Resonance Raman studies of catecholate and phenolate complexes of recombinant human tyrosine hydroxylase

Unusual Self-Hydroxylation in 4-Histidine Tetrad-Supporting Nonheme Iron Center

The TM1459 protein from Thermotoga maritima is a member of the cupin protein superfamily and contains a mononuclear metal center. Structural information has been obtained using X-ray crystallography; however, its physiological role remains unknown. The metal-binding site has an octahedral coordination geometry ligated by four histidine imidazoles and two terminal water molecules present in the cis position. This protein had the ability to bind Mn, Fe, and Zn ions; additionally, a self-hydroxylation reaction occurred in the Fe-TM1459 C106V mutant. Namely, one of the tyrosine residues (Tyr7) was hydroxylated to generate the green form. Spectroscopic analyses using Vis-NIR, MALDI-TOF/MS, and resonance Raman spectroscopy confirmed that Tyr7 was hydroxylated to 3,4-dihydroxyphenylalanine giving an iron-catecholate complex. Because the Y7A/C106V mutant did not produce this green form, the mutation of Cys106 to Val was assumed to have induced a conformational change in Tyr7 that facilitated its approach to the metal center promoting the self-hydroxylation reaction. Thus, these results demonstrated that Fe-TM1459 protein has monooxygenase activity.

Formation mechanism of iron-catechol complexes in the colored periostracum of Corbicula spp

The shell color of Corbicula clams, which are globally distributed, is roughly divided into yellowish and blackish depending on the environmental conditions of the sediment. The formation of an iron-L-3,4-dihydroxyphenylalanine (DOPA) complex in a thin organic layer, called the periostracum, on a calcareous layer causes the blackening of the clamshell. However, the iron-DOPA complex formation mechanism is unclear. To reveal how the iron is transported from the aquatic environment to the periostracum, cross-sectional analyses of the shell were conducted using an electron probe microanalyzer and Raman spectroscopy to investigate the distribution and structure of iron in the shell. Iron was only present in the periostracum, excluding deposition, and all iron was in the form of an iron-DOPA complex. Attenuated total reflection infrared spectroscopy and oxygen K-edge X-ray absorption fine structure spectroscopy revealed that the molecular structure of the native periostracum is independent of shell color. These results indicate that dissolved iron-organic complexes diffuse from the aqueous environment to the periostracum, forming iron-DOPA complex through ligand exchange. Because the iron-DOPA complex color depends on the pH, the shell color can serve as a historical indicator of the shell's growth environment.

SERS Sensing of Dopamine with Fe(III)-Sensitized Nanogaps in Recleanable AuNP Monolayer Films

Sensing of neurotransmitters (NTs) down to nm concentrations is demonstrated by utilizing self-assembled monolayers of plasmonic 60 nm Au nanoparticles in close-packed arrays immobilized onto glass substrates. Multiplicative surface-enhanced Raman spectroscopy enhancements are achieved by integrating Fe(III) sensitizers into the precisely-defined <1 nm nanogaps, to target dopamine (DA) sensing. The transparent glass substrates allow for efficient access from both sides of the monolayer aggregate films by fluid and light, allowing repeated sensing in different analytes. Repeated reusability after analyte sensing is shown through oxygen plasma cleaning protocols, which restore pristine conditions for the nanogaps. Examining binding competition in multiplexed sensing of two catecholamine NTs, DA and epinephrine, reveals their bidentate binding and their interactions. These systems are promising for widespread microfluidic integration enabling a wide range of continuous biofluid monitoring for applications in precision health.

Bivalent Gadolinium Ions Forming Injectable Hydrogels for Simultaneous In Situ Vaccination Therapy and Imaging of Soft Tissue Sarcoma

Doxorubicin (DOX) is the classic soft tissue sarcomas (STS) first-line treatment drug, while dose-dependent myelosuppression and cardiotoxicity limit its application in clinic. This research intends to apply DOX, which is also an inducer of immunogenic cell death as a part for "in situ vaccination" and conjointly uses PD-1 inhibitors to enhance antitumor efficacy. In order to achieve the sustained vaccination effect and real-time monitoring of distribution in vivo, the in situ forming and injectable hydrogel platform with the function of visualization is established for local delivery. The hydrogel platform is synthesized by hyaluronic acid-dopamine coordinated with gadolinium ions (Gd2+ ). Gd2+ provides the ability of magnetic resonance imaging, meanwhile further cross-linking the hydrogel network. Experiments show excellent ability of sustained release and imaging tracking for the hydrogel platform. In mouse STS models, the "in situ vaccination" hydrogels show the best effect of inhibiting tumor growth. Further analysis of tumor tissues show that "in situ vaccination" group can increase T cell infiltration, promote M1-type macrophage polarization and block elevated PD-1/PD-L1 pathway caused by DOX. These results are expected to prove the potential for synthesized hydrogels to achieve a universal platform for "in situ vaccination" strategies on STS treatments.

Spectroscopic studies for identifying the chemical states of the periostracum of the Corbicula species in Lake Biwa

Corbicula clam shells consist of thin periostracum and calcareous layers made of calcium carbonate (CaCO₃). Depending on habitat conditions, the shell exhibits various colorations, such as yellow, brown, and black. The chemical state of the periostracum of the Corbicula species in Lake Biwa was studied by X-ray absorption fine structure (XAFS) and Raman scattering spectroscopies. Fe K-edge X-ray absorption near edge structure (XANES) revealed that the Fe³⁺ intensity increases as the color of the shell changes from yellow to black. Raman spectra suggested that quinone-based polymers cover the yellow shell, and the black shell is further covered by dihydroxyphenylalanine (DOPA) rings of amino acid derivatives. From Fe K-edge extended X-ray absorption fine structure (EXAFS), it was found that Fe³⁺ in the periostracum was surrounded by five to six oxygen atoms with an average Fe-O ligand distance of 2.0 Å. Accordingly, a tris-DOPA-Fe³⁺ complex is formed, which is responsible for the periostracum's black color.

Nitrite/Nitrous Acid Generation from the Reaction of Nitrate and Fe(II) Promoted by Photolysis of Iron-Organic Complexes

Gaseous nitrous acid (HONO) has the potential to greatly contribute to the atmospheric oxidation capacity. Increased attention has been paid to in-particle nitrite or nitrous acid, N(III), as one of the HONO sources. However, sources and formation mechanisms of N(III) remain uncertain. Here, we study a much less examined reaction of Fe(II) and nitrate as a source of N(III). The N(III) production was indirectly probed by its multiphase reaction with SO₂ for sulfate production. Particles containing nitrate and Fe(III) were irradiated for generating Fe(II). Sulfate production was enhanced by the presence of UV and organic compounds likely because of the enhanced redox cycle between Fe(II) and Fe(III). Sulfate production rate increases with the concentration of iron-organic complexes in nitrate particles. Similarly, higher concentrations of iron-organic complexes yield higher nitrate decay rates. The estimated production rates of N(III) under simulated conditions in our study vary from 0.1 to 3.0 μg m^-3 of air h^-1. These values are comparable to HONO production rates of 0.2-1.6 ppbv h^-1, which fall in the values reported in laboratory and field studies. The present study highlights a synergistic effect of the coexistence of iron-organic complexes and nitrate under irradiation as a source of N(III).

Personalized Medicine to Improve Treatment of Dopa-Responsive Dystonia-A Focus on Tyrosine Hydroxylase Deficiency

Dopa-responsive dystonia (DRD) is a rare movement disorder associated with defective dopamine synthesis. This impairment may be due to the fact of a deficiency in GTP cyclohydrolase I (GTPCHI, GCH1 gene), sepiapterin reductase (SR), tyrosine hydroxylase (TH), or 6-pyruvoyl tetrahydrobiopterin synthase (PTPS) enzyme functions. Mutations in GCH1 are most frequent, whereas fewer cases have been reported for individual SR-, PTP synthase-, and TH deficiencies. Although termed DRD, a subset of patients responds poorly to L-DOPA. As this is regularly observed in severe cases of TH deficiency (THD), there is an urgent demand for more adequate or personalized treatment options. TH is a key enzyme that catalyzes the rate-limiting step in catecholamine biosynthesis, and THD patients often present with complex and variable phenotypes, which results in frequent misdiagnosis and lack of appropriate treatment. In this expert opinion review, we focus on THD pathophysiology and ongoing efforts to develop novel therapeutics for this rare disorder. We also describe how different modeling approaches can be used to improve genotype to phenotype predictions and to develop in silico testing of treatment strategies. We further discuss the current status of mathematical modeling of catecholamine synthesis and how such models can be used together with biochemical data to improve treatment of DRD patients.

Microfluidic-like fabrication of metal ion-cured bioadhesives by mussels

[Figure: see text].

Levalbuterol lowers the feedback inhibition by dopamine and delays misfolding and aggregation in tyrosine hydroxylase

Tyrosine hydroxylase (TH) catalyses the (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4)-dependent conversion of L-tyrosine to L-3,4-dihydroxyphenylalanine (L-Dopa), which is the rate-limiting step in the synthesis of dopamine and other catecholamine neurotransmitters and hormones. Dysfunctional mutant TH causes tyrosine hydroxylase deficiency (THD), characterized by symptoms ranging from mild l-Dopa responsive dystonia to severe neuropathy. THD-associated mutations often present misfolding and a propensity to aggregate, characteristics that can also be manifested by dysregulated wild-type TH. TH - and subsequently dopamine - is also reduced in Parkinson's disease (PD) due to the selective death of dopaminergic neurons. Thus, TH is a target for stabilizing small molecular weight compounds that can function as pharmacological chaperones, restoring enzyme folding and function. In this work we carried out a screening of a compound library with 1280 approved drugs and we identified levalbuterol, a beta2-adrenergic agonist that is broadly used in asthma treatment, as an interesting validated binder of human TH. Levalbuterol stabilized TH with reduced affinity compared to dopamine, the end-product and regulatory feedback inhibitor of TH, but without compromising enzymatic activity. Moreover, levalbuterol also delays the formation of TH aggregates and makes the enzyme less sensitive to dopamine, effects that could contribute to ameliorate disorders related to TH, such as THD and PD.

Synthesis and Characterization of Catecholato Copper(II) Complexes with Sterically Hindered Neutral and Anionic N3 Type Ligands: Tris(3,5-diisopropyl-1-pyrazolyl)methane and Hydrotris(3,5-diisopropyl-1-pyrazolyl)borate

Three catecholato copper(II) complexes, [Cu(catCl4)(L1′)], [Cu(catBr4)(L1′)], and [Cu(catCl4)(L1H)], supported by sterically hindered neutral and anionic N3 type ligands: tris(3,5-diisopropyl-1-pyrazolyl)methane (referred to as L1′) and hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (referred to as L1−), are synthesized and characterized in detail. Their X-ray structures reveal that both [Cu(catCl4)(L1′)] and [Cu(catBr4)(L1′)] complexes have a five-coordinate square-pyramidal geometry and [Cu(catCl4)(L1H)] complex has a four-coordinate square-planar geometry. The L1H is unusual protonated ligand that controls its overall charge. For the three catecholato copper(II) complexes, the oxidation state of copper is divalent, and catechol exists in catecholate as two minus anion. This difference in coordination geometry affects their d-d and CT transitions energy and ESR parameters.

Compartmentalized processing of catechols during mussel byssus fabrication determines the destiny of DOPA

Inspired largely by the role of the posttranslationally modified amino acid dopa (DOPA) in mussel adhesion, catechol functional groups have become commonplace in medical adhesives, tissue scaffolds, and advanced smart polymers. Yet, the complex redox chemistry of catechol groups complicates cross-link regulation, hampering fabrication and the long-term stability/performance of mussel-inspired polymers. Here, we investigated the various fates of DOPA residues in proteins comprising mussel byssus fibers before, during, and after protein secretion. Utilizing a combination of histological staining and confocal Raman spectroscopy on native tissues, as well as peptide-based cross-linking studies, we have identified at least two distinct DOPA-based cross-linking pathways during byssus fabrication, achieved by oxidative covalent cross-linking or formation of metal coordination interactions under reducing conditions, respectively. We suggest that these end states are spatiotemporally regulated by the microenvironments in which the proteins are stored prior to secretion, which are retained after formation-in particular, due to the presence of reducing moieties. These findings provide physicochemical pathways toward greater control over properties of synthetic catechol-based polymers and adhesives.

Preparation of Au nanoparticles on a magnetically responsive support via pyrolysis of a Prussian blue composite

A strategy is described for the direct preparation of Au nanoparticles (AuNPs) on a Fe-based support, coated with porous carbon (PC), via pyrolysis of an AuCN functionalised Prussian Blue (PB) metal organic framework (MOF). The composite starting material was prepared with an even distribution of AuCN on the surface via galvanic exchange of PB with a gold salt in solution. The resulting structures after pyrolysis were shown to be active Au-based nanomaterials for model applications including catalysis (4-nitrophenol reduction) and electroanalysis (arsenic (III) detection), suggesting broad application where Au nanoparticles are required at a liquid-solid interface. The Fe based support was seen to consist of Fe, Fe₃C and Fe₄C phases, and the carbon coating increased the stability and improved the conductivity of the materials. The temperature of pyrolysis was seen to affect the activity of the supported nanoparticles, with an increased Au surface area obtained at the higher pyrolysis temperature (650 °C) tested. A general strategy is thus confirmed for preparation of noble metal nanoparticles evenly distributed on a magnetic support, allowing easy separation of catalysts from products in heterogeneous applications.

Calcium-Mediated Control of Polydopamine Film Oxidation and Iron Chelation

The facile preparation of conformal polydopamine (PDA) films on broad classes of materials has prompted extensive research into a wide variety of potential applications for PDA. The constituent molecular species in PDA exhibit diverse chemical moieties, and therefore highly variable properties of PDA-based devices may evolve with post-processing conditions. Here we report the use of redox-inactive cations for oxidative post-processing of deposited PDA films. PDA films incubated in alkaline CaCl₂ solutions exhibit accelerated oxidative evolution in a dose-dependent manner. PDA films incubated in CaCl₂ solutions exhibit 53% of the oxidative charge transfer compared to pristine PDA films. Carboxylic acid groups generated from the oxidation process lower the isoelectric point of PDA films from pH = 4.0 ± 0.2 to pH = 3.1 ± 0.3. PDA films exposed to CaCl₂ solutions during post-processing also enhance Fe2+/Fe3+ chelation compared to pristine PDA films. These data illustrate that the molecular heterogeneity and non-equilibrium character of as-deposited PDA films afford control over the final composition by choosing post-processing conditions, but also demands forethought into how the performance of PDA-incorporated devices may change over time in salt solutions.

A seawater triggered dynamic coordinate bond and its application for underwater self-healing and reclaiming of lipophilic polymer

In this work, water triggered dynamic catechol-Fe3+ coordinate bonds are revealed and studied at atomic, molecular and macroscopic levels using Mössbauer spectroscopy, rheological analysis, etc. DOPA-iron complexation is found to be dynamic in the presence of water, and this dynamic manner is immobilized after removing water. Accordingly, a water saturated lipophilic polymer containing catechol-Fe3+ crosslinks, rather than the dry version, exhibits dynamic coordination-dissociation behavior. In addition, a migration of iron proves to be enabled in the catechol-Fe3+ crosslinked polymer immersed in seawater. Rearrangement of the dynamic catechol-Fe3+ coordinate bonds among different molecules is thus favored. Based on these results, we develop a bulk lipophilic polymer solid capable of repeated autonomic recovery of strength in seawater without manual intervention. When the polymer is damaged in seawater, reshuffling of the mobile hyperbranched polymer networks across the crack interface, owing to the dynamic catechol-Fe3+ crosslinkages activated by the alkaline circumstances, rebinds the damaged site. By taking advantage of the same mechanism, the polymer can be remolded with the help of seawater and this recycled polymer is still self-healable in seawater. Unlike in the case of conventional polymers where water would shield macromolecules from interacting, here, seawater is a necessary environmental assistant for the material interaction to take effect. The outcomes are beneficial for deepening the understanding of coordinate bonds, and the development of robust underwater self-healing lipophilic polymers.

Mechanical homeostasis of a DOPA-enriched biological coating from mussels in response to metal variation

Protein-metal coordination interactions were recently found to function as crucial mechanical cross-links in certain biological materials. Mussels, for example, use Fe ions from the local environment coordinated to DOPA-rich proteins to stiffen the protective cuticle of their anchoring byssal attachment threads. Bioavailability of metal ions in ocean habitats varies significantly owing to natural and anthropogenic inputs on both short and geological spatio-temporal scales leading to large variations in byssal thread metal composition; however, it is not clear how or if this affects thread performance. Here, we demonstrate that in natural environments mussels can opportunistically replace Fe ions in the DOPA coordination complex with V and Al. In vitro removal of the native DOPA-metal complexes with ethylenediaminetetraacetic acid and replacement with either Fe or V does not lead to statistically significant changes in cuticle performance, indicating that each metal ion is equally sufficient as a DOPA cross-linking agent, able to account for nearly 85% of the stiffness and hardness of the material. Notably, replacement with Al ions also leads to full recovery of stiffness, but only 82% recovery of hardness. These findings have important implications for the adaptability of this biological material in a dynamically changing and unpredictable habitat.

High-performance TiO2 nanoparticle/DOPA-polymer composites

Many natural materials are complex composites whose mechanical properties are often outstanding considering the weak constituents from which they are assembled. Nacre, made of inorganic (CaCO3 ) and organic constituents, is a textbook example because of its strength and toughness, which are related to its hierarchical structure and its well-defined organic-inorganic interface. Emulating the construction principles of nacre using simple inorganic materials and polymers is essential for understanding how chemical composition and structure determine biomaterial functions. A hard multilayered nanocomposite is assembled based on alternating layers of TiO2 nanoparticles and a 3-hydroxy-tyramine (DOPA) substituted polymer (DOPA-polymer), strongly cemented together by chelation through infiltration of the polymer into the TiO2 mesocrystal. With a Young's modulus of 17.5 ± 2.5 GPa and a hardness of 1.1 ± 0.3 GPa the resulting material exhibits high resistance against elastic as well as plastic deformation. A key feature leading to the high strength is the strong adhesion of the DOPA-polymer to the TiO2 nanoparticles.

Green coating by coordination of tannic acid and iron ions for antioxidant nanofiltration membranes

A novel green coating method was proposed to prepare composite nanofiltration (NF) membranes with good structural stability and oxidation resistance ability.

Metal-coordination: Using one of nature's tricks to control soft material mechanics

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion1. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise2. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.

Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands

This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and [Fe(2+)((Ph2)TIP)((tBu)PDA)](OTf)2, [4](OTf)2 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species [Fe(3+)((Ph2)TIP)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to give [Fe(2+)((Ph2)TIP)((tBu)DIBQ)](2+) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2](+) with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as [Fe(3+)((Ph2)TIP)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe(3+)(L(2-))](+) (3(ox)) to [Fe(3+)(L(•-))](2+) (2(ox)) to [Fe(2+)(L(0))](2+) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order [3](+) > [2](+) > [4](2+), even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.

Evidence for a dual role of an active site histidine in α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase

The previously reported crystal structures of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) show a five-coordinate Zn(II)(His)(3)(Asp)(OH(2)) active site. The water ligand is H-bonded to a conserved His228 residue adjacent to the metal center in ACMSD from Pseudomonas fluorescens (PfACMSD). Site-directed mutagenesis of His228 to tyrosine and glycine in this study results in a complete or significant loss of activity. Metal analysis shows that H228Y and H228G contain iron rather than zinc, indicating that this residue plays a role in the metal selectivity of the protein. As-isolated H228Y displays a blue color, which is not seen in wild-type ACMSD. Quinone staining and resonance Raman analyses indicate that the blue color originates from Fe(III)-tyrosinate ligand-to-metal charge transfer. Co(II)-substituted H228Y ACMSD is brown in color and exhibits an electron paramagnetic resonance spectrum showing a high-spin Co(II) center with a well-resolved (59)Co (I = 7/2) eight-line hyperfine splitting pattern. The X-ray crystal structures of as-isolated Fe-H228Y (2.8 Å) and Co-substituted (2.4 Å) and Zn-substituted H228Y (2.0 Å resolution) support the spectroscopic assignment of metal ligation of the Tyr228 residue. The crystal structure of Zn-H228G (2.6 Å) was also determined. These four structures show that the water ligand present in WT Zn-ACMSD is either missing (Fe-H228Y, Co-H228Y, and Zn-H228G) or disrupted (Zn-H228Y) in response to the His228 mutation. Together, these results highlight the importance of His228 for PfACMSD's metal specificity as well as maintaining a water molecule as a ligand of the metal center. His228 is thus proposed to play a role in activating the metal-bound water ligand for subsequent nucleophilic attack on the substrate.

Mussel foot protein-1 (mcfp-1) interaction with titania surfaces()

Marine mussels utilize a variety of DOPA-rich proteins for purposes of underwater adhesion, as well as for creating hard and flexible surface coatings for their tough and stretchy byssal fibers. In the present study, moderately strong, yet reversible wet adhesion between the protective mussel coating protein, mcfp-1, and amorphous titania was measured with a surface force apparatus (SFA). In parallel, resonance Raman spectroscopy was employed to identify the presence of bidentate DOPA-Ti coordination bonds at the TiO(2)-protein interface, suggesting that catechol-TiO(2) complexation contributes to the observed reversible wet adhesion. These results have important implications for the design of protective coatings on TiO(2).

pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jérôme C, Stephen JS (Academic Press, London), pp 75-133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216-220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe(3+) interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe(3+) cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G') that approach covalently cross-linked gels as well as self-healing properties.

In vivo self-hydroxylation of an iron-substituted manganese-dependent extradiol cleaving catechol dioxygenase

The homoprotocatechuate 2,3-dioxygenase from Arthrobacter globiformis (MndD) catalyzes the oxidative ring cleavage reaction of its catechol substrate in an extradiol fashion. Although this reactivity is more typically associated with non-heme iron enzymes, MndD exhibits an unusual specificity for manganese(II). MndD is structurally very similar to the iron(II)-dependent homoprotocatechuate 2,3-dioxygenase from Brevibacterium fuscum (HPCD), and we have previously shown that both MndD and HPCD are equally active towards substrate turnover with either iron(II) or manganese(II) (Emerson et al. in Proc. Natl. Acad. Sci. USA 105:7347-7352, 2008). However, expression of MndD in Escherichia coli under aerobic conditions in the presence of excess iron results in the isolation of inactive blue-green iron-substituted MndD. Spectroscopic studies indicate that this form of iron-substituted MndD contains an iron(III) center with a bound catecholate, which is presumably generated by in vivo self-hydroxylation of a second-sphere tyrosine residue, as found for other self-hydroxylated non-heme iron oxygenases. The absence of this modification in either the native manganese-containing MndD or iron-containing HPCD suggests that the metal center of iron-substituted MndD is able to bind and activate O(2) in the absence of its substrate, employing a high-valence oxoiron oxidant to carry out the observed self-hydroxylation chemistry. These results demonstrate that the active site metal in MndD can support two dramatically different O(2) activation pathways, further highlighting the catalytic flexibility of enzymes containing a 2-His-1-carboxylate facial triad metal binding motif.

Modification of bone-like apatite nanoparticle size and growth kinetics by alizarin red S

The formation of nanocrystals in biomineralization such as in bone occurs under the influence of organic molecules. Prompted by this fact, the effect of alizarin red S, a dye used in in vivo bone labeling methods, on bone-like carbonated apatite nanocrystal formation was investigated as a function of alizarin red S additive concentration. The obtained nanoparticles were investigated by powder X-ray diffraction (XRD), FTIR as well thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) while the kinetics of nanoparticle formation was investigated by in situ pH and synchrotron XRD measurements. Increasing alizarin red S concentration lead to amorphous particles over a threshold concentration and to smaller crystallites in a dose-dependent fashion. Alizarin red S induced a macroscopic lattice strain that scaled linearly with the alizarin red S concentration; this effect is reminiscent of that seen in biogenic calcium carbonates. TGA showed that the amorphous particles contained significantly more water than the crystalline samples and the DSC data showed that crystallization occurs after loss of most of the included organic material. The in situ studies showed that the formation of apatite goes via the very rapid formation of an amorphous precursor that after a certain nucleation time crystallizes into apatite. This nucleation time increased exponentially with alizarin red S concentration showing that this additive strongly stabilizes the amorphous precursor phase.

Iron-clad fibers: a metal-based biological strategy for hard flexible coatings

The extensible byssal threads of marine mussels are shielded from abrasion in wave-swept habitats by an outer cuticle that is largely proteinaceous and approximately fivefold harder than the thread core. Threads from several species exhibit granular cuticles containing a protein that is rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (dopa) as well as inorganic ions, notably Fe3+. Granular cuticles exhibit a remarkable combination of high hardness and high extensibility. We explored byssus cuticle chemistry by means of in situ resonance Raman spectroscopy and demonstrated that the cuticle is a polymeric scaffold stabilized by catecholato-iron chelate complexes having an unusual clustered distribution. Consistent with byssal cuticle chemistry and mechanics, we present a model in which dense cross-linking in the granules provides hardness, whereas the less cross-linked matrix provides extensibility.

Raman, IR, UV-vis and EPR characterization of two copper dioxolene complexes derived from L-dopa and dopamine

The anionic complexes [Cu(L(1-))3](1-), L(-)=dopasemiquinone or L-dopasemiquinone, were prepared and characterized. The complexes are stable in aqueous solution showing intense absorption bands at ca. 605 nm for Cu(II)-L-dopasemiquinone and at ca. 595 nm for Cu(II)-dopasemiquinone in the UV-vis spectra, that can be assigned to intraligand transitions. Noradrenaline and adrenaline, under the same reaction conditions, did not yield Cu-complexes, despite the bands in the UV region showing that noradrenaline and adrenaline were oxidized during the process. The complexes display a resonance Raman effect, and the most enhanced bands involve ring modes and particularly the nuCC+nuCO stretching mode at ca. 1384 cm(-1). The free radical nature of the ligands and the oxidation state of the Cu(II) were confirmed by the EPR spectra that display absorptions assigned to organic radicals with g=2.0005 and g=2.0923, and for Cu(II) with g=2.008 and g=2.0897 for L-dopasemiquinone and dopasemiquinone, respectively. The possibility that dopamine and L-dopa can form stable and aqueous-soluble copper complexes at neutral pH, whereas noradrenaline and adrenaline cannot, may be important in understanding how Cu(II)-dopamine crosses the cellular membrane as proposed in the literature to explain the role of copper in Wilson disease.

Preparation, UV-vis, IR, EPR and resonance Raman study of Fe, Ni, Co and Zn dioxolene complexes

An UV-vis, Raman, IR and EPR spectroscopic study was performed for the water soluble complexes of Fe(III), Ni(II), Co(II) and Zn(II) coordinated to dioxolene ligands derived from oxidized dopamine. The complexes were obtained and stabilized at neutral pH by the strong reducing agent sodium thiosulfate. Iron(III) stabilizes the ligand in catecholate form as [Fe(III)(Cat)2]1-, Cat=dopacatecholate, and the divalent metals as dopasemiquinone (SQ): [Ni(SQ)3]1-, [Co(SQ)3]1- and [Zn(SQ)3]1-. The resonance Raman spectra of the solid complexes as [CAT][Ni(SQ)3], [CTA][Co(SQ)3] and [CTA][Zn(SQ)3], CTA is the cetyltrimethylammonium, are very similar to the spectra of the complexes in solution, while the Fe(III) complex is a mixture of two iron complexes, with catecholate or dopasemiquinone ligands.

Role for an essential tyrosine in peptide amidation

The catalytic core of the peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL) domain of peptidylglycine alpha-amidating monooxygenase was investigated with respect to its ability to function as a ureidoglycolate lyase and the identity and role of its bound metal ions. The purified PAL catalytic core (PALcc) contains molar equivalents of calcium and zinc along with substoichiometric amounts of iron and functions as a ureidoglycolate lyase. Limiting iron availability in the cells synthesizing PALcc reduces the specific activity of the enzyme produced. Concentrated samples of native PALcc have an absorption maximum at 560 nm, suggestive of a phenolate-Fe(III) charge transfer complex. An essential role for a Tyr residue was confirmed by elimination of PAL activity following site-directed mutagenesis. Purified PALcc in which the only conserved Tyr residue (Tyr(654)) was mutated to Phe was secreted normally, but was catalytically inactive and lacked bound iron and bound zinc. Our data demonstrate an essential role for Tyr(654) and suggest that it serves as an Fe(III) ligand in an essential iron-zinc bimetallic site.

Molecular mechanism for pterin-mediated inactivation of tyrosine hydroxylase: formation of insoluble aggregates of tyrosine hydroxylase

Tyrosine hydroxylase (TH), an iron-containing enzyme, catalyzes the first and rate-limiting step of catecholamine biosynthesis, and requires tetrahydrobiopterin (BH4) as a cofactor. We found that preincubation of recombinant human TH with BH4 results in the irreversible inactivation of the enzyme at a concentration far less than the Km value toward BH4 in spite of its cofactor role, whereas oxidized biopterin, which has no cofactor activity, does not affect the enzyme activity. We show that TH is inactivated by BH4 in competition with the binding of dopamine. The sequential addition of BH4 to TH results in a gradual decrease in the intensity of the fluorescence and CD spectra without changing their overall profiles. Sedimentation velocity analysis demonstrated an association of TH molecules with each other in the presence of BH4, and studies using gel-permeation chromatography, turbidity measurements, and transmission electron microscopy demonstrated the formation of amorphous aggregates with large molecular weights following the association of the TH proteins. These results suggest that BH4 not only acts as a cofactor, but also accelerates the aggregation of TH. We propose a novel mechanism for regulating the amount of TH protein, and discuss its physiological significance.

Self-hydroxylation of taurine/alpha-ketoglutarate dioxygenase: evidence for more than one oxygen activation mechanism

2-Aminoethanesulfonic acid (taurine)/alpha-ketoglutarate (alphaKG) dioxygenase (TauD) is a mononuclear non-heme iron enzyme that catalyzes the hydroxylation of taurine to generate sulfite and aminoacetaldehyde in the presence of O2, alphaKG, and Fe(II). Fe(II)TauD complexed with alphaKG or succinate, the decarboxylated product of alphaKG, reacts with O2 in the absence of prime substrate to generate 550- and 720-nm chromophores, respectively, that are interconvertible by the addition or removal of bound bicarbonate and have resonance Raman features characteristic of an Fe(III)-catecholate complex. Mutagenesis studies suggest that both reactions result in the self-hydroxylation of the active-site residue Tyr73, and liquid chromatography nano-spray mass spectrometry/mass spectrometry evidence corroborates this result for the succinate reaction. Furthermore, isotope-labeling resonance Raman studies demonstrate that the oxygen atom incorporated into the tyrosyl residue derives from H2 18O and 18O2 for the alphaKG and succinate reactions, respectively, suggesting distinct mechanistic pathways. Whereas the alphaKG-dependent hydroxylation likely proceeds via an Fe(IV) = O intermediate that is known to be generated during substrate hydroxylation, we propose Fe(III)-OOH (or Fe(V) = O) as the oxygenating species in the succinate-dependent reaction. These results demonstrate the two oxygenating mechanisms available to enzymes with a 2-His-1-carboxylate triad, depending on whether the electron source donates one or two electrons.

Electronic, Vibrational, and Structural Properties of a Spin-Crossover Catecholato-Iron System in the Solid State: Theoretical Study of the Electronic Nature of the Doublet and Sextet States

As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form to be identified. In addition, the vibrational progression observed in the NIR absorption of the LS form was assigned to a five-membered chelate ring mode. The X-ray crystal structure solved at two different temperatures, shows the presence of highly distorted pseudo-octahedral ferric complexes that occupy two nonequivalent crystalline sites. The variation of the molecular parameters as a function of temperature strongly suggests that the two-step transition proceeds by a successive transition of the species in the two nonequivalent sites. The thermal dependence of the high-spin fraction of metal ions determined by Mössbauer experiments is consistent with the magnetic data, except for slight deviations in the high temperature range. The optimized geometries, the electronic transitions, vibrational frequencies, and thermodynamic functions were calculated with the B3LYP density functional method for the doublet and the sextet states. The finding of a ground state that possesses a significant mixture of Fe(III)-catecholate and FeII-semiquinonate configurations is discussed with regard to the set of experimental and theoretical data.

Oxygenase activity in the self-hydroxylation of (s)-2-hydroxypropylphosphonic acid epoxidase involved in fosfomycin biosynthesis

The last step of the biosynthesis of fosfomycin is the conversion of (S)-2-hydroxypropylphosphonic acid (HPP) to fosfomycin by HPP epoxidase (HppE), which is a mononuclear non-heme iron-dependent enzyme. The apo-HppE from Streptomyces wedmorensis is colorless, but turns green with broad absorption bands at 430 and 680 nm after reconstitution with ferrous ion under aerobic conditions. Resonance Raman studies showed that this green chromophore arises from a bidentate iron(III)-catecholate (DOPA) complex, and the most likely site of modification is at Tyr105 on the basis of site-specific mutagenesis results. It was also found that reconstitution in the presence of ascorbate leads to the formation of additional DOPA that shows (18)O-incorporation from (18)O(2). Thus, HppE can act as an oxygenase via a putative high valent iron-oxo or an iron-hydroperoxo intermediate, just like other members of the family of non-heme iron enzymes. The oxygen activation mechanism for catalytic turnover is proposed to parallel that for self-hydroxylation.

Effects of phosphorylation by protein kinase A on binding of catecholamines to the human tyrosine hydroxylase isoforms

Tyrosine hydroxylase (TyrH), the catalyst for the key regulatory step in catecholamine biosynthesis, is phosphorylated by cAMP-dependent protein kinase A (PKA) on a serine residue in a regulatory domain. In the case of the rat enzyme, phosphorylation of Ser40 by PKA is critical in regulating the enzyme activity; the effect of phosphorylation is to relieve the enzyme from inhibition by dopamine and dihydroxyphenylalanine (DOPA). There are four isoforms of human tyrosine hydroxylase (hTyrH), differing in the size of an insertion after Met30. The effects of phosphorylation by PKA on the binding of DOPA and dopamine have now been determined for all four human isoforms. There is an increase of about two-fold in the Kd value for DOPA for isoform 1 upon phosphorylation, from 4.4 to 7.4 microM; this effect decreases with the larger isoforms such that there is no effect of phosphorylation on the Kd value for isoform 4. Dopamine binds more much tightly, with Kd values less than 3 nM for all four unphosphorylated isoforms. Phosphorylation decreases the affinity for dopamine at least two orders of magnitude, resulting in Kd values of about 0.1 microM for the phosphorylated human enzymes, due primarily to increases in the rate constant for dissociation of dopamine. Dopamine binds about two-fold less tightly to the phosphorylated isoform 1 than to the other three isoforms. The results extend the regulatory model developed for the rat enzyme, in which the activity is regulated by the opposing effects of catecholamine binding and phosphorylation by PKA. The small effects on the relatively high Kd values for DOPA suggest that DOPA levels do not regulate the activity of hTyrH.

Characterization of the Fe(III)-binding Site in Sepia Eumelanin by Resonance Raman Confocal Microspectroscopy¶

The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)-like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm(-1); the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm(-1) bands are assigned to Fe-OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin-binding site in melanin is composed of CAT-like structural units.

Interconversion of two oxidized forms of taurine/alpha-ketoglutarate dioxygenase, a non-heme iron hydroxylase: evidence for bicarbonate binding

Taurinealpha-ketoglutarate (alphaKG) dioxygenase, or TauD, is a mononuclear non-heme iron hydroxylase that couples the oxidative decarboxylation of alphaKG to the decomposition of taurine, forming sulfite and aminoacetaldehyde. Prior studies revealed that taurine-free TauD catalyzes an O(2)- and alphaKG-dependent self-hydroxylation reaction involving Tyr-73, yielding an Fe(III)-catecholate chromophore with a lambda(max) of 550 nm. Here, a chromophore (lambda(max) 720 nm) is described and shown to arise from O(2)-dependent self-hydroxylation of TauD in the absence of alphaKG, but requiring the product succinate. A similar chromophore rapidly develops with the alternative oxidant H(2)O(2). Resonance Raman spectra indicate that the approximately 700-nm chromophore also arises from an Fe(III)-catecholate species, and site-directed mutagenesis studies again demonstrate Tyr-73 involvement. The approximately 700-nm and 550-nm species are shown to interconvert by the addition or removal of bicarbonate, consistent with the alphaKG-derived CO(2) remaining tightly bound to the oxidized metal site as bicarbonate. The relevance of the metal-bound bicarbonate in TauD to reactions of other members of this enzyme family is discussed.

Modeled ligand-protein complexes elucidate the origin of substrate specificity and provide insight into catalytic mechanisms of phenylalanine hydroxylase and tyrosine hydroxylase

NMR spectroscopy and X-ray crystallography have provided important insight into structural features of phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH). Nevertheless, significant problems such as the substrate specificity of PAH and the different susceptibility of TH to feedback inhibition by l-3,4-dihydroxyphenylalanine (l-DOPA) compared with dopamine (DA) remain unresolved. Based on the crystal structures 5pah for PAH and 2toh for TH (Protein Data Bank), we have used molecular docking to model the binding of 6(R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) and the substrates phenylalanine and tyrosine to the catalytic domains of PAH and TH. The amino acid substrates were placed in positions common to both enzymes. The productive position of tyrosine in TH.BH4 was stabilized by a hydrogen bond with BH4. Despite favorable energy scores, tyrosine in a position trans to PAH residue His290 or TH residue His336 interferes with the access of the essential cofactor dioxygen to the catalytic center, thereby blocking the enzymatic reaction. DA and l-DOPA were directly coordinated to the active site iron via the hydroxyl residues of their catechol groups. Two alternative conformations, rotated 180 degrees around an imaginary iron-catecholamine axis, were found for DA and l-DOPA in PAH and for DA in TH. Electrostatic forces play a key role in hindering the bidentate binding of the immediate reaction product l-DOPA to TH, thereby saving the enzyme from direct feedback inhibition.

The effect of substrate, dihydrobiopterin, and dopamine on the EPR spectroscopic properties and the midpoint potential of the catalytic iron in recombinant human phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) is a tetrahydrobiopterin (BH(4)) and non-heme iron-dependent enzyme that hydroxylates L-Phe to L-Tyr. The paramagnetic ferric iron at the active site of recombinant human PAH (hPAH) and its midpoint potential at pH 7.25 (E(m)(Fe(III)/Fe(II))) were studied by EPR spectroscopy. Similar EPR spectra were obtained for the tetrameric wild-type (wt-hPAH) and the dimeric truncated hPAH(Gly(103)-Gln(428)) corresponding to the "catalytic domain." A rhombic high spin Fe(III) signal with a g value of 4.3 dominates the EPR spectra at 3.6 K of both enzyme forms. An E(m) = +207 +/- 10 mV was measured for the iron in wt-hPAH, which seems to be adequate for a thermodynamically feasible electron transfer from BH(4) (E(m) (quinonoid-BH(2)/BH(4)) = +174 mV). The broad EPR features from g = 9.7-4.3 in the spectra of the ligand-free enzyme decreased in intensity upon the addition of L-Phe, whereas more axial type signals were observed upon binding of 7,8-dihydrobiopterin (BH(2)), the stable oxidized form of BH(4), and of dopamine. All three ligands induced a decrease in the E(m) value of the iron to +123 +/- 4 mV (L-Phe), +110 +/- 20 mV (BH(2)), and -8 +/- 9 mV (dopamine). On the basis of these data we have calculated that the binding affinities of L-Phe, BH(2), and dopamine decrease by 28-, 47-, and 5040-fold, respectively, for the reduced ferrous form of the enzyme, with respect to the ferric form. Interestingly, an E(m) value comparable with that of the ligand-free, resting form of wt-hPAH, i.e. +191 +/- 11 mV, was measured upon the simultaneous binding of both L-Phe and BH(2), representing an inactive model for the iron environment under turnover conditions. Our findings provide new information on the redox properties of the active site iron relevant for the understanding of the reductive activation of the enzyme and the catalytic mechanism.