Alison Butler

Competition among marine phytoplankton for different chelated iron species

Dissolved-iron availability plays a critical role in controlling phytoplankton growth in the oceans,. The dissolved iron is overwhelmingly (∼99%) bound to organic ligands with a very high affinity for iron, but the origin, chemical identity and biological availability of this organically complexed Fe is largely unknown. The release into sea water of complexes that strongly chelate iron could result from the inducible iron-uptake systems of prokaryotes (siderophore complexes) or by processes such as zooplankton-mediated degradation and release of intracellular material (porphyrin complexes). Here we compare the uptake of siderophore- and porphyrin-complexed 55Fe by phytoplankton, using both cultured organisms and natural assemblages. Eukaryotic phytoplankton efficiently assimilate porphyrin-complexed iron, but this iron source is relatively unavailable to prokaryotic picoplankton (cyanobacteria). In contrast, iron bound to a variety of siderophores is relatively more available to cyanobacteria than to eukaryotes, suggesting that the two plankton groups exhibit fundamentally different iron-uptake strategies. Prokaryotes utilize iron complexed to either endogenous or exogenous siderophores, whereas eukaryotes may rely on a ferrireductase system, that preferentially accesses iron chelated by tetradentate porphyrins, rather than by hexadentate siderophores. Competition between prokaryotes and eukaryotes for organically-bound iron may therefore depend on the chemical nature of available iron complexes, with consequences for ecological niche separation, plankton community size-structure and carbon export in low-iron waters.

Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands.

Iron is a limiting nutrient for primary production in large areas of the oceans. Dissolved iron(iii) in the upper oceans occurs almost entirely in the form of complexes with strong organic ligands presumed to be of biological origin. Although the importance of organic ligands to aquatic iron cycling is becoming clear, the mechanism by which they are involved in this process remains uncertain. Here we report observations of photochemical reactions involving Fe(iii) bound to siderophores—high-affinity iron(iii) ligands produced by bacteria to facilitate iron acquisition. We show that photolysis of Fe(iii)–siderophore complexes leads to the formation of lower-affinity Fe(iii) ligands and the reduction of Fe(iii), increasing the availability of siderophore-bound iron for uptake by planktonic assemblages. These photochemical reactions are mediated by the α-hydroxy acid moiety, a group which has generally been found to be present in the marine siderophores that have been characterized. We suggest that Fe(iii)-binding ligands can enhance the photolytic production of reactive iron species in the euphotic zone and so influence iron availability in aquatic systems.

Microbial Iron Acquisition: Marine and Terrestrial Siderophores

The vast majority of bacteria require iron for growth.1,2 Iron is an essential element required for key biological processes including amino acid synthesis, oxygen transport, respiration, nitrogen fixation, methanogenesis, the citric acid cycle, photosynthesis and DNA biosynthesis. However, obtaining iron presents challenges for the majority of microorganisms. While iron is the fourth most abundant transition metal in the Earths crust, the insolubility of iron(III) [Ksp of Fe(OH)3 = 10-39] at physiological pH in aerobic environments severely limits the availability of this essential nutrient. Pathogenic and marine bacteria face similar challenges for obtaining iron because both live in very low iron environments. Bacteria typically require micromolar levels of total iron for growth, yet the iron concentration in the surface waters of the oceans is only 0.01-2 nM.3-7 In humans cellular iron is also very low and is sequestered by lactoferrin, transferrin, and ferritin as a primary defense mechanism at the onset of infection.8 Given its cellular importance, it is not surprising that microbes have evolved multiple pathways designed to extract iron from their surrounding environments, tailored to the molecular constraints of the iron pool (Figure 1). Figure 1 Microbial (Gram negative) iron uptake pathways. In this review the general pathways by which bacteria acquire iron are considered first as an overview to illustrate the singular importance of iron for microbial growth. The focus of this review is on siderophore-mediated iron uptake, particularly structural characteristics of marine siderophores and the reactivity that these characteristics confer. Relatively little is known about marine microbial iron transport compared to that for terrestrial and pathogenic microbes, yet comparison of the structures and reactivity may hint at the biological advantage that these structural traits confer to marine microbes and very possibly provide insights to siderophore-mediated iron uptake in some pathogens.

COORDINATION CHEMISTRY OF VANADIUM IN BIOLOGICAL SYSTEMS

F. Organisms that accumulate vanadium ......... (i) Amanita muscaria (ii) Tunicates G. Vanadium-protein interactions (i) Vanadium transferrin (ii) Vanadoenzymes (a) Vanadium bromoperoxidase (b) Vanadium nitrogenase H. Other systems (i) Vanadyl-substituted bleomycin (ii) Vanadyl-substituted dopamine /J-monooxygenase (iii) Vanadocene interaction with nucleic acids .... I. Summary and a look to the future Acknowledgements References

Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry

Brown algae of the Laminariales (kelps) are the strongest accumulators of iodine among living organisms. They represent a major pump in the global biogeochemical cycle of iodine and, in particular, the major source of iodocarbons in the coastal atmosphere. Nevertheless, the chemical state and biological significance of accumulated iodine have remained unknown to this date. Using x-ray absorption spectroscopy, we show that the accumulated form is iodide, which readily scavenges a variety of reactive oxygen species (ROS). We propose here that its biological role is that of an inorganic antioxidant, the first to be described in a living system. Upon oxidative stress, iodide is effluxed. On the thallus surface and in the apoplast, iodide detoxifies both aqueous oxidants and ozone, the latter resulting in the release of high levels of molecular iodine and the consequent formation of hygroscopic iodine oxides leading to particles, which are precursors to cloud condensation nuclei. In a complementary set of experiments using a heterologous system, iodide was found to effectively scavenge ROS in human blood cells.

Catalytic activity of mesoporous silicate-immobilized chloroperoxidase

A versatile enzyme, FeHeme chloroperoxidase (CPO) from Caldariomyces fumago, is immobilized in the mesoporous silicate material, mesocellular foam (MCF). MCF is a promising material for immobilizing enzymes, due to its large pore structure and high loading capacity compared to other mesoporous materials, such as MCM-48, SBA-16 and SBA-15. The immobilized CPO in MCF retains its activity. The optimal pH at which the maximum amount of enzyme is immobilized was determined to be pH 3.4, slightly below the isoelectric point of the enzyme. A weak ionic interaction between the enzyme and the surface of the inorganic substrate is thought to be critical in maintaining the activity of the immobilized enzyme. The loading capacity of MCF is 122 mg protein per 1 g of MCF. We demonstrate the advantage of MCF as an inorganic substrate for immobilization of enzymes.

The role of vanadium bromoperoxidase in the biosynthesis of halogenated marine natural products

Halogenated natural products are frequently reported metabolites in marine seaweeds. These compounds span a range from halogenated indoles, terpenes, acetogenins, phenols, etc., to volatile halogenated hydrocarbons that are produced on a very large scale. In many cases these halogenated marine metabolites possess biological activities of pharmacological interest. Given the abundance of halogenated marine natural products found in marine organisms and their potentially important biological activities, the biogenesis of these compounds has intrigued marine natural product chemists for decades. Over a quarter of a century ago, a possible role for haloperoxidase enzymes was first suggested in the biogenesis of certain halogenated marine natural products, although this was long before haloperoxidases were discovered in marine organisms. Since that time, FeHeme- and Vanadium-haloperoxidases (V-HPO) have been discovered in many marine organisms. The structure and catalytic activity of vanadium haloperoxidases is reviewed herein, including the importance of V-HPO-catalyzed bromination and cyclization of terpene substrates.

MECHANISTIC CONSIDERATIONS OF THE VANADIUM HALOPEROXIDASES

Abstract Haloperoxidases are enzymes which catalyze the oxidation of halide ions (i.e. chloride, bromide and iodide) by hydrogen peroxide. These enzymes usually contain the FeHeme moiety or vanadium as an essential constituent at their active site, however, a few haloperoxidases which lack a metal cofactor are known. This review will examine (1) the reactivity of the vanadium haloperoxidases, particularly the mechanism of halide oxidation by hydrogen peroxide, and the mechanism of halogenation and sulfoxidation, including the newly reported regioselectivity and enantioselectivity of the vanadium haloperoxidases; (2) the X-ray structure of vanadium chloroperoxidase, the vanadium(V) active site and the role of critical amino acid side chains for catalysis and (3) functional biomimetic systems, with specific relevance to the mechanism of the vanadium haloperoxidase enzymes.

Determination of conditional stability constants and kinetic constants for strong model Fe-binding ligands in seawater

Abstract Conditional stability constants and the rates of formation and dissociation for Fe3+ complexation with nine model ligands were measured in chelexed, photo-oxidized seawater. The ligands were chosen to represent Fe-binding organic functional groups that are present in seawater as a result of siderophore production by marine prokaryotes, or as a result of release during cell lysis or grazing. Four Fe-chelating moieties were studied including: tetrapyrrole ligands (i.e., phaeophytin and protoporphyrin IX (and its dimethyl ester); a terrestrial catecholate siderophore (i.e., enterobactin); terrestrial hydroxamate siderophores (i.e., ferrichrome and desferrioxamine) and marine siderophores containing a mixed functional moiety: β-hydroxyaspartate/catecholate (i.e., Alterobactin A) and the bis-catecholate siderophore (i.e., Alterobactin B). Also considered were the Fe storage protein apoferritin, and the Fe-complexing ligand inositol hexaphosphate (phytic acid). The competitive ligand 1-nitroso-2-naphthol (1N2N) was used with cathodic stripping voltammetry (CLE-CSV) to determine conditional stability constants for these FeL complexes. Conditional stability constants (log KFe3+L) for the nine ligands ranged from log KFe3+L=21.6 to greater than 24.0, remarkably close to the values that have been reported for natural ligands in seawater. Formation rate constants, kf, for inorganic Fe′ complexation by these Fe-binding ligands varied by a factor of 21 and ranged from 0.93×105 M−1 s−1 (apoferritin) to 19.6×105 (desferrioxamine). Dissociation rate constants, kd, of the model FeL complexes varied by a factor of 316 and ranged from 0.05×10−6 s−1 (ferrichrome) to 15.8×10−6 s−1 (enterobactin). Kinetic measurements showed log KFe3+L values ranging between 20.8 and 22.9. Results suggest that the CLE-CSV method cannot distinguish between different organic moieties that may be present in seawater, because the measured conditional stability constants do not vary in a systematic manner with Fe-binding ligand structure. The dissociation rate constant does provide structural information on the organic compounds binding Fe3+ in seawater, and its variation for model ligands appears to correlate with changes in ligand structure.

Mechanistic considerations of halogenating enzymes

In nature, halogenation is a strategy used to increase the biological activity of secondary metabolites, compounds that are often effective as drugs. However, halides are not particularly reactive unless they are activated, typically by oxidation. The pace of discovery of new enzymes for halogenation is increasing, revealing new metalloenzymes, flavoenzymes, S-adenosyl-L-methionine (SAM)-dependent enzymes and others that catalyse halide oxidation using dioxygen, hydrogen peroxide and hydroperoxides, or that promote nucleophilic halide addition reactions.