Stearate-Coated Biogenic Calcium Carbonate from Waste Seashells: A Sustainable Plastic Filler.

Waste seashells from aquaculture are a massive source of biogenic calcium carbonate (bCC) that can be a potential substitute for ground calcium carbonate and precipitated calcium carbonate. These last materials find several applications in industry after a surface coating with hydrophobic molecules, with stearate as the most used. Here, we investigate for the first time the capability of aqueous stearate dispersions to coat bCC powders from seashells of market-relevant mollusc aquaculture species, namely the oyster Crassostrea gigas, the scallop Pecten jacobaeus, and the clam Chamelea gallina. The chemical-physical features of bCC were extensively characterized by different analytical techniques. The results of stearate adsorption experiments showed that the oyster shell powder, which is the bCC with a higher content of the organic matrix, showed the highest adsorption capability (about 23 wt % compared to 10 wt % of geogenic calcite). These results agree with the mechanism proposed in the literature in which stearate adsorption mainly involves the formation of calcium stearate micelles in the dispersion before the physical adsorption. The coated bCC from oyster shells was also tested as fillers in an ethylene vinyl acetate compound used for the preparation of shoe soles. The obtained compound showed better mechanical performance than the one prepared using ground calcium. In conclusion, we can state that bCC can replace ground and precipitated calcium carbonate and has a higher stearate adsorbing capability. Moreover, they represent an environmentally friendly and sustainable source of calcium carbonate that organisms produce by high biological control over composition, polymorphism, and crystal texture. These features can be exploited for applications in fields where calcium carbonate with selected features is required.

Structural and biochemical characterization of Arabidopsis alcohol dehydrogenases reveals distinct functional properties but similar redox sensitivity.

Alcohol dehydrogenases (ADHs) are a group of zinc-binding enzymes belonging to the medium-length dehydrogenase/reductase (MDR) protein superfamily. In plants, these enzymes fulfill important functions involving the reduction of toxic aldehydes to the corresponding alcohols (as well as catalyzing the reverse reaction, i.e., alcohol oxidation; ADH1) and the reduction of nitrosoglutathione (GSNO; ADH2/GSNOR). We investigated and compared the structural and biochemical properties of ADH1 and GSNOR from Arabidopsis thaliana. We expressed and purified ADH1 and GSNOR and determined two new structures, NADH-ADH1 and apo-GSNOR, thus completing the structural landscape of Arabidopsis ADHs in both apo- and holo-forms. A structural comparison of these Arabidopsis ADHs revealed a high sequence conservation (59% identity) and a similar fold. In contrast, a striking dissimilarity was observed in the catalytic cavity supporting substrate specificity and accommodation. Consistently, ADH1 and GSNOR showed strict specificity for their substrates (ethanol and GSNO, respectively), although both enzymes had the ability to oxidize long-chain alcohols, with ADH1 performing better than GSNOR. Both enzymes contain a high number of cysteines (12 and 15 out of 379 residues for ADH1 and GSNOR, respectively) and showed a significant and similar responsivity to thiol-oxidizing agents, indicating that redox modifications may constitute a mechanism for controlling enzyme activity under both optimal growth and stress conditions.

Nanocrystalline and Amorphous Calcium Carbonate from Waste Seashells by Ball Milling Mechanochemistry Processes.

Nanocrystalline calcium carbonate (CaCO3) and amorphous CaCO3 (ACC) are materials of increasing technological interest. Nowadays, they are mainly synthetically produced by wet reactions using CaCO3 reagents in the presence of stabilizers. However, it has recently been discovered that ACC can be produced by ball milling calcite. Calcite and/or aragonite are the mineral phases of mollusk shells, which are formed from ACC precursors. Here, we investigated the possibility to convert, on a potentially industrial scale, the biogenic CaCO3 (bCC) from waste mollusk seashells into nanocrystalline CaCO3 and ACC. Waste seashells from the aquaculture species, namely oysters (Crassostrea gigas, low-Mg calcite), scallops (Pecten jacobaeus, medium-Mg calcite), and clams (Chamelea gallina, aragonite) were used. The ball milling process was carried out by using different dispersing solvents and potential ACC stabilizers. Structural, morphological, and spectroscopic characterization techniques were used. The results showed that the mechanochemical process produced a reduction of the crystalline domain sizes and formation of ACC domains, which coexisted in microsized aggregates. Interestingly, bCC behaved differently from the geogenic CaCO3 (gCC), and upon long milling times (24 h), the ACC reconverted into crystalline phases. The aging in diverse environments of mechanochemically treated bCC produced a mixture of calcite and aragonite in a species-specific mass ratio, while the ACC from gCC converted only into calcite. In conclusion, this research showed that bCC can produce nanocrystalline CaCO3 and ACC composites or mixtures having species-specific features. These materials can enlarge the already wide fields of applications of CaCO3, which span from medical to material science.

Characterization of chloroplast ribulose-5-phosphate-3-epimerase from the microalga Chlamydomonas reinhardtii.

Carbon fixation relies on Rubisco and 10 additional enzymes in the Calvin-Benson-Bassham cycle. Epimerization of xylulose-5-phosphate (Xu5P) into ribulose-5-phosphate (Ru5P) contributes to the regeneration of ribulose-1,5-bisphosphate, the substrate of Rubisco. Ribulose-5-phosphate-3-epimerase (RPE, EC 5.1.3.1) catalyzes the formation of Ru5P, but it can also operate in the pentose-phosphate pathway by catalyzing the reverse reaction. Here, we describe the structural and biochemical properties of the recombinant RPE isoform 1 from Chlamydomonas (Chlamydomonas reinhardtii) (CrRPE1). The enzyme is a homo-hexamer that contains a zinc ion in the active site and exposes a catalytic pocket on the top of an α8ß8 triose isomerase-type barrel as observed in structurally solved RPE isoforms from both plant and non-plant sources. By optimizing and developing enzyme assays to monitor the reversible epimerization of Ru5P to Xu5P and vice versa, we determined the catalytic parameters that differ from those of other plant paralogs. Despite being identified as a putative target of multiple thiol-based redox modifications, CrRPE1 activity is not affected by both reductive and oxidative treatments, indicating that enzyme catalysis is insensitive to possible redox alterations of cysteine residues. We mapped phosphorylation sites on the crystal structure, and the specific location at the entrance of the catalytic cleft supports a phosphorylation-based regulatory mechanism. This work provides an accurate description of the structural features of CrRPE1 and an in-depth examination of its catalytic and regulatory properties highlighting the physiological relevance of this enzyme in the context of photosynthetic carbon fixation.

Assembly of the Intraskeletal Coral Organic Matrix during Calcium Carbonate Formation.

Scleractinia coral skeleton formation occurs by a heterogeneous process of nucleation and growth of aragonite in which intraskeletal soluble organic matrix molecules, usually referred to as SOM, play a key role. Several studies have demonstrated that they influence the shape and polymorphic precipitation of calcium carbonate. However, the structural aspects that occur during the growth of aragonite have received less attention. In this research, we study the deposition of calcium carbonate on a model substrate, silicon, in the presence of SOM extracted from the skeleton of two coral species representative of different living habitats and colonization strategies, which we previously characterized. The study is performed mainly by grazing incidence X-ray diffraction with the support of Raman spectroscopy and electron and optical microscopies. The results show that SOM macromolecules once adsorbed on the substrate self-assembled in a layered structure and induced the oriented growth of calcite, inhibiting the formation of vaterite. Differently, when SOM macromolecules were dispersed in solution, they induced the deposition of amorphous calcium carbonate (ACC), still preserving a layered structure. The entity of these effects was species-dependent, in agreement with previous studies. In conclusion, we observed that in the setup required by the experimental procedure, the SOM from corals appears to present a 2D lamellar structure. This structure is preserved when the SOM interacts with ACC but is lost when the interaction occurs with calcite. This knowledge not only is completely new for coral biomineralization but also has strong relevance in the study of biomineralization on other organisms.

Conformational Disorder Analysis of the Conditionally Disordered Protein CP12 from Arabidopsis thaliana in Its Different Redox States.

CP12 is a redox-dependent conditionally disordered protein universally distributed in oxygenic photosynthetic organisms. It is primarily known as a light-dependent redox switch regulating the reductive step of the metabolic phase of photosynthesis. In the present study, a small angle X-ray scattering (SAXS) analysis of recombinant Arabidopsis CP12 (AtCP12) in a reduced and oxidized form confirmed the highly disordered nature of this regulatory protein. However, it clearly pointed out a decrease in the average size and a lower level of conformational disorder upon oxidation. We compared the experimental data with the theoretical profiles of pools of conformers generated with different assumptions and show that the reduced form is fully disordered, whereas the oxidized form is better described by conformers comprising both the circular motif around the C-terminal disulfide bond detected in previous structural analysis and the N-terminal disulfide bond. Despite the fact that disulfide bridges are usually thought to confer rigidity to protein structures, in the oxidized AtCP12, their presence coexists with a disordered nature. Our results rule out the existence of significant amounts of structured and compact conformations of free AtCP12 in a solution, even in its oxidized form, thereby highlighting the importance of recruiting partner proteins to complete its structured final folding.

Ribulose-1,5-bisphosphate regeneration in the Calvin-Benson-Bassham cycle: Focus on the last three enzymatic steps that allow the formation of Rubisco substrate.

The Calvin-Benson-Bassham (CBB) cycle comprises the metabolic phase of photosynthesis and is responsible for carbon fixation and the production of sugar phosphates. The first step of the cycle involves the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) which catalyzes the incorporation of inorganic carbon into 3-phosphoglyceric acid (3PGA). The following steps include ten enzymes that catalyze the regeneration of ribulose-1,5-bisphosphate (RuBP), the substrate of Rubisco. While it is well established that Rubisco activity acts as a limiting step of the cycle, recent modeling studies and experimental evidence have shown that the efficiency of the pathway is also impacted by the regeneration of the Rubisco substrate itself. In this work, we review the current understanding of the structural and catalytic features of the photosynthetic enzymes that catalyze the last three steps of the regeneration phase, namely ribose-5-phosphate isomerase (RPI), ribulose-5-phosphate epimerase (RPE), and phosphoribulokinase (PRK). In addition, the redox- and metabolic-based regulatory mechanisms targeting the three enzymes are also discussed. Overall, this review highlights the importance of understudied steps in the CBB cycle and provides direction for future research aimed at improving plant productivity.

Recovering and Exploiting Aragonite and Calcite Single Crystals with Biologically Controlled Shapes from Mussel Shells.

Control over the shape and morphology of single crystals is a theme of great interest in fundamental science and for technological application. Many synthetic strategies to achieve this goal are inspired by biomineralization processes. Indeed, organisms are able to produce crystals with high fidelity in shape and morphology utilizing macromolecules that act as modifiers. An alternative strategy can be the recovery of crystals from biomineralization products, in this case, seashells. In particular, waste mussel shells from aquaculture are considered. They are mainly built up of single crystals of calcite fibers and aragonite tablets forming an outer and an inner layer, respectively. A simple mechanochemical treatment has been developed to separate and recover these two typologies of single crystals. The characterization of these single crystals showed peculiar properties with respect to the calcium carbonate from quarry or synthesis. We exploited these biomaterials in the water remediation field using them as substrate adsorbing dyes. We found that these substrates show a high capability of adsorption for anionic dye, such as Eosin Y, but a low capability of adsorption for cationic dyes, such as Blue Methylene. The adsorption was reversible at pH 5.6. This application represents just an example of the potential use of these biogenic single crystals. We also envision potential applications as reinforcing fillers and optical devices.

Unravelling the regulation pathway of photosynthetic AB-GAPDH.

Oxygenic phototrophs perform carbon fixation through the Calvin-Benson cycle. Different mechanisms adjust the cycle and the light-harvesting reactions to rapid environmental changes. Photosynthetic glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a key enzyme in the cycle. In land plants, different photosynthetic GAPDHs exist: the most abundant isoform is formed by A2B2 heterotetramers and the least abundant by A4 homotetramers. Regardless of the subunit composition, GAPDH is the major consumer of photosynthetic NADPH and its activity is strictly regulated. While A4-GAPDH is regulated by CP12, AB-GAPDH is autonomously regulated through the C-terminal extension (CTE) of its B subunits. Reversible inhibition of AB-GAPDH occurs via the oxidation of a cysteine pair located in the CTE and the substitution of NADP(H) with NAD(H) in the cofactor-binding site. These combined conditions lead to a change in the oligomerization state and enzyme inhibition. SEC-SAXS and single-particle cryo-EM analysis were applied to reveal the structural basis of this regulatory mechanism. Both approaches revealed that spinach (A2B2)n-GAPDH oligomers with n = 1, 2, 4 and 5 co-exist in a dynamic system. B subunits mediate the contacts between adjacent tetramers in A4B4 and A8B8 oligomers. The CTE of each B subunit penetrates into the active site of a B subunit of the adjacent tetramer, which in turn moves its CTE in the opposite direction, effectively preventing the binding of the substrate 1,3-bisphosphoglycerate in the B subunits. The whole mechanism is made possible, and eventually controlled, by pyridine nucleotides. In fact, NAD(H), by removing NADP(H) from A subunits, allows the entrance of the CTE into the active site of the B subunit, hence stabilizing inhibited oligomers.

Calcium carbonate: controlled synthesis, surface functionalization, and nanostructured materials.

Calcium carbonate (CaCO3) is an important inorganic mineral in biological and geological systems. Traditionally, it is widely used in plastics, papermaking, ink, building materials, textiles, cosmetics, and food. Over the last decade, there has been rapid development in the controlled synthesis and surface modification of CaCO3, the stabilization of amorphous CaCO3 (ACC), and CaCO3-based nanostructured materials. In this review, the controlled synthesis of CaCO3 is first examined, including Ca2+-CO32- systems, solid-liquid-gas carbonation, water-in-oil reverse emulsions, and biomineralization. Advancing insights into the nucleation and crystallization of CaCO3 have led to the development of efficient routes towards the controlled synthesis of CaCO3 with specific sizes, morphologies, and polymorphs. Recently-developed surface modification methods of CaCO3 include organic and inorganic modifications, as well as intensified surface reactions. The resultant CaCO3 can then be further engineered via template-induced biomineralization and layer-by-layer assembly into porous, hollow, or core-shell organic-inorganic nanocomposites. The introduction of CaCO3 into nanostructured materials has led to a significant improvement in the mechanical, optical, magnetic, and catalytic properties of such materials, with the resultant CaCO3-based nanostructured materials showing great potential for use in biomaterials and biomedicine, environmental remediation, and energy production and storage. The influences that the preparation conditions and additives have on ACC preparation and stabilization are also discussed. Studies indicate that ACC can be used to construct environmentally-friendly hybrid films, supramolecular hydrogels, and drug vehicles. Finally, the existing challenges and future directions of the controlled synthesis and functionalization of CaCO3 and its expanding applications are highlighted.

Structural snapshots of nitrosoglutathione binding and reactivity underlying S-nitrosylation of photosynthetic GAPDH.

S-nitrosylation is a redox post-translational modification widely recognized to play an important role in cellular signaling as it can modulate protein function and conformation. At the physiological level, nitrosoglutathione (GSNO) is considered the major physiological NO-releasing compound due to its ability to transfer the NO moiety to protein thiols but the structural determinants regulating its redox specificity are not fully elucidated. In this study, we employed photosynthetic glyceraldehyde-3-phosphate dehydrogenase from Chlamydomonas reinhardtii (CrGAPA) to investigate the molecular mechanisms underlying GSNO-dependent thiol oxidation. We first observed that GSNO causes reversible enzyme inhibition by inducing S-nitrosylation. While the cofactor NADP+ partially protects the enzyme from GSNO-mediated S-nitrosylation, protein inhibition is not observed in the presence of the substrate 1,3-bisphosphoglycerate, indicating that the S-nitrosylation of the catalytic Cys149 is responsible for CrGAPA inactivation. The crystal structures of CrGAPA in complex with NADP+ and NAD+ reveal a general structural similarity with other photosynthetic GAPDH. Starting from the 3D structure, we carried out molecular dynamics simulations to identify the protein residues involved in GSNO binding. The reaction mechanism of GSNO with CrGAPA Cys149 was investigated by quantum mechanical/molecular mechanical calculations, which permitted to disclose the relative contribution of protein residues in modulating the activation barrier of the trans-nitrosylation reaction. Based on our findings, we provide functional and structural insights into the response of CrGAPA to GSNO-dependent regulation, possibly expanding the mechanistic features to other protein cysteines susceptible to be oxidatively modified by GSNO.

Multiscale analysis on otolith structural features reveals differences in ontogenesis and sex in Merluccius merluccius in the western Adriatic Sea.

Otolith biomineralization results from biochemical processes regulated by the interaction of internal (physiological) and external (environmental) factors which lead to morphological and ultrastructural variability at intra- and interspecific levels. The aim of this study was to conduct a multi-scale analysis of the sagittal otoliths of the Merlucius merlucius (European hake) from the western Adriatic Sea in order to correlate otolith features with fish ontogeny and sex. We show that otoliths of sexually undifferentiated (non-sexed) individuals having a fish body total length (TL) less than 15 cm had faster growth in length, width, area, perimeter, volume and weight and a higher amount of organic matrix compared with otoliths of sexually differentiated individuals (females and males) having a fish size range of 15-50 cm. Most importantly, with increasing fish TL, female saccular otoliths contained a higher number of protuberances and rougher surface compared with male specimens, which showed more uniform mean curvature density. The differences between females and males discovered in this study could be associated with fish hearing adaptation to reproductive behavioural strategies during the spawning season. The outcomes of this research provide insights on how size and sex-related variations in otolith features may be affected by fish ecological and behavioural patterns.

Coral micro- and macro-morphological skeletal properties in response to life-long acclimatization at CO2 vents in Papua New Guinea.

This study investigates the effects of long-term exposure to OA on skeletal parameters of four tropical zooxanthellate corals naturally living at CO2 seeps and adjacent control sites from two locations (Dobu and Upa Upasina) in the Papua New Guinea underwater volcanic vent system. The seeps are characterized by seawater pH values ranging from 8.0 to about 7.7. The skeletal porosity of Galaxea fascicularis, Acropora millepora, massive Porites, and Pocillopora damicornis was higher (up to ~ 40%, depending on the species) at the seep sites compared to the control sites. Pocillopora damicornis also showed a decrease of micro-density (up to ~ 7%). Thus, further investigations conducted on this species showed an increase of the volume fraction of the larger pores (up to ~ 7%), a decrease of the intraskeletal organic matrix content (up to ~ 15%), and an increase of the intraskeletal water content (up to ~ 59%) at the seep sites. The organic matrix related strain and crystallite size did not vary between seep and control sites. This multi-species study showed a common phenotypic response among different zooxanthellate corals subjected to the same environmental pressures, leading to the development of a more porous skeletal phenotype under OA.

The skeleton of Balanophyllia coral species suggests adaptive traits linked to the onset of mixotrophy.

The diversity in the skeletal features of coral species is an outcome of their evolution, distribution and habitat. Here, we explored, from macro- to nano-scale, the skeletal structural and compositional characteristics of three coral species belonging to the genus Balanophyllia having different trophic strategies. The goal is to address whether the onset of mixotrophy influenced the skeletal features of B. elegans, B. regia, and B. europaea. The macroscale data suggest that the presence of symbiotic algae in B. europaea can lead to a surplus of energy input that increases its growth rate and skeletal bulk density, leading to larger and denser corals compared to the azooxanthellate ones, B. regia and B. elegans. The symbiosis would also explain the higher intra-skeletal organic matrix (OM) content, which is constituted by macromolecules promoting the calcification, in B. europaea compared to the azooxanthellate species. The characterization of the soluble OM also revealed differences between B. europaea and the azooxanthellate species, which may be linked to diverse macromolecular machineries responsible for skeletal biosynthesis and final morphology. Differently, the crystallographic features were homogenous among species, suggesting that the basic building blocks of skeletons remained a conserved trait in these related species, regardless of the trophic strategy. These results show changes in skeletal phenotype that could be triggered by the onset of mixotrophy, as a consequence of the symbiotic association, displaying remarkable plasticity of coral skeletons which repeatedly allowed this coral group to adapt to a range of changing environments throughout its geological history.

Wavy graphene sheets from electrochemical sewing of corannulene.

The presence of non-hexagonal rings in the honeycomb carbon arrangement of graphene produces rippled graphene layers with valuable chemical and physical properties. In principle, a bottom-up approach to introducing distortion from planarity of a graphene sheet can be achieved by careful insertion of curved polyaromatic hydrocarbons during the growth of the lattice. Corannulene, the archetype of such non-planar polyaromatic hydrocarbons, can act as an ideal wrinkling motif in 2D carbon nanostructures. Herein we report an electrochemical bottom-up method to obtain egg-box shaped nanographene structures through a polycondensation of corannulene that produces a new conducting layered material. Characterization of this new polymeric material by electrochemistry, spectroscopy, electron microscopy (SEM and TEM), scanning probe microscopy, and laser desorption-ionization time of flight mass spectrometry provides strong evidence that the anodic polymerization of corannulene, combined with electrochemically induced oxidative cyclodehydrogenations (Scholl reactions), leads to polycorannulene with a wavy graphene-like structure.

Calvin-Benson cycle regulation is getting complex.

Oxygenic phototrophs use the Calvin-Benson cycle to fix CO2 during photosynthesis. In the dark, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK), two enzymes of the Calvin-Benson cycle, form an inactive complex with the regulatory protein CP12, mainly under the control of thioredoxins and pyridine nucleotides. In the light, complex dissociation allows GAPDH and PRK reactivation. The GAPDH/CP12/PRK complex is conserved from cyanobacteria to angiosperms and coexists in land plants with an autoassembling GAPDH that is analogously regulated. With the recently described 3D structures of PRK and GAPDH/CP12/PRK, the structural proteome of this ubiquitous regulatory system has been completed. This outcome opens a new avenue for understanding the regulatory potential of photosynthetic carbon fixation by laying the foundation for its knowledge-based manipulation.

Climate variation during the Holocene influenced the skeletal properties of Chamelea gallina shells in the North Adriatic Sea (Italy).

Understanding how marine taxa will respond to near-future climate changes is one of the main challenges for management of coastal ecosystem services. Ecological studies that investigate relationships between the environment and shell properties of commercially important marine species are commonly restricted to latitudinal gradients or small-scale laboratory experiments. This paper aimed to explore the variations in shell features and growth of the edible bivalve Chamelea gallina from the Holocene sedimentary succession to present-day thanatocoenosis of the Po Plain-Adriatic Sea system (Italy). Comparing the Holocene sub-fossil record to modern thanatocoenoses allowed obtaining an insight of shell variations dynamics on a millennial temporal scale. Five shoreface-related assemblages rich in C. gallina were considered: two from the Middle Holocene, when regional sea surface temperatures were higher than today, representing a possible analogue for the near-future global warming, one from the Late Holocene and two from the present-day. We investigated shell biometry and skeletal properties in relation to the valve length of C. gallina. Juveniles were found to be more porous than adults in all horizons. This suggested that C. gallina promoted an accelerated shell accretion with a higher porosity and lower density at the expense of mechanically fragile shells. A positive correlation between sea surface temperature and both micro-density and bulk density were found, with modern specimens being less dense, likely due to lower aragonite saturation state at lower temperature, which could ultimately increase the energetic costs of shell formation. Since no variation was observed in shell CaCO3 polymorphism (100% aragonite) or in compositional parameters among the analyzed horizons, the observed dynamics in skeletal parameters are likely not driven by a diagenetic recrystallization of the shell mineral phase. This study contributes to understand the response of C. gallina to climate-driven environmental shifts and offers insights for assessing anthropogenic impacts on this economic relevant species.

Structural and functional insights into nitrosoglutathione reductase from Chlamydomonas reinhardtii.

Protein S-nitrosylation plays a fundamental role in cell signaling and nitrosoglutathione (GSNO) is considered as the main nitrosylating signaling molecule. Enzymatic systems controlling GSNO homeostasis are thus crucial to indirectly control the formation of protein S-nitrosothiols. GSNO reductase (GSNOR) is the key enzyme controlling GSNO levels by catalyzing its degradation in the presence of NADH. Here, we found that protein extracts from the microalga Chlamydomonas reinhardtii catabolize GSNO via two enzymatic systems having specific reliance on NADPH or NADH and different biochemical features. Scoring the Chlamydomonas genome for orthologs of known plant GSNORs, we found two genes encoding for putative and almost identical GSNOR isoenzymes. One of the two, here named CrGSNOR1, was heterologously expressed and purified. Its kinetic properties were determined and the three-dimensional structures of the apo-, NAD+- and NAD+/GSNO-forms were solved. These analyses revealed that CrGSNOR1 has a strict specificity towards GSNO and NADH, and a conserved folding with respect to other plant GSNORs. The catalytic zinc ion, however, showed an unexpected variability of the coordination environment. Furthermore, we evaluated the catalytic response of CrGSNOR1 to thermal denaturation, thiol-modifying agents and oxidative modifications as well as the reactivity and position of accessible cysteines. Despite being a cysteine-rich protein, CrGSNOR1 contains only two solvent-exposed/reactive cysteines. Oxidizing and nitrosylating treatments have null or limited effects on CrGSNOR1 activity and folding, highlighting a certain resistance of the algal enzyme to redox modifications. The molecular mechanisms and structural features underlying the response to thiol-based modifications are discussed.

Influence of proteins on mechanical properties of a natural chitin-protein composite.

In many biogenic materials, chitin chains are assembled in fibrils that are wrapped by a protein fold. In them, the mechanical properties are supposed to be related to intra- and inter- interactions among chitin and proteins. This hypothesis has been poorly investigated. Here, this research theme is studied using the pen of Loligo vulgaris as a model material of chitin-protein composites. Chemical treatments were used to change the interactions involving only the proteic phase, through unfolding and/or degradation processes. Successively, structural and mechanical parameters were examined using spectroscopy, microscopy, X-ray diffractometry, and tensile tests. The data analysis showed that chemical treatments did not modify the structure of the chitin matrix. This allowed to derive from the mechanical test analysis the following conclusions: (i) the maximum stress (σmax) relies on the presence of the disulfide bonds; (ii) the Young's modulus (E) relies on the overall correct folding of the proteins; (iii) the whole removal of proteins induces a decrease of E (> 90%) and σmax (> 80%), and an increase in the maximum elongation. These observations indicate that in the chitin matrix the proteins act as a strengthener, which efficacy is controlled by the presence of disulfide bridges. This reinforcement links the chitin fibrils avoiding them to slide one on the other and maximizing their resistance and stiffness. In conclusion, this knowledge can explain the physio-chemical properties of other biogenic polymeric composites and inspire the design of new materials. STATEMENT OF SIGNIFICANCE: To date, no study has addressed on how proteins influence chitin-composite material's mechanical properties. Here we show that the Young's modulus and the maximum stress mainly rely on protein disulfide bonds, the inter-proteins ones and those controlling the folding of chitin-binding domains. The removal of protein matrix induce a reduction of Young's modulus and maximum stress, leaving the chitin matrix structurally unaltered. The measure of the maximum elongation shows that the chitin fibrils slide on each other only after removing the protein matrix. In conclusion, this research shows that the proteins act as a stiff matrix reinforced by di-sulfide bridges that link crystalline chitin fibrils avoiding them to slide one on the other.

Hierarchical chitinous matrices byssus-inspired with mechanical properties tunable by Fe(III) and oxidation.

In this study the multi-scale hierarchical structure of the ß-chitin matrix from squid pen of Loligo vulgaris was used as substrate to synthesize new bio-inspired materials. Aiming to mimic the byssus peculiar mechanical properties, we chemically functionalized the ß-chitin matrix with catechols, one of the main functional groups of the byssus. The obtained matrix preserved its multi-scale structural organization and was able to chelate reversibly Fe(III). Thus, it behaved as the byssus, acting as a metal cross-linkable matrix that upon metalation increased its Young's modulus, E (>10 times). The functionalized matrix was also cross-linked by oxidation provoking an increase of the E (>10 times) and first failure stress (>5 times). The oxidation of the functionalized matrix followed by metalation slightly increased the material mechanical properties. In conclusion, we added specific bio-functionalities in a natural matrix tuning its mechanical properties without altering its multi-scale organization.