SummaryBacterial peptidoglycan (PG or murein) is a single, large, covalently cross-linked macromolecule and forms a mesh-like sacculus that completely encases the cytoplasmic membrane. Hence, growth of a bacterial cell is intimately coupled to expansion of murein sacculus and requires cleavage of pre-existing crosslinks for incorporation of new murein material. Although, conceptualized nearly five decades ago, the mechanism of such essential murein cleavage activity has not been studied so far. Here, we identify three new murein hydrolytic enzymes in Escherichia coli, two (Spr and YdhO) belonging to the NlpC/P60 peptidase superfamily and the third (YebA) to the lysostaphin family of proteins that cleave peptide cross-bridges between glycan chains. We show that these hydrolases are redundantly essential for bacterial growth and viability as a conditional mutant lacking all the three enzymes is unable to incorporate new murein and undergoes rapid lysis upon shift to restrictive conditions. Our results indicate the step of cross-link cleavage as essential for enlargement of the murein sacculus, rendering it a novel target for development of antibacterial therapeutic agents.

The oxygen reduction reaction (ORR) is a core reaction for electrochemical energy technologies such as fuel cells and metal–air batteries. ORR catalysts have been limited to platinum, which meets the requirements of high activity and durability. Over the last few decades, a variety of materials have been tested as non‐Pt catalysts, from metal–organic complex molecules to metal‐free catalysts. In particular, nitrogen‐doped graphitic carbon materials, including N‐doped graphene and N‐doped carbon nanotubes, have been extensively studied. However, due to the lack of understanding of the reaction mechanism and conflicting knowledge of the catalytic active sites, carbon‐based catalysts are still under the development stage of achieving a performance similar to Pt‐based catalysts. In addition to the catalytic viewpoint, designing mass transport pathways is required for O2. Recently, the importance of pyridinic N for the creation of active sites for ORR and the requirement of hydrophobicity near the active sites have been reported. Based on the increased knowledge in controlling ORR performances, bottom‐up preparation of N‐doped carbon catalysts, using N‐containing conjugative molecules as the assemblies of the catalysts, is promising. Here, the recent understanding of the active sites and the mechanism of ORRs on N‐doped carbon catalysts are reviewed.

Bacterial growth and morphogenesis are intimately coupled to expansion of peptidoglycan (PG), an extensively cross-linked macromolecule that forms a protective mesh-like sacculus around the cytoplasmic membrane. Growth of the PG sacculus is a dynamic event requiring the concerted action of hydrolases that cleave the cross-links for insertion of new material and synthases that catalyze cross-link formation; however, the factors that regulate PG expansion during bacterial growth are poorly understood. Here, we show that the PG hydrolase MepS (formerly Spr), which is specific to cleavage of cross-links during PG expansion in Escherichia coli, is modulated by proteolysis. Using combined genetic, molecular, and biochemical approaches, we demonstrate that MepS is rapidly degraded by a proteolytic system comprising an outer membrane lipoprotein of unknown function, NlpI, and a periplasmic protease, Prc (or Tsp). In summary, our results indicate that the NlpI-Prc system contributes to growth and enlargement of the PG sacculus by modulating the cellular levels of the cross-link-cleaving hydrolase MepS. Overall, this study signifies the importance of PG cross-link cleavage and its regulation in bacterial cell wall biogenesis.bacterial morphogenesis | peptidoglycan | regulated proteolysis | MepS | NlpI-Prc P eptidoglycan (PG or murein) is a unique and essential constituent of eubacterial cell walls, thus making it an excellent target for several antimicrobial agents. It is a single, large, extensively cross-linked macromolecule that forms a mesh-like sacculus protecting cells against intracellular turgor pressure in addition to conferring cell shape. Structurally, the PG sacculus is made up of linear glycan strands cross-linked to each other by short peptide chains forming a continuous layer around the cytoplasmic membrane. The glycan strands are made up of alternating N-acetyl muramic acid (NAM) and N-acetyl glucosamine (NAG) disaccharide units in which NAM is covalently attached to a peptide chain containing 2-to 5-amino acid residues, with the pentapeptide consisting of L-alanine (ala)−D-glutamic acid (glu)−meso-diaminopimelic acid (mDAP)−D-ala−D-ala. Normally, D-ala of one peptide chain is cross-linked to mDAP of another peptide chain of an adjacent glycan strand, resulting in an extensively cross-linked single-or multilayered sacculus (1).Because the murein sacculus totally encircles the cytoplasmic membrane, growth of a cell is tightly coupled to expansion of PG. Growth of the PG sacculus is a dynamic and coordinated event requiring concerted action both of murein hydrolases that facilitate cleavage of cross-links for the insertion of nascent murein material and of synthases that catalyze cross-link formation between adjacent glycan strands (Fig. 1) (2, 3).Escherichia coli encodes multiple PG synthases that catalyze the formation of D-ala−mDAP cross-links in the PG sacculus. The class I enzymes (PBP1a and PBP1b, encoded by mrcA and mrcB, respectively) are bifunctional and possess both glycosyl transferase (GT) and tr...

Covalent organic frameworks (COFs) have structures and morphologies closely resembling graphenes, whose modular construction permits atomiclevel manipulations. This, combined with their porous structure, makes them excellent catalyst supports. Here, the high electrocatalytic activity of a composite, formed by supporting Ni 3 N nanoparticles on a benzimidazole COF, for oxygen evolution reaction is shown. The composite oxidizes alkaline water with a near-record low overpotential of 230 mV @ 10 mA cm −2 (η 10 ). This high activity is attributed to the ability of the COF to confine the Ni 3 N nanoparticles to size regimes otherwise difficult to obtain and to its low band gap character (1.49 eV) arising from the synergy between the conducting Ni 3 N nanoparticles and the π-conjugated COF. The COF itself, as a metalfree self-standing framework, has an oxygen evolution reaction activity with η 10 of 400 mV. The periodic structure of the COF makes it serve as a matrix to disperse the catalytically active Ni 3 N nanoparticles favoring their high accessibility and thereby good charge-transport within the composite. This is evident from the amount of O 2 evolved (230 mmol h −1 g −1 ), which, to the best of our knowledge, is the highest reported. The work reveals the emergence of COF as supports for electrocatalysts.

Open-shell phenalenyl chemistry started more than half a century back, and the first solid-state phenalenyl radical was realized only 15 years ago highlighting the synthetic challenges associated in stabilizing carbon-based radical chemistry, though it has great promise as building blocks for molecular electronics and multifunctional materials. Alternatively, stable closed-shell phenalenyl has tremendous potential as it can be utilized to create an in situ open-shell state by external spin injection. In the present study, we have designed a closed-shell phenalenyl-based iron(III) complex, Fe(III)(PLY)3 (PLY-H = 9-hydroxyphenalenone) displaying an excellent electrocatalytic property as cathode material for one compartment membraneless H2O2 fuel cell. The power density output of Fe(III)(PLY)3 is nearly 15-fold higher than the structurally related model compound Fe(III)(acac)3 (acac = acetylacetonate) and nearly 140-fold higher than an earlier reported mononuclear Fe(III) complex, Fe(III)(Pc)Cl (Pc = pthalocyaninate), highlighting the role of switchable closed-shell phenalenyl moiety for electron-transfer process in designing electroactive materials.