Richard K. Watt

Synthesis of peptide-nanotube platinum-nanoparticle composites

Nanotubes prepared by the self-assembly of D-Phe-D-Phe molecules are investigated by electron microscopy and Monte Carlo simulations; the nanotubes appear to be porous and are capable of forming novel peptide-nanotube platinum-nanoparticle composites.

The many faces of the octahedral ferritin protein.

Iron is an essential trace nutrient required for the active sites of many enzymes, electron transfer and oxygen transport proteins. In contrast, to its important biological roles, iron is a catalyst for reactive oxygen species (ROS). Organisms must acquire iron but must protect against oxidative damage. Biology has evolved siderophores, hormones, membrane transporters, and iron transport and storage proteins to acquire sufficient iron but maintain iron levels at safe concentrations that prevent iron from catalyzing the formation of ROS. Ferritin is an important hub for iron metabolism because it sequesters iron during times of iron excess and releases iron during iron paucity. Ferritin is expressed in response to oxidative stress and is secreted into the extracellular matrix and into the serum. The iron sequestering ability of ferritin is believed to be the source of the anti-oxidant properties of ferritin. In fact, ferritin has been used as a biomarker for disease because it is synthesized in response to oxidative damage and inflammation. The function of serum ferritin is poorly understood, however serum ferritin concentrations seem to correlate with total iron stores. Under certain conditions, ferritin is also associated with pro-oxidant activity. The source of this switch from anti-oxidant to pro-oxidant has not been established but may be associated with unregulated iron release from ferritin. Recent reports demonstrate that ferritin is involved in other aspects of biology such as cell activation, development, immunity and angiogenesis. This review examines ferritin expression and secretion in correlation with anti-oxidant activity and with respect to these new functions. In addition, conditions that lead to pro-oxidant conditions are considered.

Oxido-reduction is not the only mechanism allowing ions to traverse the ferritin protein shell.

BACKGROUND Most models for ferritin iron release are based on reduction and chelation of iron. However, newer models showing direct Fe(III) chelation from ferritin have been proposed. Fe(III) chelation reactions are facilitated by gated pores that regulate the opening and closing of the channels. SCOPE OF REVIEW Results suggest that iron core reduction releases hydroxide and phosphate ions that exit the ferritin interior to compensate for the negative charge of the incoming electrons. Additionally, chloride ions are pumped into ferritin during the reduction process as part of a charge balance reaction. The mechanism of anion import or export is not known but is a natural process because phosphate is a native component of the iron mineral core and non-native anions have been incorporated into ferritin in vitro. Anion transfer across the ferritin protein shell conflicts with spin probe studies showing that anions are not easily incorporated into ferritin. To accommodate both of these observations, ferritin must possess a mechanism that selects specific anions for transport into or out of ferritin. Recently, a gated pore mechanism to open the 3-fold channels was proposed and might explain how anions and chelators can penetrate the protein shell for binding or for direct chelation of iron. CONCLUSIONS AND GENERAL SIGNIFICANCE These proposed mechanisms are used to evaluate three in vivo iron release models based on (1) equilibrium between ferritin iron and cytosolic iron, (2) iron release by degradation of ferritin in the lysosome, and (3) metallo-chaperone mediated iron release from ferritin.

A Unified Model for Ferritin Iron Loading by the Catalytic Center: Implications for Controlling “Free Iron” during Oxidative Stress

IRONING OUT THE DIFFERENCES: A unified model shows that the catalytic centers of human H ferritin and archaeal P. furiosus ferritin load iron according to the same mechanism. This model could help our understanding of the processes of controlling the various subcellular concentrations of iron during inflammation.

Sensitive detection of surface- and size-dependent direct and indirect band gap transitions in ferritin

Ferritin is a protein nano-cage that encapsulates minerals inside an 8 nm cavity. Previous band gap measurements on the native mineral, ferrihydrite, have reported gaps as low as 1.0 eV and as high as 2.5-3.5 eV. To resolve this discrepancy we have used optical absorption spectroscopy, a well-established technique for measuring both direct and indirect band gaps. Our studies included controls on the protein nano-cage, ferritin with the native ferrihydrite mineral, and ferritin with reconstituted ferrihydrite cores of different sizes. We report measurements of an indirect band gap for native ferritin of 2.140 ± 0.015 eV (579.7 nm), with a direct transition appearing at 3.053 ± 0.005 eV (406.1 nm). We also see evidence of a defect-related state having a binding energy of 0.220 ± 0.010 eV . Reconstituted ferrihydrite minerals of different sizes were also studied and showed band gap energies which increased with decreasing size due to quantum confinement effects. Molecules that interact with the surface of the mineral core also demonstrated a small influence following trends in ligand field theory, altering the native minerals band gap up to 0.035 eV.

Ferritin and metallothionein: dangerous liaisons

Ferritin (Ft) interaction with the Zn-complexes of mammalian MT1, MT2 and MT3 metallothioneins (MT) leads to simultaneous Fe(II) and Zn(II) release.

Ferritin as a model for developing 3rd generation nano architecture organic/inorganic hybrid photo catalysts for energy conversion

Solar power is the best option to replace fossil fuels to meet global energy demands. Current photovoltaic and artificial photosynthetic systems require improvements and include the development of: 1) inexpensive, abundant, non-toxic charge separation catalysts that absorb visible light; 2) stable catalysts with high turnover numbers that possess self-healing mechanisms to prevent photo corrosion; 3) catalysts capable of oxidizing a broad range of electron donors; and 4) hybrid organic/inorganic nano architectures that bridge charge flow from electron donors to electron acceptors. The ferritin nanocage is a model system of such an organic/inorganic hybrid catalyst capable of overcoming these photochemical limitations.

Anion deposition into ferritin

When the iron core of equine spleen ferritin is reduced, anions in solution cross the protein shell and enter the ferritin interior as part of a charge balancing reaction. Anion sequestration inside ferritin during iron core reduction was monitored using ion selective electrodes, inductively coupled plasma emission, and energy-dispersive X-ray spectroscopy. The requirement for anion translocation to the ferritin interior occurs because upon iron core reduction, two OH(-) ions per iron are released or neutralized inside ferritin leaving a net positive charge. Halides and oxoanions were tested as anionic substrates for this reaction. A general trend for the halides showed that the smaller halides accumulated inside ferritin in greater abundance than larger halides, presumably because the protein channels restrict the transfer of the larger anionic species. In contrast, oxoanion accumulation inside ferritin did not show selectivity based on size or charge. Vanadate and molybdate accumulated to the highest concentrations and nitrate, phosphate and tungstate showed poor accumulation inside ferritin. Fe(II) remains stably sequestered inside ferritin, as shown by electron microscopy and by column chromatography. Upon oxidation of the iron core, the anions are expelled from ferritin, and OH(-) ions coordinate to the Fe(III) to form the original Fe(O)OH mineral. Anion transport across the ferritin protein shell represents an important mechanism by which ferritin maintains proper charge balance inside the protein cavity.

Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)-phosphate complex: a potential non-transferrin bound iron species.

In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe(3+) loading into apo-transferrin by forming Fe(III)-phosphate species. We report that phosphate competes with apo-transferrin for Fe(3+) by forming a soluble Fe(III)-phosphate complex. Once formed, the Fe(III)-phosphate complex is not a substrate for donating Fe(3+) to apo-transferrin. Phosphate (1-10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe(3+), such as iron nitrilotriacetic acid (Fe(3+)-NTA), and Fe(III)-citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe(3+) to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe(3+) and phosphate, the addition of Fe(2+) to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe(3+) into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.

A ferritin mediated photochemical method to synthesize biocompatible catalytically active gold nanoparticles: size control synthesis for small (∼2 nm), medium (∼7 nm) or large (∼17 nm) nanoparticles

Ferritin (Ftn) undergoes photo-induced charge separation reactions that oxidize organic substrates. The liberated electrons are transferred through the protein shell to reduce Au ions to gold nanoparticles (AuNPs). We systematically varied the concentrations of citrate (electron donor), Au3+ or Au+ (electron acceptor), and ferritin (photo catalyst) to determine if careful control of these reactant concentrations would: (1) provide size control; (2) alter the morphology of the resulting AuNPs; and (3) alter the catalytic activity of the resulting AuNPs. The size and phosphate content of the ferritin iron core was also evaluated for its influence in this photocatalysis reaction. We report that as the Ftn concentration was increased to an optimal range, the number of AuNPs increased and showed smaller size, more spherical shape, and narrower distribution. Increasing the citrate concentration (electron donor) increased the rate of AuNP formation producing more spherical, uniform sized AuNPs. Increasing the Au3+ concentrations increased the number and sizes of the AuNPs. Since Au3+ reduction requires 3-electrons we proposed that using Au+ would increase the rate of the reaction. The photochemical reaction with Au+ was faster and produced 2.4 ± 1.0 nm diameter AuNPs providing another method of size control. AuNPs were tested as reduction catalysts to convert 4-nitrophenol into 4-aminophenol. The smaller spherical AuNPs were better reduction catalysts than the larger AuNPs. In summary, using a single photochemical synthesis method we can reproducibly control the size, uniformity and catalytic activity of the resulting AuNPs simply by varying the concentrations or oxidation states of the reactants.