Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen.

Since the characterization of cytochrome c552 as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH4+ produced from NO2- is released as NH3 leading to nitrogen loss, similar to denitrification which generates NO, N2O, and N2. NH4+ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO2- to NH4+, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.

A [3Cu:2S] cluster provides insight into the assembly and function of the CuZ site of nitrous oxide reductase.

Nitrous oxide reductase (N2OR) is the only known enzyme reducing environmentally critical nitrous oxide (N2O) to dinitrogen (N2) as the final step of bacterial denitrification. The assembly process of its unique catalytic [4Cu:2S] cluster CuZ remains scarcely understood. Here we report on a mutagenesis study of all seven histidine ligands coordinating this copper center, followed by spectroscopic and structural characterization and based on an established, functional expression system for Pseudomonas stutzeri N2OR in Escherichia coli. While no copper ion was found in the CuZ binding site of variants H129A, H130A, H178A, H326A, H433A and H494A, the H382A variant carried a catalytically inactive [3Cu:2S] center, in which one sulfur ligand, SZ2, had relocated to form a weak hydrogen bond to the sidechain of the nearby lysine residue K454. This link provides sufficient stability to avoid the loss of the sulfide anion. The UV-vis spectra of this cluster are strikingly similar to those of the active enzyme, implying that the flexibility of SZ2 may have been observed before, but not recognized. The sulfide shift changes the metal coordination in CuZ and is thus of high mechanistic interest.

Histidine-Gated Proton-Coupled Electron Transfer to the CuA Site of Nitrous Oxide Reductase.

Copper-containing nitrous oxide reductase (N2OR) is the only known enzyme to catalyze the conversion of the environmentally critical greenhouse gas nitrous oxide (N2O) to dinitrogen (N2) as the final step of bacterial denitrification. Other than its unique tetranuclear active site CuZ, the binuclear electron entry point CuA is also utilized in other enzymes, including cytochrome c oxidase. In the CuA site of Pseudomonas stutzeri N2OR, a histidine ligand was found to undergo a conformational flip upon binding of the substrate N2O between the two copper centers. Here we report on the systematic mutagenesis and spectroscopic and structural characterization of this histidine and surrounding H-bonding residues, based on an established functional expression system for PsN2OR in E. coli. A single hydrogen bond from Ser550 is sufficient to stabilize an unbound conformation of His583, as shown in a Asp576Ala variant, while the additional removal of the hydrogen bond in a Asp576Ala/Ser550Ala double variant compelled His583 to stay in a bound conformation as a ligand to CuA. Systematic mutagenesis of His583 to Ala, Asp, Asn, Glu, Gln, Lys, Phe, Tyr, and Trp showed that although both the CuZ and CuA sites were present in all the variants, only the ones with a protonable side chain, i.e., His, Asp, and Glu, were able to mediate electron transfer at physiological pH. This observation is in line with a proton-coupled electron transfer mechanism at the CuA site of N2OR.

Introduction: Transition Metals and Sulfur.

The number of transition metal ions which are essential to life - also often called trace elements - increased steadily over the years. In parallel, the list of biological functions in which transition metals are involved, has grown, and is still growing tremendously. Significant progress has been made in understanding the chemistry operating at the biological sites where metal ions have been discovered. Early on, based on the application of physical, chemical, and biological techniques, it became likely that numerous of these metal centers carry sulfur ligands in their coordination sphere, such as sulfide (S2-), cysteine (RS-), or methionine (RSCH3). Notably, the structure and the reactivity of the metal active sites turned out to be quite different from anything previously observed in simple coordination complexes. Consequently, the prediction of active-site structures, based on known properties of transition metal ion complexes, turned out to be difficult and incorrect in many cases. Yet, biomimetic inorganic chemistry, via synthesis and detailed structural and electronic characterization of synthetic analogues, became an important factor and helped to understand the properties of the metal active sites. Striking advances came from molecular biology techniques and protein crystallography, as documented by the publication of the first high-resolution structures of iron-sulfur proteins and the blue copper protein plastocyanin approximately five decades ago. In this volume of METAL IONS IN LIFE SCIENCES the focus will be on some of the most intriguing, in our view, transition metal-sulfur sites discovered in living organisms. These include the type 1 Cu mononuclear center, the purple mixed-valent [Cu1.5+-(Cys)2-Cu1.5+] CuA, the tetranuclear copper-sulfide catalytic center of nitrous oxide reductase, the heme-thiolate site in cytochrome P450, the iron-sulfur proteins with bound inorganic (S2-) and organic (Cys-) sulfur, the pterin dithiolene cofactor (Moco) coordinated to either molybdenum or tungsten, the [8Fe-7S] P-cluster and the [Mo-7Fe-9S-C]-homocitrate catalytic site of nitrogenase, the siroheme-[4Fe-4S] center involved in the reduction of sulfite (SO32-) to hydrogen sulfide (H2S), the NiFeS sites of hydrogenases and CO dehydrogenase, and the zinc finger domains. We apologize to all researchers and their associates who have made tremendous contributions to our current knowledge of the steadily increasing transition metal sulfur sites in proteins and enzymes but are not mentioned here. These omissions are by no means intentional but merely the consequence of time and space. We are fully aware of the excellent books and authoritative reviews on various aspects of the subject, however, it is our motivation to cover in one single volume this exciting domain of bioinorganic chemistry.

Sulfur, the Versatile Non-metal.

The non-metallic chemical element sulfur, 3216S , referred to in Genesis as brimstone and identified as element by Lavoisier, is the tenth most abundant element in the universe and the fifth most common element on Earth. Important inorganic forms of sulfur in the biosphere are elemental sulfur (S8), sulfate (SO2-4), and sulfide (S2-), sulfite (SO2-3), thiosulfate, (S2O23), and polythionates (S3O62-; S4O62-). Because of its wide range of stable oxidation states, from +6to -2, sulfur plays important roles in central biochemistry as a structural and redoxactive element and is intimately related to life on Earth. Unusual reaction pathways involving sulfur compounds become possible by the specific properties of this element. Sulfur occurs in all the major classes of biomolecules, including enzymes, proteins, sugars, nucleic acids, vitamin cofactors, and metabolites. The flexibility of these biomolecules follows from its versatile chemistry. The best known sulfur mineral is perhaps pyrite (Fool's gold), with the chemical formula, FeS2. Sulfur radical anions, such as [S3].-, are responsible for the intense blue color of lapis lazuli, one of the most desired and expensive artists' materials. In the microbial world, inorganic sulfur compounds, e.g., elemental sulfur and sulfate, belong to the most important electron acceptors. Studies on microbial sulfur metabolism revealed many novel enzymes and pathways and advanced the understanding on metabolic processes used for energy conservation, not only of the microbes, but of biology in general. Transition metal sulfur complexes display intriguing catalytic activities, they provide surfaces and complex cavities in metalloenzymes that activate inert molecules such as H2, CO, N2 or N2O, and they catalyze the transformations of numerous organic molecules. Both thiamine diphosphate- (ThDP) and S-adenosyl- L-methionine- (SAM) dependent enzymes belong to Nature's most powerful catalysts with a remarkable spectrum of catalytic activities. In conclusion, given sulfur's diverse properties, evolution made an excellent choice in selecting sulfur as one the basic elements of life.

Cytochrome P450. The Dioxygen-Activating Heme Thiolate.

Cytochromes P450 (CYPs) are heme b-binding enzymes and belong to Nature's most versatile catalysts. They participate in countless essential life processes, and exist in all domains of life, Bacteria, Archaea, and Eukarya, and in viruses. CYPs attract the interest of researchers active in fields as diverse as biochemistry, chemistry, biophysics, molecular biology, pharmacology, and toxicology. CYPs fight chemicals such as drugs, poisonous compounds in plants, carcinogens formed during cooking, and environmental pollutants. They represent the first line of defense to detoxify and solubilize poisonous substances by modifying them with dioxygen. The heme iron is proximally coordinated by a thiolate residue, and this ligation state represents the active form of the enzyme. The Fe(III) center displays characteristic UV/Vis and EPR spectra (Soret maximum at 418 nm; g-values at 2.41, 2.26, 1.91). The Fe(II) state binds the inhibitor carbon monoxide (CO) to produce a Fe(II)-CO complex, with the major absorption maximum at 450 nm, hence, its name P450. CYPs are flexible proteins in order to allow a vast range of substrates to enter and products to leave. Two extreme forms exist: substrate-bound (closed) and substrate-free (open). CYPs share a sophisticated catalytic cycle that involves a series of consecutive transformations of the heme thiolate active site, with the strong oxidants compound I and II as key intermediates. Each of these high-valent Fe(IV) species has its characteristic features and chemical properties, crucial for the activation of dioxygen and cleavage of strong C-H bonds.

Walking the seven lines: binuclear copper A in cytochrome c oxidase and nitrous oxide reductase.

The enzymes nitrous oxide reductase (N2OR) and cytochrome c oxidase (COX) are constituents of important biological processes. N2OR is the terminal reductase in a respiratory chain converting N2O to N2 in denitrifying bacteria; COX is the terminal oxidase of the aerobic respiratory chain of certain bacteria and eukaryotic organisms transforming O2 to H2O accompanied by proton pumping. Different spectroscopies including magnetic resonance techniques, were applied to show that N2OR has a mixed-valent Cys-bridged [Cu1.5+(CyS)2Cu1.5+] copper site, and that such a binuclear center, called CuA, does also exist in COX. A sequence motif shared between the CuA center of N2OR and the subunit II of COX raises the issue of a putative evolutionary relationship of the two enzymes. The suggestion of a binuclear CuA in COX, with one unpaired electron delocalized between two equivalent Cu nuclei, was difficult to accept originally, even though regarded as a clever solution to many experimental observations. This minireview in honor of Helmut Sigel traces several of the critical steps forward in understanding the nature of CuA in N2OR and COX, and discusses its unique electronic features to some extent including the contributions made by the development of methodology and the discovery of a novel multi-copper enzyme. Left: X-band (9.130 GHz) and C-band (4.530 GHz, 1st harmonic display of experimental spectrum) EPR spectra of bovine heart cytochrome c oxidase, recorded at 20K. Right: Ribbon presentation of the CuA domain in cytochrome c oxidase and nitrous oxide reductase.

Anaerobic Microbial Degradation of Hydrocarbons: From Enzymatic Reactions to the Environment.

Hydrocarbons are abundant in anoxic environments and pose biochemical challenges to their anaerobic degradation by microorganisms. Within the framework of the Priority Program 1319, investigations funded by the Deutsche Forschungsgemeinschaft on the anaerobic microbial degradation of hydrocarbons ranged from isolation and enrichment of hitherto unknown hydrocarbon-degrading anaerobic microorganisms, discovery of novel reactions, detailed studies of enzyme mechanisms and structures to process-oriented in situ studies. Selected highlights from this program are collected in this synopsis, with more detailed information provided by theme-focused reviews of the special topic issue on 'Anaerobic biodegradation of hydrocarbons' [this issue, pp. 1-244]. The interdisciplinary character of the program, involving microbiologists, biochemists, organic chemists and environmental scientists, is best exemplified by the studies on alkyl-/arylalkylsuccinate synthases. Here, research topics ranged from in-depth mechanistic studies of archetypical toluene-activating benzylsuccinate synthase, substrate-specific phylogenetic clustering of alkyl-/arylalkylsuccinate synthases (toluene plus xylenes, p-cymene, p-cresol, 2-methylnaphthalene, n-alkanes), stereochemical and co-metabolic insights into n-alkane-activating (methylalkyl)succinate synthases to the discovery of bacterial groups previously unknown to possess alkyl-/arylalkylsuccinate synthases by means of functional gene markers and in situ field studies enabled by state-of-the-art stable isotope probing and fractionation approaches. Other topics are Mo-cofactor-dependent dehydrogenases performing O2-independent hydroxylation of hydrocarbons and alkyl side chains (ethylbenzene, p-cymene, cholesterol, n-hexadecane), degradation of p-alkylated benzoates and toluenes, glycyl radical-bearing 4-hydroxyphenylacetate decarboxylase, novel types of carboxylation reactions (for acetophenone, acetone, and potentially also benzene and naphthalene), W-cofactor-containing enzymes for reductive dearomatization of benzoyl-CoA (class II benzoyl-CoA reductase) in obligate anaerobes and addition of water to acetylene, fermentative formation of cyclohexanecarboxylate from benzoate, and methanogenic degradation of hydrocarbons.

Structure and Function of the Unusual Tungsten Enzymes Acetylene Hydratase and Class II Benzoyl-Coenzyme A Reductase.

In biology, tungsten (W) is exclusively found in microbial enzymes bound to a bis-pyranopterin cofactor (bis-WPT). Previously known W enzymes catalyze redox oxo/hydroxyl transfer reactions by directly coordinating their substrates or products to the metal. They comprise the W-containing formate/formylmethanofuran dehydrogenases belonging to the dimethyl sulfoxide reductase (DMSOR) family and the aldehyde:ferredoxin oxidoreductase (AOR) families, which form a separate enzyme family within the Mo/W enzymes. In the last decade, initial insights into the structure and function of two unprecedented W enzymes were obtained: the acetaldehyde forming acetylene hydratase (ACH) belongs to the DMSOR and the class II benzoyl-coenzyme A (CoA) reductase (BCR) to the AOR family. The latter catalyzes the reductive dearomatization of benzoyl-CoA to a cyclic diene. Both are key enzymes in the degradation of acetylene (ACH) or aromatic compounds (BCR) in strictly anaerobic bacteria. They are unusual in either catalyzing a nonredox reaction (ACH) or a redox reaction without coordinating the substrate or product to the metal (BCR). In organic chemical synthesis, analogous reactions require totally nonphysiological conditions depending on Hg2+ (acetylene hydration) or alkali metals (benzene ring reduction). The structural insights obtained pave the way for biological or biomimetic approaches to basic reactions in organic chemistry.

Acetylene hydratase: a non-redox enzyme with tungsten and iron-sulfur centers at the active site.

In living systems, tungsten is exclusively found in microbial enzymes coordinated by the pyranopterin cofactor, with additional metal coordination provided by oxygen and/or sulfur, and/or selenium atoms in diverse arrangements. Prominent examples are formate dehydrogenase, formylmethanofuran dehydrogenase, and aldehyde oxidoreductase all of which catalyze redox reactions. The bacterial enzyme acetylene hydratase (AH) stands out of its class as it catalyzes the conversion of acetylene to acetaldehyde, clearly a non-redox reaction and a reaction distinct from the reduction of acetylene to ethylene by nitrogenase. AH harbors two pyranopterins bound to W, and a [4Fe-4S] cluster. W is coordinated by four dithiolene sulfur atoms, one cysteine sulfur, and one oxygen ligand. AH activity requires a strong reductant suggesting W(IV) as the active oxidation state. Two different types of reaction pathways have been proposed. The 1.26 Å structure reveals a water molecule coordinated to W which could gain a partially positive net charge by the adjacent protonated Asp-13, enabling a direct attack of C2H2. To access the W-Asp site, a substrate channel was evolved distant from where it is found in other members of the DMSOR family. Computational studies of this second shell mechanism led to unrealistically high energy barriers, and alternative pathways were proposed where C2H2 binds directly to W. The architecture of the catalytic cavity, the specificity for C2H2 and the results from site-directed mutagenesis do not support this first shell mechanism. More investigations including structural information on the binding of C2H2 are needed to present a conclusive answer.

The role of molecular oxygen in the iron(III)-promoted oxidative dehydrogenation of amines.

A mechanistic study is presented of the oxidative dehydrogenation of the iron(III) complex [Fe(III)L(3)](3+), 1, (L(3) = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex [Fe(II)L(4)](2+), 2, (L(4) = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(III) complex 1. Reduced oxygen species, e.g., superoxide, (O2˙(-)) and peroxide (O2(2-)), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, -d[(Fe(III)L(3))(3+)]/dt = kOD[(Fe(III)L(3))(3+)][EtO(-)][O2], with kOD = 3.80 ± 0.09 × 10(7) M(-2) s(-1) (60 °C, µ = 0.01 M). The reduction O2 → O2˙(-) represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H2O2, replacing O2 as the oxidant, the experimental rate law depended on [H2O2]: -d[(Fe(III)L(3))(3+)]/dt = kH2O2[(Fe(III)L(3))(3+)][H2O2]), with kH2O2 = 6.25 ± 0.02 × 10(-3) M(-1) s(-1). In contrast to the reaction performed under N2, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(III) complex 1 with O2. Under N2, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(II)-monoimine complex [Fe(II)L(4)](2+) and the iron(II)-amine complex [Fe(II)L(3)](2+) in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O2, with kEtO(-) = 3.02 ± 0.09 × 10(5) M(-1) s(-1) (O2) compared to kEtO(-) = 4.92 ± 0.01 × 10(4) M(-1) s(-1) (N2), documenting the role of molecular oxygen in the dehydrogenation reaction.

The magic of dioxygen.

Oxygen has to be considered one of the most important elements on Earth. Earlier, some dispute arose as to which of the three scientists, Carl Wilhelm Scheele (Sweden), Joseph Priestley (United Kingdom) or Antoine Lavoisier (France), should get credit for the air of life.Today it is agreed that the Swede discovered it first, the fire air in 1772. The British chemist published it first, the dephlogisticated air in 1775, and the Frenchman understood it first, the oxygen in 1775-1778. Surely, there is credit enough for all three to split the "Nobel Prize" awarded by Carl Djerassi and Roald Hoffmann in their play Oxygen. Molecular oxygen means life. So-called aerobes - these include humans, animals, and plants - need O2 to conserve the energy they have to gain from their environment. Eliminate O2 and these organisms cannot support an active lifestyle. What makes dioxygen that special? It is a non-metal and oxidizing agent that readily reacts with most elements to form compounds, notably oxides. From a biological point of view, the most important compound of course is water, H2O, which provides an excellent solvent for biomolecules. It influences the climate of the Earth, and it is the source of almost all of the molecular oxygen in the atmosphere.

The oxidative fermentation of ethanol in Gluconacetobacter diazotrophicus is a two-step pathway catalyzed by a single enzyme: alcohol-aldehyde Dehydrogenase (ADHa).

Gluconacetobacter diazotrophicus is a N2-fixing bacterium endophyte from sugar cane. The oxidation of ethanol to acetic acid of this organism takes place in the periplasmic space, and this reaction is catalyzed by two membrane-bound enzymes complexes: the alcohol dehydrogenase (ADH) and the aldehyde dehydrogenase (ALDH). We present strong evidence showing that the well-known membrane-bound Alcohol dehydrogenase (ADHa) of Ga. diazotrophicus is indeed a double function enzyme, which is able to use primary alcohols (C2-C6) and its respective aldehydes as alternate substrates. Moreover, the enzyme utilizes ethanol as a substrate in a reaction mechanism where this is subjected to a two-step oxidation process to produce acetic acid without releasing the acetaldehyde intermediary to the media. Moreover, we propose a mechanism that, under physiological conditions, might permit a massive conversion of ethanol to acetic acid, as usually occurs in the acetic acid bacteria, but without the transient accumulation of the highly toxic acetaldehyde.

The production of ammonia by multiheme cytochromes C.

The global biogeochemical nitrogen cycle is essential for life on Earth. Many of the underlying biotic reactions are catalyzed by a multitude of prokaryotic and eukaryotic life forms whereas others are exclusively carried out by microorganisms. The last century has seen the rise of a dramatic imbalance in the global nitrogen cycle due to human behavior that was mainly caused by the invention of the Haber-Bosch process. Its main product, ammonia, is a chemically reactive and biotically favorable form of bound nitrogen. The anthropogenic supply of reduced nitrogen to the biosphere in the form of ammonia, for example during environmental fertilization, livestock farming, and industrial processes, is mandatory in feeding an increasing world population. In this chapter, environmental ammonia pollution is linked to the activity of microbial metalloenzymes involved in respiratory energy metabolism and bioenergetics. Ammonia-producing multiheme cytochromes c are discussed as paradigm enzymes.

Catalytic scope of the thiamine-dependent multifunctional enzyme cyclohexane-1,2-dione hydrolase.

The thiamine diphosphate (ThDP)-dependent enzyme cyclohexane-1,2-dione hydrolase (CDH) was expressed in Escherichia coli and purified by affinity chromatography (Ni-NTA). Recombinant CDH showed the same C-C bond-cleavage and C-C bond-formation activities as the native enzyme. Furthermore, we have shown that CDH catalyzes the asymmetric cross-benzoin reaction of aromatic aldehydes and (decarboxylated) pyruvate (up to quantitative conversion, 92-99 % ee). CDH accepts also hydroxybenzaldehydes and nitrobenzaldehydes; these previously have not (or only in rare cases) been known as substrates of other ThDP-dependent enzymes. On a semipreparative scale, sterically demanding 4-(tert-butyl)benzaldehyde and 2-naphthaldehyde were transformed into the corresponding 2-hydroxy ketone products in high yields. Additionally, certain benzaldehydes with electron withdrawing substituents were identified as potential inhibitors of the ligase activity of CDH.

Toxicity and deficiency of copper in Elsholtzia splendens affect photosynthesis biophysics, pigments and metal accumulation.

Elsholtzia splendens is a copper-tolerant plant species growing on copper deposits in China. Spatially and spectrally resolved kinetics of in vivo absorbance and chlorophyll fluorescence in mesophyll of E. splendens were used to investigate the copper-induced stress from deficiency and toxicity as well as the acclimation to excess copper stress. The plants were cultivated in nutrient solutions containing either Fe(III)-EDTA or Fe(III)-EDDHA. Copper toxicity affected light-acclimated electron flow much stronger than nonphotochemical quenching (NPQ) or dark-acclimated photochemical efficiency of PSIIRC (Fv/Fm). It also changed spectrally resolved Chl fluorescence kinetics, in particular by strengthening the short-wavelength (<700 nm) part of NPQ altering light harvesting complex II (LHCII) aggregation. Copper toxicity reduced iron accumulation, decreased Chls and carotenoids in leaves. During acclimation to copper toxicity, leaf copper decreased but leaf iron increased, with photosynthetic activity and pigments recovering to normal levels. Copper tolerance in E. splendens was inducible; acclimation seems be related to homeostasis of copper and iron in E. splendens. Copper deficiency appeared at 10 mg copper per kg leaf DW, leading to reduced growth and decreased photosynthetic parameters (F0, Fv/Fm, ΦPSII). The importance of these results for evaluating responses of phytoremediation plants to stress in their environment is discussed.

In vivo speciation studies and antioxidant properties of bromine in Laminaria digitata reinforce the significance of iodine accumulation for kelps.

The metabolism of bromine in marine brown algae remains poorly understood. This contrasts with the recent finding that the accumulation of iodide in the brown alga Laminaria serves the provision of an inorganic antioxidant - the first case documented from a living system. The aim of this study was to use an interdisciplinary array of techniques to study the chemical speciation, transformation, and function of bromine in Laminaria and to investigate the link between bromine and iodine metabolism, in particular in the antioxidant context. First, bromine and iodine levels in different Laminaria tissues were compared by inductively coupled plasma MS. Using in vivo X-ray absorption spectroscopy, it was found that, similarly to iodine, bromine is predominantly present in this alga in the form of bromide, albeit at lower concentrations, and that it shows similar behaviour upon oxidative stress. However, from a thermodynamic and kinetic standpoint, supported by in vitro and reconstituted in vivo assays, bromide is less suitable than iodide as an antioxidant against most reactive oxygen species except superoxide, possibly explaining why kelps prefer to accumulate iodide. This constitutes the first-ever study exploring the potential antioxidant function of bromide in a living system and other potential physiological roles. Given the tissue-specific differences observed in the content and speciation of bromine, it is concluded that the bromide uptake mechanism is different from the vanadium iodoperoxidase-mediated uptake of iodide in L. digitata and that its function is likely to be complementary to the iodide antioxidant system for detoxifying superoxide.