Structure, biosynthesis and possible function of tunichromes and related compounds

Catecholamine Derivatives as Novel Crosslinkers for the Synthesis of Versatile Biopolymers

Catecholamine metabolites are not only involved in primary metabolism, but also in secondary metabolism, serving a diverse array of physiologically and biochemically important functions. Melanin, which originates from dopa and dopamine, found in the hair, eye, and skin of all animals, is an important biopolymeric pigment. It provides protection against damaging solar radiation to animals. N-Acetyldopamine and N-β-alanyldopamine play a crucial role in the hardening of the exoskeletons of all insects. In addition, insects and other arthropods utilize the melanogenic process as a key component of their defense systems. Many marine organisms utilize dopyl peptides and proteins as bonding materials to adhere to various substrata. Moreover, the complex dopa derivatives that are precursors to the formation of the exoskeletons of numerous marine organisms also exhibit antibiotic properties. The biochemistry and mechanistic transformations of different catecholamine derivatives to produce various biomaterials with antioxidant, antibiotic, crosslinking, and gluing capabilities are highlighted. These reactivities are exhibited through the transient and highly reactive quinones, quinone methides, and quinone methide imine amide intermediates, as well as chelation to metal ions. A careful consideration of the reactivities summarized in this review will inspire numerous strategies for synthesizing novel biomaterials for future medical and industrial use.

A Molecular Modeling Investigation of the Therapeutic Potential of Marine Compounds as DPP-4 Inhibitors

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder characterized by elevated levels of blood glucose due to insulin resistance or insulin-secretion defects. The development of diabetes is mainly attributed to the interaction of several complex pathogenic, genetic, environmental and metabolic processes. Dipeptidyl peptidase-4 (DPP-4) is a serine protease that cleaves X-proline dipeptides from the N-terminus of several polypeptides, including natural hypoglycemic incretin hormones. Inhibition of this enzyme restores and maintains glucose homeostasis, making it an attractive drug target for the management of T2DM. Natural products are important sources of bioactive agents for anti-T2DM drug discovery. Marine ecosystems are a rich source of bioactive products and have inspired the development of drugs for various human disorders, including diabetes. Here, structure-based virtual screening and molecular docking were performed to identify antidiabetic compounds from the Comprehensive Marine Natural Products Database (CMNPD). The binding characteristics of two shortlisted compounds, CMNPD13046 and CMNPD17868, were assessed using molecular dynamics simulations. Thus, this study provides insights into the potential antidiabetic activity and the underlying molecular mechanism of two compounds of marine origin. These compounds could be investigated further for the development of potent DPP-4 inhibitors.

Evaluation of the Physicochemical, Antioxidant, and Antibacterial Properties of Tunichrome Released from Phallusia nigra Persian Gulf Marine Tunicate

The aim of this study was to evaluate the physicochemical, nutraceutical, antioxidant, and antibacterial properties of tunichrome released from Persian Gulf tunicate (Phallusia nigra). For this purpose, molecular weight (SDS-PAGE), amino acid profile, chemical composition (GC-MS), mineral composition, functional groups (FTIR), total phenol content (TPC), total flavonoid content (TFC), antioxidant activity, and antimicrobial properties were investigated. The results showed that tunichrome contained a high amount of essential amino acids (i.e., Lys = 32.24 mg/100 g) and essential minerals. According to GC-MS results, tunichrome had different antioxidant and antimicrobial components. The TPC and TFC of tunichrome were 0.55 mg GA/g and 0.21 mg quercetin/100 g, respectively. Tunichrome showed higher antioxidant activity than ascorbic acid, and its radical scavenging activity values were increased from 30.28 to 82.08% by increasing concentration from 50 to 200 ppm. Inhibition zones of Staphylococcus aureus, Bacillus cereus, Salmonella enterica, and Escherichia coli O157:H7 were 14, 18, 17, and 15 mm, respectively. Moreover, the minimum inhibitory concentration values of tunichrome for S. aureus, Bacillus cereus, S. enterica, and E. coli O157:H7 were 1.17, 0.59, 0.59, and 1.17 mg/ml, respectively. The minimum bacterial concentrations were 2.34, 1.17, 1.17, and 2.34 mg/ml for S. aureus, Bacillus cereus, S. enterica, and E. coli O157:H7, respectively. These results showed that tunichrome of Phallusia nigra has excellent biological effects as a bioactive compound for food fortification.

Density Functional Theory-Based Calculation Shed New Light on the Bizarre Addition of Cysteine Thiol to Dopaquinone

Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of l-cysteine, DQ immediately binds to the –SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. l-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of l-cysteine thiolate (Cys–S⁻) to DQ. Interestingly, the C2–S bonded intermediate was less energetically stable than the C6–S bonded case. Furthermore, the most preferred Cys–S⁻-attacked intermediate is at the carbon-carbon bridge between the two carbonyls (C3–C4 bridge site) but not on the C5 site. This structure allows the Cys–S⁻ to migrate onto the adjacent C5 or C2 with small activation energies. Further simulation demonstrated a possible conversion pathway of the C5–S (and C2–S) intermediate into 5-S-cysteinyldopa (and 2-S-cysteinyldopa), which is the experimentally identified major (and minor) product. Based on the results, we propose that the binding of Cys–S⁻ to DQ proceeds via the following path: (i) coordination of Cys–S⁻ to C3–C4 bridge, (ii) migration of Cys–S⁻ to C5 (C2), (iii) proton rearrangement from cysteinyl –NH₃⁺ to O4 (O3), and (iv) proton rearrangement from C5 (C2) to O3 (O4).

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

The enduring legacy of Koji Nakanishi's research on bioorganic chemistry and natural products. Part 1: Isolation, structure determination and mode of action

In this brief review on Koji Nakanishi's remarkable career in natural products chemistry, we have highlighted a number of his accomplishments that illustrate the broad diversity of his interests. These include the isolation, structure determination, and biological mechanism of action of many natural products including the triterpenoid pristimerin; the diterpenoid ginkgolides; insect and crustacean molting hormones; phytoalexins; the toxic red tide principle brevetoxin; the vanadium tunicate pigments; philanthotoxin from killer wasps; antisickling agents; mitomycin DNA adducts; insect antifeedants; a mitotic hormone, the small molecule fish attractants from the sea anemone; new isolation and purification technologies; molecular chemistry of vision; age-related macular degeneration; and the development of the exciton circular dichroism (CD) chirality method for microscale determination of absolute configuration of natural products and chirality of other chiral molecules and supramolecular assembly.

Expanding the DOPA Universe with Genetically Encoded, Mussel-Inspired Bioadhesives for Material Sciences and Medicine

Catechols are a biologically relevant group of aromatic diols that have attracted much attention as mediators of adhesion of "bio-glue" proteins in mussels of the genus Mytilus. These organisms use catechols in the form of the noncanonical amino acid l-3,4-dihydroxyphenylalanine (DOPA) as a building block for adhesion proteins. The DOPA is generated post-translationally from tyrosine. Herein, we review the properties, natural occurrence, and reactivity of catechols in the design of bioinspired materials. We also provide a basic description of the mussel's attachment apparatus, the interplay between its different molecules that play a crucial role in adhesion, and the role of post-translational modifications (PTMs) of these proteins. Our focus is on the microbial production of mussel foot proteins with the aid of orthogonal translation systems (OTSs) and the use of genetic code engineering to solve some fundamental problems in the bioproduction of these bioadhesives and to expand their chemical space. The major limitation of bacterial expression systems is their intrinsic inability to introduce PTMs. OTSs have the potential to overcome these challenges by replacing canonical amino acids with noncanonical ones. In this way, PTM steps are circumvented while the genetically programmed precision of protein sequences is preserved. In addition, OTSs should enable spatiotemporal control over the complex adhesion process, because the catechol function can be masked by suitable chemical protection. Such caged residues can then be noninvasively unmasked by, for example, UV irradiation or thermal treatment. All of these features make OTSs based on genetic code engineering in reprogrammed microbial strains new and promising tools in bioinspired materials science.

Fueling a Hot Debate on the Application of TiO2 Nanoparticles in Sunscreen

Titanium is one of the most abundant elements in the earth's crust and while there are many examples of its bioactive properties and use by living organisms, there are few studies that have probed its biochemical reactivity in physiological environments. In the cosmetic industry, TiO2 nanoparticles are widely used. They are often incorporated in sunscreens as inorganic physical sun blockers, taking advantage of their semiconducting property, which facilitates absorbing ultraviolet (UV) radiation. Sunscreens are formulated to protect human skin from the redox activity of the TiO2 nanoparticles (NPs) and are mass-marketed as safe for people and the environment. By closely examining the biological use of TiO2 and the influence of biomolecules on its stability and solubility, we reassess the reactivity of the material in the presence and absence of UV energy. We also consider the alarming impact that TiO2 NP seepage into bodies of water can cause to the environment and aquatic life, and the effect that it can have on human skin and health, in general, especially if it penetrates into the human body and the bloodstream.

Crystal structures of sodium-, lithium-, and ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) hydrates

The solid-state structures of the Na+, Li+, and NH4+ salts of the 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) dianion are reported, namely disodium 4,5-di-hydroxy-benzene-1,3-di-sulfonate, 2Na+·C6H4O8S22-, μ-4,5-di-hydroxy-benzene-1,3-di-sulfonato-bis-[aqua-lithium(I)] hemihydrate, [Li2(C6H4O8S2)(H2O)2]·0.5H2O, and di-ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate monohydrate, 2NH4+·C6H4O8S22-·H2O. Inter-molecular inter-actions vary with the size of the cation, and the asymmetric unit cell, and the macromolecular features are also affected. The sodium in Na2(tiron) is coordinated in a distorted octa-hedral environment through the sulfonate oxygen and hydroxyl oxygen donors on tiron, as well as an inter-stitial water mol-ecule. Lithium, with its smaller ionic radius, is coordinated in a distorted tetra-hedral environment by sulfonic and phenolic O atoms, as well as water in Li2(tiron). The surrounding tiron anions coordinating to sodium or lithium in Na2(tiron) and Li2(tiron), respectively, result in a three-dimensional network held together by the coordinate bonds to the alkali metal cations. The formation of such a three-dimensional network for tiron salts is relatively rare and has not been observed with monovalent cations. Finally, (NH4)2(tiron) exhibits extensive hydrogen-bonding arrays between NH4+ and the surrounding tiron anions and inter-stitial water mol-ecules. This series of structures may be valuable for understanding charge transfer in a putative solid-state fuel cell utilizing tiron.

From sea squirts to squirrelfish: facultative trace element hyperaccumulation in animals

The hyperaccumulation of trace elements is a widely characterized phenomenon in plants, bacteria, and fungi, but has received little attention in animals. However, there are numerous examples of animals that specifically and facultatively accumulate trace elements in the absence of elevated environmental concentrations. Metal hyperaccumulating animals are usually marine invertebrates, likely owing to environmental (e.g. constant exposure via the water) and physiological (e.g. osmoconforming and reduced integument permeability) factors. However, there are examples of terrestrial animals (insect larvae) and marine vertebrates (e.g. squirrelfish) that accumulate high body and/or tissue metal burdens. This review examines examples of animal hyperaccumulation of the elements arsenic, copper, iron, titanium, vanadium and zinc, describing mechanisms by which accumulation occurs and, where possible, hypothesizing functional roles. Groups such as the ascidians (sea squirts), molluscs (gastropods, bivalves and cephalopods) and polychaete annelids feature prominently as animals with hyperaccumulating capacity. Many of these species are potential model organisms offering insight into fundamental processes underlying metal handling, with relevance to human disease and aquatic metal toxicity, and some offer promise in applied fields such as bioremediation.

The haemocytes of the colonial aplousobranch ascidian Diplosoma listerianum: Structural, cytochemical and functional analyses

Diplosoma listerianum is a colonial aplousobranch ascidian of the family Didemnidae that is native to the northeast Atlantic and exhibits a cosmopolitan distribution in temperate waters. It lacks a shared colonial circulation crossing the tunic, and the zooids are connected only by the common tunic. In the present study, the haemocytes of this ascidian were analysed via light and electron microscopy. Their phagocytic and enzymatic activities, staining and immunostaining properties, and lectin affinity were examined with various classical methods reconsidered and modified for small marine invertebrates. Eight morphotypes were identified in reference to corresponding cell types described in other ascidians: undifferentiated cells (haemoblasts), storage cells for nitrogenous catabolites (nephrocytes) and immunocytes. The immunocytes are involved in immune responses, acting as (1) phagocytes, rich in hydrolases and involved in the clearance of both foreign particles and effete cells (hyaline amoebocytes and macrophage-like cells); (2) cytotoxic cells, able to degranulate and induce cytotoxicity through the release of the enzyme phenoloxidase after an immune stimulus (granular amoebocytes and morula cells); and (3) basophilic cells with an affinity for ConA and NPA that contain heparin and histamine and that show sensitivity to the compound 48/80, promoting their degranulation (mast cell-like granulocytes). In addition, a particular cell type showing exceptional development of the Golgi apparatus and large vacuoles containing a filamentous material has been recognised (spherule cell), for which a role in tunic repair and fibrogenesis has been hypothesised.

Marine Pharmacology in 2012-2013: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, and Antiviral Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The peer-reviewed marine pharmacology literature from 2012 to 2013 was systematically reviewed, consistent with the 1998–2011 reviews of this series. Marine pharmacology research from 2012 to 2013, conducted by scientists from 42 countries in addition to the United States, reported findings on the preclinical pharmacology of 257 marine compounds. The preclinical pharmacology of compounds isolated from marine organisms revealed antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral and anthelmitic pharmacological activities for 113 marine natural products. In addition, 75 marine compounds were reported to have antidiabetic and anti-inflammatory activities and affect the immune and nervous system. Finally, 69 marine compounds were shown to display miscellaneous mechanisms of action which could contribute to novel pharmacological classes. Thus, in 2012–2013, the preclinical marine natural product pharmacology pipeline provided novel pharmacology and lead compounds to the clinical marine pharmaceutical pipeline, and contributed significantly to potentially novel therapeutic approaches to several global disease categories.

Immunity in Protochordates: The Tunicate Perspective

Tunicates are the closest relatives of vertebrates, and their peculiar phylogenetic position explains the increasing interest toward tunicate immunobiology. They are filter-feeding organisms, and this greatly influences their defense strategies. The majority of the studies on tunicate immunity were carried out in ascidians. The tunic acts as a first barrier against pathogens and parasites. In addition, the oral siphon and the pharynx represent two major, highly vascularized, immune organs, where circulating hemocytes can sense non-self material and trigger immune responses that, usually, lead to inflammation and phagocytosis. Inflammation involves the recruitment of circulating cytotoxic, phenoloxidase (PO)-containing cells in the infected area, where they degranulate as a consequence of non-self recognition and release cytokines, complement factors, and the enzyme PO. The latter, acting on polyphenol substrata, produces cytotoxic quinones, which polymerize to melanin, and reactive oxygen species, which induce oxidative stress. Both the alternative and the lectin pathways of complement activation converge to activate C3: C3a and C3b are involved in the recruitment of hemocytes and in the opsonization of foreign materials, respectively. The interaction of circulating professional phagocytes with potentially pathogenic foreign material can be direct or mediated by opsonins, either complement dependent or complement independent. Together with cytotoxic cells, phagocytes are active in the encapsulation of large materials. Cells involved in immune responses, collectively called immunocytes, represent a large fraction of hemocytes, and the presence of a cross talk between cytotoxic cells and phagocytes, mediated by secreted humoral factors, was reported. Lectins play a pivotal role as pattern-recognition receptors and opsonizing agents. In addition, variable region-containing chitin-binding proteins, identified in the solitary ascidian Ciona intestinalis, control the settlement and colonization of bacteria in the gut.

Synthesis and biological evaluation of the ascidian blood-pigment halocyamine A

The first synthesis of the (Z)-indolic enamide-containing antibacterial marine natural product halocyamine A is reported.

Contemplating a role for titanium in organisms

Titanium is the ninth most abundant element in the Earth's crust and some organisms sequester it avidly, though no essential biological role has yet been recognized. This Minireview addresses how the properties of titanium, especially in an oxic aqueous environment, might make a biological role difficult to recognize. It further considers how new -omic technologies might overcome the limitations of the past and help to reveal a specific role for this metal. While studies with well established model organisms have their rightful place, organisms that are known avid binders or sequesterers of titanium should be promising places to investigate a biological role.

Reactivities of Quinone Methides versus o-Quinones in Catecholamine Metabolism and Eumelanin Biosynthesis

Melanin is an important biopolymeric pigment produced in a vast majority of organisms. Tyrosine and its hydroxylated product, dopa, form the starting material for melanin biosynthesis. Earlier studies by Raper and Mason resulted in the identification of dopachrome and dihydroxyindoles as important intermediates and paved way for the establishment of well-known Raper-Mason pathway for the biogenesis of brown to black eumelanins. Tyrosinase catalyzes the oxidation of tyrosine as well as dopa to dopaquinone. Dopaquinone thus formed, undergoes intramolecular cyclization to form leucochrome, which is further oxidized to dopachrome. Dopachrome is either converted into 5,6-dihydroxyindole by decarboxylative aromatization or isomerized into 5,6-dihydroxyindole-2-carboxylic acid. Oxidative polymerization of these two dihydroxyindoles eventually produces eumelanin pigments via melanochrome. While the role of quinones in the biosynthetic pathway is very well acknowledged, that of isomeric quinone methides, however, remained marginalized. This review article summarizes the key role of quinone methides during the oxidative transformation of a vast array of catecholamine derivatives and brings out the importance of these transient reactive species during the melanogenic process. In addition, possible reactions of quinone methides at various stages of melanogenesis are discussed.

Novel post-translational oligomerization of peptidyl dehydrodopa model compound, 1,2-dehydro-N-acetyldopa methyl ester

Post-translational modification of peptidyl tyrosine to peptidyl dopa is widely observed in different marine organisms. While peptidyl dopas are oxidatively converted to dehydrodopa derivatives, nothing is known about the further fate of dehydrodopyl compounds. To fill this void, we studied the oxidation chemistry of a peptidyl dehydrodopa mimic, 1,2-dehydro-N-acetyldopa methyl ester with mushroom tyrosinase. We employed both routine biochemical studies and reversed phase liquid chromatography mass spectrometry to investigate the course of the reaction. Tyrosinase catalyzed the oxidation of 1,2-dehydro-N-acetyldopa methyl ester readily generating its typical o-quinone as the transient two-electron oxidation product. This quinone was extremely unstable and rapidly reacted with the parent compound forming benzodioxan type oligomeric products. Reaction mixture containing chemically made o-benzoquinone and 1,2-dehydro-N-acetyldopa methyl ester generated a mixed adduct of benzoquinone and 1,2-dehydro-N-acetyldopa methyl ester. Based on this finding, we propose that peptidyl dehydrodopa also exhibits a similar transformation accounting partially for the adhesive and cementing properties of dopyl proteins in nature.

Marine peptides and their anti-infective activities

Marine bioresources are a valuable source of bioactive compounds with industrial and nutraceutical potential. Numerous clinical trials evaluating novel chemotherapeutic agents derived from marine sources have revealed novel mechanisms of action. Recently, marine-derived bioactive peptides have attracted attention owing to their numerous beneficial effects. Moreover, several studies have reported that marine peptides exhibit various anti-infective activities, such as antimicrobial, antifungal, antimalarial, antiprotozoal, anti-tuberculosis, and antiviral activities. In the last several decades, studies of marine plants, animals, and microbes have revealed tremendous number of structurally diverse and bioactive secondary metabolites. However, the treatments available for many infectious diseases caused by bacteria, fungi, and viruses are limited. Thus, the identification of novel antimicrobial peptides should be continued, and all possible strategies should be explored. In this review, we will present the structures and anti-infective activity of peptides isolated from marine sources (sponges, algae, bacteria, fungi and fish) from 2006 to the present.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.