A highly convergent approach for the synthesis of disaccharide repeating units of peptidoglycan

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

Assembly of Peptidoglycan Fragments-A Synthetic Challenge

Peptidoglycan (PGN) is a major constituent of most bacterial cell walls that is recognized as a primary target of the innate immune system. The availability of pure PGN molecules has become key to different biological studies. This review aims to (1) provide an overview of PGN biosynthesis, focusing on the main biosynthetic intermediates; (2) focus on the challenges for chemical synthesis posed by the unique and complex structure of PGN; and (3) cover the synthetic routes of PGN fragments developed to date. The key difficulties in the synthesis of PGN molecules mainly involve stereoselective glycosylation involving NAG derivatives. The complex synthesis of the carbohydrate backbone commonly involves multistep sequences of chemical reactions to install the lactyl moiety at the O-3 position of NAG derivatives and to control enantioselective glycosylation. Recent advances are presented and synthetic routes are described according to the main strategy used: (i) based on the availability of starting materials such as glucosamine derivatives; (ii) based on a particular orthogonal synthesis; and (iii) based on the use of other natural biopolymers as raw materials.

Revisiting peptidoglycan sensing: interactions with host immunity and beyond

The interaction between host immunity and bacterial cells plays a pivotal role in a variety of human diseases. The bacterial cell wall component peptidoglycan (PG) is known to stimulate an immune response, which makes PG a distinctive recognition element for unveiling these complicated molecular interactions. Pattern recognition receptor (PRR) proteins are among the critical components of this system that initially recognize molecular patterns associated with microorganisms such as bacteria and fungi. These molecular patterns are mostly embedded in the bacterial or fungal cell wall structure and can be released and presented to the immune system in various situations. Nonetheless, detailed knowledge of this recognition is limited due to the diversity among the PG polymer and its fragments; the subsequent responses by multiple hosts add more complexity. Here, we discuss how our understanding of the role and molecular mechanisms of the well-studied PRR, the NOD-like receptors (NLRs), in the human immune system has evolved in recent years. We highlight the instances of other classes of proteins with similar behavior in the recognition of PG that have been identified in other microorganisms such as yeasts. These proteins are particularly interesting because a network of cellular interactions exists between human host cells, bacteria and yeast as a part of the normal human flora. To support our understanding of these interactions, we provide insight into the chemist's toolbox of peptidoglycan probes that aid in the investigations of the behaviors of these proteins and other biological contexts relevant to the sensing and recognition of peptidoglycan. The importance of these interactions in human health for the development of biomarkers and biotherapy is highlighted.

A target oriented expeditious approach towards synthesis of certain bacterial rare sugar derivatives

A 3-step sequential one-pot protection profile manipulation on suitable d-glucosamine/d-mannose derivatives led to the diversity oriented synthesis of rare sugar derivatives.

The extracytoplasmic domain of the Mycobacterium tuberculosis Ser/Thr kinase PknB binds specific muropeptides and is required for PknB localization

The Mycobacterium tuberculosis Ser/Thr kinase PknB has been implicated in the regulation of cell growth and morphology in this organism. The extracytoplasmic domain of this membrane protein comprises four penicillin binding protein and Ser/Thr kinase associated (PASTA) domains, which are predicted to bind stem peptides of peptidoglycan. Using a comprehensive library of synthetic muropeptides, we demonstrate that the extracytoplasmic domain of PknB binds muropeptides in a manner dependent on the presence of specific amino acids at the second and third positions of the stem peptide, and on the presence of the sugar moiety N-acetylmuramic acid linked to the peptide. We further show that PknB localizes strongly to the mid-cell and also to the cell poles, and that the extracytoplasmic domain is required for PknB localization. In contrast to strong growth stimulation by conditioned medium, we observe no growth stimulation of M. tuberculosis by a synthetic muropeptide with high affinity for the PknB PASTAs. We do find a moderate effect of a high affinity peptide on resuscitation of dormant cells. While the PASTA domains of PknB may play a role in stimulating growth by binding exogenous peptidoglycan fragments, our data indicate that a major function of these domains is for proper PknB localization, likely through binding of peptidoglycan fragments produced locally at the mid-cell and the cell poles. These data suggest a model in which PknB is targeted to the sites of peptidoglycan turnover to regulate cell growth and cell division.

Regulation of growth by Mycobacterium tuberculosis is important in the pathogenesis of tuberculosis (TB), including asymptomatic latent TB infection and active TB disease. The M. tuberculosis kinase PknB regulates cell growth and cell division by phosphorylating proteins involved in these processes to modify their function. The activity of PknB is thought to respond to extracellular stimuli by binding specific molecules with its extracytoplasmic domain. In this work we show that cell wall fragments bind to this domain, and that strong binding requires that these interacting molecules have specific molecular features. We demonstrate that a peptidoglycan fragment that binds strongly can stimulate growth of dormant bacteria, but that it does not affect growth of non-dormant bacteria. We also show that PknB localizes to the site of cell division and to the growing tip of the bacterium, where cell wall synthesis and degradation occur, and that the extracytoplasmic domain is required for this localization. These findings indicate that a major function of the extracytoplasmic domain of PknB is to place it at the sites of cell wall turnover, and suggest a model by which PknB can regulate growth and cell division, and thereby contribute to the pathogenesis of TB.

Binding and Cellular Activation Studies Reveal That Toll-like Receptor 2 Can Differentially Recognize Peptidoglycan from Gram-positive and Gram-negative Bacteria

Although much progress has been made toward the identification of innate immune receptors, far less is known about how these receptors recognize specific microbial products. Such studies have been hampered by the need to purify compounds from microbial sources and a reliance on biological assays rather than direct binding to monitor recognition. We have employed surface plasmon resonance (SPR) binding studies using a wide range of well defined synthetic muropeptides derived from Gram-positive (lysine-containing) and Gram-negative (diaminopimelic acid (DAP)-containing) bacteria to demonstrate that Toll-like receptor 2 can recognize peptidoglycan (PGN). In the case of lysine-containing muropeptides, a limited number of compounds, which were derived from PGN remodeled by bacterial autolysins, was recognized. However, a wider range of DAP-containing muropeptides was bound with high affinity, and these compounds were derived from nascent and remodeled PGN. The difference in recognition of the two classes of muropeptides is proposed to be a strategy by the host to respond appropriately to Gram-negative and -positive bacteria, which produce vastly different quantities of PGN. It was also found that certain modifications of the carboxylic acids of isoglutamine and DAP can dramatically reduce binding, and thus, bacterial strains may employ such modifications to evade innate immune detection. Cellular activation studies employing highly purified PGN from Bacillus licheniformis, Bacillus subtilis, Escherichia coli, Lactobacillus plantarum, Micrococcus luteus, and Staphylococcus aureus support the structure binding relationship. The data firmly establish Toll-like receptor 2 as an innate immune sensor for PGN and provides an understanding of host-pathogen interactions at the molecular level.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Modification of the structure of peptidoglycan is a strategy to avoid detection by nucleotide-binding oligomerization domain protein 1

Nucleotide-binding oligomerization domain (NOD) protein 1 (NOD1) and NOD2 are pathogen recognition receptors that sense breakdown products of peptidoglycan (PGN) (muropeptides). It is shown that a number of these muropeptides can induce tumor necrosis factor alpha (TNF-alpha) gene expression without significant TNF-alpha translation. This translation block is lifted when the muropeptides are coincubated with lipopolysaccharide (LPS), thereby accounting for an apparently synergistic effect of the muropeptides with LPS on TNF-alpha protein production. The compounds that induced synergistic effects were also able to activate NF-kappaB in a NOD1- or NOD2-dependent manner, implicating these proteins in synergistic TNF-alpha secretion. It was found that a diaminopimelic acid (DAP)-containing muramyl tetrapeptide could activate NF-kappaB in a NOD1-dependent manner, demonstrating that an exposed DAP is not essential for NOD1 sensing. The activity was lost when the alpha-carboxylic acid of iso-glutamic acid was modified as an amide. However, agonists of NOD2, such as muramyl dipeptide and lysine-containing muramyl tripeptides, were not affected by amidation of the alpha-carboxylic acid of iso-glutamic acid. Many pathogens modify the alpha-carboxylic acid of iso-glutamic acid of PGN, and thus it appears this is a strategy to avoid recognition by the host innate immune system. This type of immune evasion is in particular relevant for NOD1.

Iterative one-pot syntheses of chitotetroses

Rapid syntheses of chitotetrose derivatives were achieved in good yields using the newly developed reactivity independent iterative one-pot strategy. The protective groups on donors and acceptors were independently evaluated allowing matching of the two partners in glycosylation. No anomeric reactivity adjustments or intermediate purification were necessary thus significantly improving the overall synthetic efficiency. Only near stoichiometric amounts of building blocks were required for the assembly of target molecules further highlighting the potential of the iterative one-pot method in complex oligosaccharide synthesis.

Synthesis and proinflammatory properties of muramyl tripeptides containing lysine and diaminopimelic acid moieties

The unusual amino acid diaminopimelic acid (DAP) was prepared by cross metathesis of appropriately protected vinyl glycine and allyl glycine derivatives. Catalytic hydrogenation of the cross-coupling product resulted in reduction of the double bond and the removal of protecting groups. The resulting compounds were appropriately protected for the polymer-supported and solution-phase synthesis of muramyl tripeptides 2 and 3, which differ in the amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties. Muramyl dipeptide (1, MDP), the DAP-containing muramyl tripeptide 3, and the lysine-containing muramyl tripeptides 4 and 5 induced TNF-alpha gene expression without TNF-alpha protein production in a human monocytic cell line. The observed block in translation could be removed by co-incubation with LPS, resulting in an apparent synergistic effect. Compound 2 did not induce TNF-alpha gene expression, neither did it exhibit a synergistic effect with LPS; this indicates that amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties results in a loss of biological activity. It is proposed that amidation of alpha-carboxylic acids is a strategy that may be used by pathogens to avoid detection by the innate immune system. Furthermore, the pattern recognition receptors Nod1 and Nod2 have been implicated in the possible induction of a synergistic effect of muropeptides with LPS.

Selective recognition of synthetic lysine and meso-diaminopimelic acid-type peptidoglycan fragments by human peptidoglycan recognition proteins I{alpha} and S

The interactions of a range of synthetic peptidoglycan derivatives with PGRP-Ialpha and PGRP-S have been studied in real-time using surface plasmon resonance. A dissociation constant of K(D) = 62 mum was obtained for the interaction of peptidoglycan recognition protein (PGRP)-Ialpha with the lysine-containing muramyl pentapeptide (compound 6). The normalized data for the lysine-containing muramyl tetra- (compound 5) and pentapeptide (compound 6) showed that these compounds have similar affinities, whereas a much lower affinity for muramyl tripeptide (compound 3) was measured. Similar affinities were obtained when the lysine moiety of the muramyl peptides was replaced by meso-diaminopimelic acid (DAP). Furthermore, the compounds that contained only a stem peptide (pentapeptide, compound 1) and (DAP-PP, compound 2) as well as muramyldipeptide (compound 3) exhibited no binding indicating that the muramyltripeptide (compound 4) is the smallest peptidoglycan fragment that can be recognized by PGRP-Ialpha. Surprisingly, PGRP-S derived significantly higher affinities for the DAP-containing fragments to similar lysine-containing derivatives, and the following dissociation constants were measured: muramylpentapeptide-DAP, K(D) = 104 nm; muramyltetrapeptide-DAP, 92.4 nm; and muramyltripeptide-DAP, 326 nm. The binding profiles were rationalized by using a recently reported x-ray crystal structure of PGRP-Ialpha with the lysine-containing muramyltripeptide (4).