown to phosphorylate NPr in vitro using either a [(32)P]PEP-dependent protein phosphorylation assay or a quantitative sugar phosphorylation assay. EI(Ntr) phosphorylated NPr but not HPr, whereas Enzyme I exhibited a strong preference for HPr. These two pairs of proteins (EI(Ntr)/NPr and EI/HPr) thus exhibit little cross-reactivity. Phosphoryl transfer from PEP to NPr catalyzed by EI(Ntr) has a pH optimum of 8.0, is dependent on Mg(2+), is stimulated by high ionic strength, and exhibits two K(m) values for NPr (2 and 10 microM) possibly because of negative cooperativity. The results suggest that E. coli possesses at least two distinct PTS phosphoryl transfer chains, EI(Ntr) --> NPr --> IIA(Ntr) and EI --> HPr --> IIA(sugar). Sequence comparisons allow prediction of residues likely to be important for specificity. This is the first report demonstrating specificity at the level of the energy coupling proteins of the PTS.

Identification of peptides inhibiting enzyme I of the bacterial phosphotransferase system using combinatorial cellulose-bound peptide libraries

The phosphoenolpyruvate(P-pyruvate)-dependent sugar phosphotransferase system (PTS) is a transport and signal-transduction system which is almost ubiquitous in bacteria but does not occur in eucaryotes. It catalyzes the uptake and phosphorylation of carbohydrates and is involved in signal transduction, e.g. catabolite repression, chemotaxis, and allosteric regulation of metabolic enzymes and transporters. EI (Enzyme I of the PTS) is the first and central component of the divergent PTS (P-pyruvate-dependent sugar phosphotransferase system) phosphorylation cascade. Using immobilized combinatorial peptide libraries and phosphorimaging, heptapeptides and octapeptides were identified which selectively inhibit EI in vitro. The IC50 of the best peptides is 30 microM which is close to the K(M) (6 microM) of EI for its natural substrate HPr (histidine containing phosphoryl carrier protein of the PTS). The affinity-selected peptides are better inhibitors than a peptide with the active-site sequence of HPr. The selected peptides contain several basic residues and one aromatic residue which do not occur in the active site of HPr. The large proportion of basic residues most likely reflects charge complementarity to the strongly acidic active-site pocket of EI. Guanidino groups might facilitate by complexation of the phosphoryl group the slow phosphorylation of the peptide.