Human ferritins sequester and store iron as a stable FeOOH (s) mineral core within a protein shell assembled from 24 subunits of two types, H and L. Core mineralization in recombinant H-and L-subunit homopolymer and heteropolymer ferritins and several site-directed H-subunit variants was investigated to determine the iron oxidation/hydrolysis chemistry as a function of iron flux into the protein.Stopped-flow absorption spectrometry, UV spectrometry, and electrode oximetry revealed that the mineral core forms by at least three pathways, not two as previously thought. They correspond to the ferroxidase, mineral surface, and the Fe(II) + H 2 O 2 detoxification reactions, respectively:
The health benefits of cranberries have long been recognized. However, the mechanisms behind its function are poorly understood. We have investigated the iron-binding properties of quercetin, the major phenolic phytochemical present in cranberries, and other selected phenolic compounds (chrysin, 3-hydroxyflavone, 3',4'-dihydroxy flavone, rutin, and flavone) in aqueous media using UV/vis, NMR and EPR spectroscopies and ESI-Mass spectrometry. Strong iron-binding properties have been confirmed for the compounds containing the "iron-binding motifs" identified in their structures. The apparent binding constants are estimated to be in the range of 10(6) M(-1) to 10(12) M(-2) in phosphate buffer at pH 7.2. Surprisingly, quercetin binds Fe(2+) even stronger than the well known Fe(2+)-chelator ferrozine at pH 7.2. This may be the first example of an oxygen-based ligand displaying stronger Fe(2+)-binding affinity than a strong nitrogen-based Fe(2+)-chelator. The strong Fe-binding properties of these phenolics argue that they may be effective in modulating cellular iron homeostasis under physiological conditions. Quercetin can completely suppress Fenton chemistry both at micromolar levels and in the presence of major cellular iron chelators like ATP or citrate. However, the radical scavenging activity of quercetin provides only partial protection against Fenton chemistry-mediated damage while Fe chelation by quercetin can completely inhibit Fenton chemistry, indicating that the chelation may be key to its antioxidant activity. These results demonstrate that quercetin and other phenolic compounds can effectively modulate iron biochemistry under physiologically relevant conditions, providing insight into the mechanism of action of bio-active phenolics.
Green fluorescent protein (GFP) is a widely used in vivo molecular marker. These proteins are particularly resistant, and maintain function, under a variety of cellular conditions such as pH extremes and elevated temperatures. Green fluorescent proteins are also abundant in several groups of marine invertebrates including reef-forming corals. While molecular oxygen is required for the post-translational maturation of the protein, mature GFPs are found in corals where hyperoxia and reactive oxygen species (ROS) occur due to the photosynthetic activity of algal symbionts. In vitro spin trapping electron paramagnetic resonance and spectrophotometric assays of superoxide dismutase (SOD)-like enzyme activity show that wild type GFP from the hydromedusa, Aequorea victoria, quenches superoxide radicals (O 2 •− ) and exhibits SOD-like activity by competing with cytochrome c for reaction with O 2 •− . When exposed to high amounts of O 2 •− the SOD-like activity and protein structure of GFP are altered without significant changes to the fluorescent properties of the protein. Because of the distribution of fluorescent proteins in both the epithelial and gastrodermal cells of reef-forming corals we propose that GFP, and possibly other fluorescent proteins, can provide supplementary antioxidant protection.
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