Alginate-based diblock polymers: preparation, characterization and Ca-induced self-assembly

Renewable resources can provide a range of different polysaccharide blocks that can be used to prepare new types of stimuli-responsive polysaccharide-based block copolymers. Alginates are natural polysaccharides widely used as biomaterials. Functional properties depend on the content and distribution of the two 4-linked monomers (β-D-mannuronate (M) and α-L-guluronate (G)). Blocks of L-guluronate (Gn) are responsible for cooperative binding of calcium ions and hydrogel formation. Incorporation of such blocks in block polysaccharide copolymers would represent a new class of engineered, Ca-sensitive biomacromolecules. Dioxyamines and dihydrazides have recently been shown to be well suited for preparation of block polysaccharide structures. Here we first show that when applied to alginate blocks (Gn and Mn) the two types are both very reactive, but the detailed distribution of acyclic (E)- and (Z)-forms and cyclic N-pyranosides, reaction kinetics, conjugate stability, and the rate of Schiff base reduction with α-picoline borane differ considerably, also compared to other polysaccharides. Hence, alginate specific protocols were developed. The linkers introduce a highly flexible joint in otherwise semiflexible Gn-based diblocks. This was demonstrated by SEC-MALS using a symmetrical Gn-b-Gn diblock, which in solution can best be described according to a broken rod model. Ca-Induced self-assembly of Gn-b-dextran diblocks was studied by dynamic light scattering, demonstrating that well defined nanoparticles could be prepared for certain combinations of chain lengths. Taken together, this approach provides a new class of engineered, stimuli-responsive block polysaccharide copolymers solely based on natural resources

Structural and mutational characterization of the catalytic A-module of the mannuronan C-5-epimerase AlgE4 from Azotobacter vinelandii

Alginate is a family of linear copolymers of (1→4)-linked β-d-mannuronic acid and its C-5 epimer α-l-guluronic acid. The polymer is first produced as polymannuronic acid and the guluronic acid residues are then introduced at the polymer level by mannuronan C-5-epimerases. The structure of the catalytic A-module of the Azotobacter vinelandii mannuronan C-5-epimerase AlgE4 has been determined by x-ray crystallography at 2.1-Å resolution. AlgE4A folds into a right-handed parallel β-helix structure originally found in pectate lyase C and subsequently in several polysaccharide lyases and hydrolases. The β-helix is composed of four parallel β-sheets, comprising 12 complete turns, and has an amphipathic α-helix near the N terminus. The catalytic site is positioned in a positively charged cleft formed by loops extending from the surface encompassing Asp(152), an amino acid previously shown to be important for the reaction. Site-directed mutagenesis further implicates Tyr(149), His(154), and Asp(178) as being essential for activity. Tyr(149) probably acts as the proton acceptor, whereas His(154) is the proton donor in the epimerization reaction

NMR assignments of 1H, 13C and 15N resonances of the C-terminal subunit from Azotobacter vinelandii mannuronan C5-epimerase 6 (AlgE6R3)

The 19.9 kDa C-terminal module (R3) from Azotobacter vinelandii mannronan C5-epimerase AlgE6 has been 13C, 15N isotopically labelled and recombinantly expressed. We report here the 1H, 13C, 15N resonance assignment of AlgE6R3

Bi-functional alginate oligosaccharide–polymyxin conjugates for improved treatment of multidrug-resistant gram-negative bacterial infections

The recent emergence of resistance to colistin, an antibiotic of last resort with dose-limiting toxicity, has highlighted the need for alternative approaches to combat infection. This study aimed to generate and characterise alginate oligosaccharide (“OligoG”)–polymyxin (polymyxin B and E (colistin)) conjugates to improve the effectiveness of these antibiotics. OligoG–polymyxin conjugates (amide- or ester-linked), with molecular weights of 5200–12,800 g/mol and antibiotic loading of 6.1–12.9% w/w, were reproducibly synthesised. In vitro inflammatory cytokine production (tumour necrosis factor alpha (TNFα) ELISA) and cytotoxicity (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) of colistin (2.2–9.3-fold) and polymyxin B (2.9–27.2-fold) were significantly decreased by OligoG conjugation. Antimicrobial susceptibility tests (minimum inhibitory concentration (MIC), growth curves) demonstrated similar antimicrobial efficacy of ester- and amide-linked conjugates to that of the parent antibiotic but with more sustained inhibition of bacterial growth. OligoG–polymyxin conjugates exhibited improved selectivity for Gram-negative bacteria in comparison to mammalian cells (approximately 2–4-fold). Both OligoG–colistin conjugates caused significant disruption of Pseudomonas aeruginosa biofilm formation and induced bacterial death (confocal laser scanning microscopy). When conjugates were tested in an in vitro “time-to-kill” (TTK) model using Acinetobacter baumannii, only ester-linked conjugates reduced viable bacterial counts (~2-fold) after 4 h. Bi-functional OligoG–polymyxin conjugates have potential therapeutic benefits in the treatment of multidrug-resistant (MDR) Gram-negative bacterial infections, directly reducing toxicity whilst retaining antimicrobial and antibiofilm activities

The Structure and Regulation of Human Muscle α-Actinin

SummaryThe spectrin superfamily of proteins plays key roles in assembling the actin cytoskeleton in various cell types, crosslinks actin filaments, and acts as scaffolds for the assembly of large protein complexes involved in structural integrity and mechanosensation, as well as cell signaling. α-actinins in particular are the major actin crosslinkers in muscle Z-disks, focal adhesions, and actin stress fibers. We report a complete high-resolution structure of the 200 kDa α-actinin-2 dimer from striated muscle and explore its functional implications on the biochemical and cellular level. The structure provides insight into the phosphoinositide-based mechanism controlling its interaction with sarcomeric proteins such as titin, lays a foundation for studying the impact of pathogenic mutations at molecular resolution, and is likely to be broadly relevant for the regulation of spectrin-like proteins

Solution Structure of Selenoprotein W and NMR Analysis of Its Interaction with 14-3-3 Proteins

Selenium is a trace element with significant biomedical potential. It is essential in mammals due to its occurrence in several proteins in the form of selenocysteine (Sec). One of the most abundant mamma¬lian Sec-containing proteins is selenoprotein W (SelW). This protein of unknown function has a broad expression pattern and contains a candidate CXXU (where U represents Sec) redox motif. Here, we re¬port the solution structure of the Sec13 →Cys variant of mouse SelW determined through high resolution NMR spectroscopy. The protein has a thioredoxin-like fold with the CXXU motif located in an exposed loop similarly to the redox-active site in thioredoxin. Protein dynam¬ics studies revealed the rigidity of the protein backbone and mobility of two external loops and suggested a role of these loops in interaction with SelW partners. Molecular modeling of structures of other mem¬bers of the Rdx family based on the SelW structure identified new con¬served features in these proteins, including an aromatic cluster and in¬teracting loops. Our previous study suggested an interaction between SelW and 14-3-3 proteins. In the present work, with the aid of NMR spectroscopy, we demonstrated specificity of this interaction and iden¬tified mobile loops in SelW as interacting surfaces. This finding sug¬gests that 14-3-3 are redox-regulated proteins

NMR assignments of \u3csup\u3e1\u3c/sup\u3eH, \u3csup\u3e13\u3c/sup\u3eC and \u3csup\u3e15\u3c/sup\u3eN spectra of methionine sulfoxide reductase B1 from \u3ci\u3eMus musculus\u3c/i\u3e

Isotopically labeled, 15N and 15N/13C forms of recombinant methionine-R-sulfoxide reductase 1 (MsrB1, SelR) from Mus musculus were produced, in which catalytic selenocysteine was replaced with cysteine. We report here the 1H, 15N and 13C NMR assignment of the reduced form of this mammalian protein