Chemically and Biologically Harmless versus Harmful Ferritin/Copper-Metallothionein Couples

Forgotten partners and function regulators of inducible metallothioneins

Metallothioneins are peculiar cysteine rich, heat resistant, small cellular plasma proteins expressed through almost all life forms. The currently established biological functions of metallothioneins are the homeostasis of essential metals and protection against toxic transitional metals (TM) alongside defence from oxidative stress by direct scavenging of reactive oxygen and nitrogen species (ROS and RNS). In mammals, among the four main evolutionary conserved forms, only the ubiquitously expressed metallothionein 1 and 2 (here abbreviated as MT) are inducible by TM, oxidative stress, glucocorticoids and starvation among various other stimuli. However, more than sixty years after being discovered, metallothioneins still bear unresolved issues about their possible physiological function and regulation. The biological function of MTs has still not been associated with the in vitro-demonstrated capacity of MT interaction with cellular molecules glutathione (GSH) or adenosine triphosphate (ATP), or with the possibility of direct iron-MT binding in the reducing intracellular environment of some organelles, e.g. lysosomes. Iron as the most abundant cellular TM is also one of the main physiological sources of ROS. Moreover, iron exhibits strain, sex and age differences that reflected ROS generation and MT induction in (patho)physiology and toxicology studies. A recent study showed that iron sex differences follows expression of both ferritin and MT leading to wide implications from essential TM interconnectivity to aging. This review places emphasis on biochemically proven but physiologically ignored interactions of MT with iron to stimulate advanced research for establishing a wide frame of the biological roles of MTs important for health and longevity.

Copper redox chemistry of plant frataxins

The presence of a conserved cysteine residue in the C-terminal amino acid sequences of plant frataxins differentiates these frataxins from those of other kingdoms and may be key in frataxin assembly and function. We report a full study on the ability of Arabidopsis (AtFH) and Zea mays (ZmFH-1 and ZmFH-2) frataxins to assemble into disulfide-bridged dimers by copper-driven oxidation and to revert to monomers by chemical reduction. We monitored the redox assembly-disassembly process by electrospray ionization mass spectrometry, electrophoresis, UV-Vis spectroscopy, and fluorescence measurements. We conclude that plant frataxins AtFH, ZmFH-1 and ZmFH-2 are oxidized by Cu2+ and exhibit redox cysteine monomer - cystine dimer interexchange. Interestingly, the tendency to interconvert is not the same for each protein. Through yeast phenotypic rescue experiments, we show that plant frataxins are important for plant survival under conditions of excess copper, indicating that these proteins might be involved in copper metabolism.

Defining the metal binding pathways of human metallothionein 1a: balancing zinc availability and cadmium seclusion

Metallothioneins (MTs) are cysteine-rich, metal-binding proteins that are found throughout Nature. This ubiquity highlights their importance in essential metal regulation, heavy metal detoxification and cellular redox chemistry. Missing from the current description of MT function is the underlying mechanism by which MTs achieve their proposed biological functions. To date, there have been conflicting reports on the mechanism of metal binding and the structures of the metal binding intermediates formed during metalation of apoMTs. The form of the metal-bound intermediates dictates the metal sequestering and metal-donating properties of the protein. Through a detailed analysis of spectral data from electrospray ionization mass spectromeric and circular dichroism methods we report that Zn(ii) and Cd(ii) metalation of the human MT1a takes place through two distinct pathways. The first pathway involves formation of beaded structures with up to five metals bound terminally to the 20 cysteines of the protein via a noncooperative mechanism. The second pathway is dominated by the formation of the four-metal domain cluster structure M4SCYS11via a cooperative mechanism. We report that there are different pathway preferences for Zn(ii) and Cd(ii) metalation of apo-hMT1a. Cd(ii) binding follows the beaded pathway above pH 7.1 but beginning below pH 7.1 the clustered (Cd4Scys11) pathway begins to dominate. In contrast, Zn(ii) binding follows the terminal, "beaded", pathway at all physiologically relevant pH (pH ≥ 5.2) only following the clustered pathway below pH 5.1. The results presented here allow us to reconcile the conflicting reports concerning the presence of different metalation intermediates of MTs. The conflict regarding cooperative versus noncooperative binding mechanisms is also reconciled with the experimental results described here. These two metal-specific pathways and the presence of radically different intermediate structures provide insight into the multi-functional nature of MT: binding Zn(ii) terminally for donation to metalloenzymes and sequestering toxic Cd(ii) in a cluster structure.