Streptococcus mutans SMU.623c codes for a functional, metal-dependent polysaccharide deacetylase that modulates interactions with salivary agglutinin.

The genome sequence of the oral pathogen Streptococcus mutans predicts the presence of two putative polysaccharide deacetylases. The first, designated PgdA in this paper, shows homology to the catalytic domains of peptidoglycan deacetylases from Streptococcus pneumoniae and Listeria monocytogenes, which are both thought to be involved in the bacterial defense mechanism against human mucosal lysozyme and are part of the CAZY family 4 carbohydrate esterases. S. mutans cells in which the pgdA gene was deleted displayed a different colony texture and a slightly increased cell surface hydrophobicity and yet did not become hypersensitive to lysozyme as shown previously for S. pneumoniae. To understand this apparent lack of activity, the high-resolution X-ray structure of S. mutans PgdA was determined; it showed the typical carbohydrate esterase 4 fold, with metal bound in a His-His-Asp triad. Analysis of the protein surface showed that an extended groove lined with aromatic residues is orientated toward the active-site residues. The protein exhibited metal-dependent de-N-acetylase activity toward a hexamer of N-acetylglucosamine. No activity was observed toward shorter chitooligosaccharides or a synthetic peptidoglycan tetrasaccharide. In agreement with the lysozyme data this would suggest that S. mutans PgdA does not act on peptidoglycan but on an as-yet-unidentified polysaccharide within the bacterial cell surface. Strikingly, the pgdA-knockout strain showed a significant increase in aggregation/agglutination by salivary agglutinin, in agreement with this gene acting as a deacetylase of a cell surface glycan.

Kinetic, inhibition and structural studies on 3-oxoacyl-ACP reductase from Plasmodium falciparum, a key enzyme in fatty acid biosynthesis.

Type II fatty acid biosynthesis represents an attractive target for the discovery of new antimalarial drugs. Previous studies have identified malarial ENR (enoyl acyl-carrier-protein reductase, or FabI) as the target for the antiseptic triclosan. In the present paper, we report the biochemical properties and 1.5 A (1 A=0.1 nm) crystal structure of OAR (3-oxoacyl acyl-carrier-protein reductase, or FabG), the second reductive step in fatty acid biosynthesis and its inhibition by hexachlorophene. Under optimal conditions of pH and ionic strength, Plasmodium falciparum OAR displays kinetic properties similar to those of OAR from bacteria or plants. Activity with NADH is <3% of that with NADPH. Fluorescence enhancement studies indicate that NADPH can bind to the free enzyme, consistent with kinetic and product inhibition studies suggesting a steady-state ordered mechanism. The crystal structure reveals a tetramer with a sulphate ion bound in the cofactor-binding site such that the side chains of the catalytic triad of serine, tyrosine and lysine are aligned in an active conformation, as previously observed in the Escherichia coli OAR-NADP+ complex. A cluster of positively charged residues is positioned at the entrance to the active site, consistent with the proposed recognition site for the physiological substrate (3-oxoacyl-acyl-carrier protein) in E. coli OAR. The antibacterial and anthelminthic agent hexachlorophene is a potent inhibitor of OAR (IC50 2.05 microM) displaying non-linear competitive inhibition with respect to NADPH. Hexachlorophene (EC50 6.2 microM) and analogues such as bithionol also have antimalarial activity in vitro, suggesting they might be useful leads for further development.

Analogues of thiolactomycin as potential antimalarial agents.

Analogues of the natural antibiotic thiolactomycin (TLM), an inhibitor of the condensing reactions of type II fatty acid synthase, were synthesized and evaluated for their ability to inhibit the growth of the malaria parasite Plasmodium falciparum. Alkylation of the C4 hydroxyl group led to the most significant increase in growth inhibition (over a 100-fold increase in activity compared to TLM). To investigate the mode of action, the P. falciparum KASIII enzyme was produced for inhibitor assay. A number of TLM derivatives were identified that showed improved inhibition of this enzyme compared to TLM. Structure-activity relationships for enzyme inhibition were identified for some series of TLM analogues, and these also showed weak correlation with inhibition of parasite growth, but this did not hold for other series. On the basis of the lack of a clear correlation between inhibition of pfKASIII activity and parasite growth, we conclude that pfKASIII is not the primary target of TLM analogues. Some of the analogues also inhibited the growth of the parasitic protozoa Trypanosoma cruzi, T. brucei, and Leishmania donovani.

Structural and functional characterization of a putative polysaccharide deacetylase of the human parasite Encephalitozoon cuniculi.

The microsporidian Encephalitozoon cuniculi is an intracellular eukaryotic parasite considered to be an emerging opportunistic human pathogen. The infectious stage of this parasite is a unicellular spore that is surrounded by a chitin containing endospore layer and an external proteinaceous exospore. A putative chitin deacetylase (ECU11_0510) localizes to the interface between the plasma membrane and the endospore. Chitin deacetylases are family 4 carbohydrate esterases in the CAZY classification, and several bacterial members of this family are involved in evading lysis by host glycosidases, through partial de-N-acetylation of cell wall peptidoglycan. Similarly, ECU11_0510 could be important for E. cuniculi survival in the host, by protecting the chitin layer from hydrolysis by human chitinases. Here, we describe the biochemical, structural, and glycan binding properties of the protein. Enzymatic analyses showed that the putative deacetylase is unable to deacetylate chitooligosaccharides or crystalline beta-chitin. Furthermore, carbohydrate microarray analysis revealed that the protein bound neither chitooligosaccharides nor any of a wide range of other glycans or chitin. The high resolution crystal structure revealed dramatic rearrangements in the positions of catalytic and substrate binding residues, which explain the loss of deacetylase activity, adding to the unusual structural plasticity observed in other members of this esterase family. Thus, it appears that the ECU11_0510 protein is not a carbohydrate deacetylase and may fulfill an as yet undiscovered role in the E. cuniculi parasite.

Analogues of thiolactomycin as potential anti-malarial and anti-trypanosomal agents.

A series of analogues of the naturally occurring antibiotic thiolactomycin (TLM) have been synthesised and evaluated for their ability to inhibit the growth of the malaria parasite, Plasmodium falciparum. Thiolactomycin is an inhibitor of Type II fatty acid synthase which is found in plants and most prokaryotes, but not an inhibitor of Type I fatty acid synthase in mammals. A number of the analogues showed inhibition equal to or greater than TLM. The introduction of hydrophobic alkyl groups at the C3 and C5 positions of the thiolactone ring lead to increased inhibition, the best showing a fourteenfold increase in activity over TLM. In addition, some of the analogues showed activity when assayed against the parasitic protozoa, Trypanosoma cruzi and Trypanosoma brucei.