We present a novel crystal structure of the IlvD/EDD family enzyme, l-arabinonate dehydratase from Rhizobium leguminosarum bv. trifolii (RlArDHT, EC 4.2.1.25), which catalyzes the conversion of l-arabinonate to 2-dehydro-3-deoxy-l-arabinonate. The enzyme is a tetramer consisting of a dimer of dimers, where each monomer is composed of two domains. The active site contains a catalytically important [2Fe-2S] cluster and Mg ion and is buried between two domains, and also at the dimer interface. The active site Lys129 was found to be carbamylated. Ser480 and Thr482 were shown to be essential residues for catalysis, and the S480A mutant structure showed an unexpected open conformation in which the active site was more accessible for the substrate. This structure showed the partial binding of l-arabinonate, which allowed us to suggest that the alkoxide ion form of the Ser480 side chain functions as a base and the [2Fe-2S] cluster functions as a Lewis acid in the elimination reaction.

Combining the star-shaped alkynyl ligands with low-nuclearity gold-copper triphosphane clusters produces 3D metallocage aggregates, which demonstrate room temperature phosphorescence in solution (max Φ =0.6). Their luminescence mainly originates from cluster-localized metal-to-ligand charge transfer excited state. These supramolecular assemblies can be easily converted into the isostructural gold-silver congeners by the direct exchange of the metal ions. Such modification of the terminal metal cores switches the emission to the intraligand (alkyne) electronic transitions of the triplet manifold, that represents an unusual optical functionality among the metallocycle/metallocage complexes.

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The ability of proteins to self--assemble into different types of fibrils with distinct morphologies has been linked to the pathological and clinical heterogeneity of amyloid diseases such as Alzheimer's and Parkinson's disease. Herein, we describe novel nanoparticles (NPs) that efficiently label amyloid fibrils produced in vitro or isolated from postmortem tissues, under hydrating conditions and in such a way to unmask their polymorphism and morphological features. Using these NPs, we show that pathological aggregates exhibit exceptional morphological homogeneity compared to amyloid fibrils produced in vitro, consistent with the emerging view that the physiologic milieu is a key determinant of amyloid fibril strains. These advances pave the way for elucidating the structural basis of amyloid strains and toxicity. AbstractThe misfolding and self--assembly of proteins into β--sheet--rich amyloid fibrils of various structures and morphologies is a hallmark of several neurodegenerative and systemic diseases. Increasing evidence suggests that amyloid polymorphism gives rise to different strains of amyloids with distinct toxicity and pathology--spreading properties. Validating this hypothesis is challenging due to a lack of tools and methods that allow for the direct characterization of amyloid polymorphism in hydrated and complex biological samples. Here, we report on the use of 11--mercapto--1--undecanesulfonate--coated gold nanoparticles (NPs) to label the edges of synthetic, recombinant and native amyloid fibrils to assess amyloid morphological polymorphism using cryogenic transmission electron microscopy (cryo--EM).The fibrils studied were derived from amyloid proteins involved in disorders of the central nervous system (amyloid--β, tau, α--synuclein) and in systemic amyloidosis (a fragment of an immunoglobulin λ light chain). The labeling efficiency enabled imaging and characterization of amyloid fibrils of different morphologies under hydrated conditions using cryo--EM. These NPs allowed for the visualization of morphological features that are not directly observed using standard imaging techniques, including TEM with use of the negative stain or cryo--EM imaging. We also demonstrate the use of these NPs to label native paired helical filaments (PHFs) from the postmortem brain of an Alzheimer's disease patient, as well as amyloid fibrils extracted from the heart tissue of a patient suffering from systemic amyloid light--chain (AL) amyloidosis. Analysis of the cryo--EM images of amyloids decorated with NPs shows

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