Investigating the influence of histidine residues on the metal ion binding ability of the wheat metallothionein γ-Ec-1 domain

Type 4 plant metallothioneins - players in zinc biofortification?

Food security is defined as uninterrupted access to food that meets people's dietary needs. One essential trace element of a complete diet is zinc, which is vital for various processes, including growth, development, and the immune response. The estimated global prevalence of zinc deficiency is around 30%. Meat and meat products provide an abundant and also bioavailable source of zinc. However, in developing countries, access to meat is restricted, and in developed countries, meat consumption has declined for ethical and environmental reasons. The potential for zinc deficiency arises from (i) low concentrations of this element in plant-based diets, (ii) poor zinc absorption from plant-based food in the human intestine, and (iii) the risk of uptake of toxic metals together with essential ones. This review summarises the current knowledge concerning type 4 metallothioneins, which represent promising targets for zinc biofortification. We describe their place in the zinc route from soil to seed, their expression patterns, their role in plants, and their three-dimensional protein structure and how this affects their selectivity towards zinc. This review aims to provide a comprehensive theoretical basis for the potential use of type 4 plant metallothioneins to create zinc-biofortified crops.

Quantitative immunodetection of metalloprotein II in Perinereis aibuhitensis after heavy metal exposure

The metal detoxification mechanism is essential for the accumulation activity of some polychaetes. In order to investigate the detoxification function of metalloprotein II in polychaete Perinereis aibuhitensis, a recombinant fusion protein of MP II was successfully expressed in vitro and an anti-MP II polyclonal antibody was got. The tissue distribution of MP II in P. aibuhitensis and the protein expression under Cd, Pb and Zn exposure were detected by ELISA with this antibody. The results showed that MP II was higher in the intestine of P. aibuhitensis, followed by the body wall and parapodium. Cd, Zn and Pb exposure can induced MP II expression, but the change trend of MP II under various heavy metal exposure was different. The amount of MP II induced in P. aibuhitensis increased with the rises of Cd concentration, but there is no significant relationship between Zn and Pb concentration and MP II level.

Secretory expression, immunoaffinity purification and metal-binding ability of recombinant metallothionein (ShMT) from freshwater crab Sinopotamon henanense

Metallothioneins (MTs) are a super-family of ubiquitous, low-molecular-weight, cysteine-rich and metal-binding proteins. They are thought to play a predominant role in mediating metal metabolism and antioxidation. However, the accurate functions of MTs remain unclear in the physiological processes due to native proteins deficiency and little information of their metal-binding character. Freshwater crab Sinopotamon henanense is a decapod crustacean widely distributed in northern China, in which only one MT isoform (ShMT) has been reported so far. In order to shed light on the accurate role of ShMT, a novel recombinant ShMT in native form was over-expressed by phoA secreted expression system in Escherichia coli (E. coli). Then the ShMT proteins were purified using a one-step gentle immunoaffinity chromatography with a polyol-responsive mAb (PR-mAb) to ShMT, which was generated by conventional hybridoma technology followed by ELISA-elution. The Zn-, Cu-, and Cd-ShMT complexes were prepared by recombinant synthesis in metal-enriched media and reconstitution with metal ions, respectively. Further analysis about metal-binding capacity showed recombinant ShMT has high ability to bind Zn, Cu and Cd metals, although the recombinantly expressed and reconstituted metal-ShMT complexes have different metal-to-protein stoichiometry. Moreover, the affinity of recombinant protein for metal ions has been analyzed using competitive reaction with 5, 5-dithiobis (2-nitrobenzoic acid) (DTNB). The results demonstrated the affinity of recombinant ShMT for metals was as follows: Cu＞Cd＞Zn. In summary, the experimental procedure we have developed facilitates production of recombinant ShMT with native characteristics for further research and the study of metal-binding ability could help further clarify the accurate functions of ShMT.

Mercury(II) Binding to Metallothionein in Mytilus edulis revealed by High Energy-Resolution XANES Spectroscopy

Of all divalent metals, mercury (HgII ) has the highest affinity for metallothioneins. HgII is considered to be enclosed in the α and β domains as tetrahedral α-type Hg4 Cys11-12 and β-type Hg3 Cys9 clusters similar to CdII and ZnII . However, neither the four-fold coordination of Hg nor the existence of Hg-Hg atomic pairs have ever been demonstrated, and the HgII partitioning among the two protein domains is unknown. Using high energy-resolution XANES spectroscopy, MP2 geometry optimization, and biochemical analysis, evidence for the coexistence of two-coordinate Hg-thiolate complex and four-coordinate Hg-thiolate cluster with a metacinnabar-type (β-HgS) structure in the α domain of separate metallothionein molecules from blue mussel under in vivo exposure is provided. The findings suggest that the CXXC claw setting of thiolate donors, which only exists in the α domain, acts as a nucleation center for the polynuclear complex and that the five CXC motifs from this domain serve as the cluster-forming motifs. Oligomerization is driven by metallophilic Hg⋅⋅⋅Hg interactions. Our results provide clues as to why Hg has higher affinity for the α than the β domain. More generally, this work provides a foundation for understanding how metallothioneins mediate mercury detoxification in the cell under in vivo conditions.

Analysis of the soybean metallothionein system under free radical stress: protein modification connected to lipid membrane damage

Metallothioneins are small Cys-rich peptides capable of coordinating metal ions, and proposed to be involved in radical stress. The four Zn(ii)-GmMT complexes of soybean (Glycine max) were recombinantly synthesised and exposed to oxidative (HO˙) and reductive (H˙ atoms and eaq-) stress conditions. Gamma-irradiation was used to simulate the endogenous formation of the reactive species in both aqueous solutions and unsaturated lipid vesicle suspensions, a biomimetic model that showed that tandem protein/lipid damage occurs, in particular under reductive radical stress. This is due to the formation of diffusible sulphur-centred radicals, which migrate from the aqueous phase to the lipid bilayer and are thus able to transform the cis double bond of the oleate moiety into the trans isomer. Among the amino acid residues present in GmMTs, Cys is one of the most sensitive residues towards the attack of free radicals, thus suggesting metal-clusters to be good interceptors of free radicals. Also Met, Tyr and Phe residues are sensitive amino acid sites of attack under both oxidative and reductive conditions. The modification of the Zn(ii)-GmMT complexes, in particular isoform 2, was monitored by Raman spectroscopy and mass spectrometry. Free radical stress on the Zn(ii)-GmMT complexes is able to induce significant structural changes such as partial deconstruction and/or rearrangement of the metal clusters, but not the complete demetallation of the proteins nor breaking of the backbone, thus confirming their capability to act as protectors under free radical stress conditions.

Localization and Spectroscopic Analysis of the Cu(I) Binding Site in Wheat Metallothionein Ec-1

The early cysteine-labeled metallothionein (MT) from Triticum aestivum (common wheat), denoted Ec-1, features two structurally well-defined domains, γ and βE, coordinating two and four Zn(II) ions, respectively. While the protein is currently assumed to function mainly in zinc homeostasis, a low amount of copper ions was also recently detected in a native Ec-1 sample. To evaluate the observed copper binding in more detail, the recombinant Zn₆Ec-1 form was exposed to different amounts of Cu(I) ions and the resulting species characterized with spectroscopic methods. Data reveal that the first Cu(I) equivalent coordinates exclusively to the N-terminal γ-domain of the protein and replaces one Zn(II) ion. To analyze the ability of the γ-domain for coordination of monovalent metal ions in more detail, the γ-Ec-1 peptide fragment was incubated with increasing amounts of Cu(I) and the process monitored with UV-VIS, circular dichroism, and luminescence spectroscopy. Closely similar spectra are observed regardless if the apo- or the metal ion-loaded and, hence, pre-folded forms, were used for the titration experiments with Cu(I). The results indicate that low amounts of Cu(I) ions displace the two metal ions subsequently and stoichiometrically, despite the different coordination geometry requirements of Cu(I) and Zn(II).