The Molecular Basis for Zinc Bioavailability

Zinc is an essential micronutrient, and its deficiency is perhaps the most prevalent and least understood worldwide. Recent advances have expanded the understanding of zinc’s unique chemistry and molecular roles in a vast array of critical functions. However, beyond the concept of zinc absorption, few studies have explored the molecular basis of zinc bioavailability that determines the proportion of dietary zinc utilized in zinc-dependent processes in the body. The purpose of this review is to merge the concepts of zinc molecular biology and bioavailability with a focus on the molecular determinants of zinc luminal availability, absorption, transport, and utilization.

An Integrated Mass Spectrometry and Molecular Dynamics Simulations Approach Reveals the Spatial Organization Impact of Metal-Binding Sites on the Stability of Metal-Depleted Metallothionein-2 Species

Mammalian metallothioneins (MTs) are a group of cysteine-rich proteins that bind metal ions in two α- and β-domains and represent a major cellular Zn(II)/Cu(I) buffering system in the cell. At cellular free Zn(II) concentrations (10-11-10-9 M), MTs do not exist in fully loaded forms with seven Zn(II)-bound ions (Zn7MTs). Instead, MTs exist as partially metal-depleted species (Zn4-6MT) because their Zn(II) binding affinities are on the nano- to picomolar range comparable to the concentrations of cellular Zn(II). The mode of action of MTs remains poorly understood, and thus, the aim of this study is to characterize the mechanism of Zn(II) (un)binding to MTs, the thermodynamic properties of the Zn1-6MT2 species, and their mechanostability properties. To this end, native mass spectrometry (MS) and label-free quantitative bottom-up and top-down MS in combination with steered molecular dynamics simulations, well-tempered metadynamics (WT-MetaD), and parallel-bias WT-MetaD (amounting to 3.5 μs) were integrated to unravel the chemical coordination of Zn(II) in all Zn1-6MT2 species and to explain the differences in binding affinities of Zn(II) ions to MTs. Differences are found to be the result of the degree of water participation in MT (un)folding and the hyper-reactive character of Cys21 and Cys29 residues. The thermodynamics properties of Zn(II) (un)binding to MT2 are found to differ from those of Cd(II), justifying their distinctive roles. The potential of this integrated strategy in the investigation of numerous unexplored metalloproteins is attested by the results highlighted in the present study.

Interplay between Carbonic Anhydrases and Metallothioneins: Structural Control of Metalation

Carbonic anhydrases (CAs) and metallothioneins (MTs) are both families of zinc metalloproteins central to life, however, they coordinate and interact with their Zn2+ ion cofactors in completely different ways. CAs and MTs are highly sensitive to the cellular environment and play key roles in maintaining cellular homeostasis. In addition, CAs and MTs have multiple isoforms with differentiated regulation. This review discusses current literature regarding these two families of metalloproteins in carcinogenesis, with a dialogue on the association of these two ubiquitous proteins in vitro in the context of metalation. Metalation of CA by Zn-MT and Cd-MT is described. Evidence for protein-protein interactions is introduced from changes in metalation profiles of MT from electrospray ionization mass spectrometry and the metalation rate from stopped-flow kinetics. The implications on cellular control of pH and metal donation is also discussed in the context of diseased states.

Metallothionein Cd4S11cluster formation dominates in the protection of carbonic anhydrase

Results from ESI-MS and stopped flow kinetics show that apo-MT protects from toxic metalation of apo-CA with Cd²⁺due to the protein–protein interactions in solution.

Kinetics of competitive Cd2+ binding pathways: the realistic structure of intrinsically disordered, partially metallated metallothioneins

The metallation of metallothionein can proceed via two different intermediate structures: a beaded structure that forms quickly (top) and a slow-forming cluster structure (bottom) before forming the fully metallated two-domain protein.

Zinc Metabolism and Metallothioneins

Among the trace elements, zinc is one of the most used elements in biological systems. Zinc is found in the structure of more than 2700 enzymes, including hydrolases, transferases, oxyreductases, ligases, isomerases, and lyases. Not surprisingly, it is present in almost all body cells. Preserving the stability and integrity of biological membranes and ion channels, zinc is also an intracellular regulator and provides structural support to proteins during molecular interactions. It acts as a structural element in nucleic acids or other gene-regulating proteins. Metallothioneins, the low molecular weight protein family rich in cysteine groups, are involved significantly in numerous physiological and pathological processes including particularly oxidative stress. A critical role of metallothioneins (MT) is to bind zinc with high affinity and to serve as an intracellular zinc reservoir. By releasing free intracellular zinc when needed, MTs mediate the unique physiological roles of zinc. MT expression is induced by zinc elevation, and thus, zinc homeostasis is maintained. That MT mediates the effects of zinc, besides having strong radical scavenging effects, points to the critical part it plays in oxidative stress. The present review aims to give information on metallothioneins, which have critical importance in the metabolism and molecular pathways of zinc.

Capturing platinum in cisplatin: kinetic reactions with recombinant human apo-metallothionein 1a

Detailed mass spectra data show the stepwise sequence of cisplatin deconstruction by apo-metallothionein.

Differential reactivity of closely related zinc(II)-binding metallothioneins from the plant Arabidopsis thaliana

The dynamics of metal binding to and transfer from metalloproteins involved in metal homeostasis are important for understanding cellular distribution of metal ions. The dicotyledonous plant Arabidopsis thaliana has two type 4 seed-specific metallothionein homologues, MT4a and MT4b, with likely roles in zinc(II) homeostasis. These two metallothioneins are 84% identical, with full conservation of all metal-binding cysteine and histidine residues. Yet, differences in their spatial and temporal expression patterns suggested divergence in their biological roles. To investigate whether biological functions are reflected in molecular properties, we compare aspects of zinc(II)-binding dynamics of full-length MT4a and MT4b, namely the pH dependence of zinc(II) binding and protein folding, and zinc(II) transfer to the chelator EDTA. UV-Vis and NMR spectroscopies as well as native electrospray ionisation mass spectrometry consistently showed that transfer from Zn6MT4a is considerably faster than from Zn6MT4b, with pseudo-first-order rate constants for the fastest observed step of k obs = 2.8 × 10-4 s-1 (MT4b) and k obs = 7.5 × 10-4 s-1 (MT4a) (5 µM protein, 500 µM EDTA, 25 mM Tris buffer, pH 7.33, 298 K). 2D heteronuclear NMR experiments allowed locating the most labile zinc(II) ions in domain II for both proteins. 3D homology models suggest that reactivity of this domain is governed by the local environment around the mononuclear Cys2His2 site that is unique to type 4 MTs. Non-conservative amino acid substitutions in this region affect local electrostatics as well as whole-domain dynamics, with both effects rendering zinc(II) ions bound to MT4a more reactive in metal transfer reactions. Therefore, domain II of MT4a is well suited to rapidly release its bound zinc(II) ions, in broad agreement with a previously suggested role of MT4a in zinc(II) transport and delivery to other proteins.

Zinc binds non-cooperatively to human liver metallothionein 2a at physiological pH

Maintenance of the homeostasis of zinc is very important in regulating bodily functions. There are over 300 Zn-dependent enzymes identified where Zn(II) plays a structural or catalytic role. However, an excess of Zn(II) in a cell is toxic and free Zn(II) is tightly controlled. Metallothioneins (MTs) are small cysteine rich proteins that can bind up to seven Zn(II) and act as a Zn(II) reservoir. The MT2a isoform is predominantly found in the liver. This study focused on designing an MT2a construct of recombinant human MT2a to determine the Zn(II) binding profile of MT2a in vitro. We analyzed the pH dependence of Zn-MT2a speciation from electrospray ionization mass spectral data. At physiological pH, Zn(II) is terminally bound to the cysteine thiols of MT2a, making bead-like structures (non-cooperative metal binding), while at low pH, Zn(II) formed Zn₄S₁₁-MT2a clusters involving bridged cysteinyl thiols to the Zn(II) (cooperative metal binding). The Zn(II) binding profile of MT2a was compared to Zn(II) binding profile of human kidney MT1a, which was reported in literature, and found that the Zn(II) binding profile of MT2a is similar to that of MT1a. The facility of forming bead-like structures at physiological pH for Zn₅-MT2a means that Zn₇-MT2a can donate up to two Zn(II) to Zn-dependent enzymes.

Proteomic High Affinity Zn2+ Trafficking: Where Does Metallothionein Fit in?

The cellular constitution of Zn-proteins and Zn-dependent signaling depend on the capacity of Zn²⁺ to find specific binding sites in the face of a plethora of other high affinity ligands. The most prominent of these is metallothionein (MT). It serves as a storage site for Zn²⁺ under various conditions, and has chemical properties that support a dynamic role for MT in zinc trafficking. Consistent with these characteristics, changing the availability of zinc for cells and tissues causes rapid alteration of zinc bound to MT. Nevertheless, zinc trafficking occurs in metallothionein-null animals and cells, hypothetically making use of proteomic binding sites to mediate the intracellular movements of zinc. Like metallothionein, the proteome contains a large concentration of proteins that strongly coordinate zinc. In this environment, free Zn²⁺ may be of little significance. Instead, this review sets forth the basis for the hypothesis that components of the proteome and MT jointly provide the platform for zinc trafficking.

Residue Modification and Mass Spectrometry for the Investigation of Structural and Metalation Properties of Metallothionein and Cysteine-Rich Proteins

Structural information regarding metallothioneins (MTs) has been hard to come by due to its highly dynamic nature in the absence of metal-thiolate cluster formation and crystallization difficulties. Thus, typical spectroscopic methods for structural determination are limited in their usefulness when applied to MTs. Mass spectrometric methods have revolutionized our understanding of protein dynamics, structure, and folding. Recently, advances have been made in residue modification mass spectrometry in order to probe the hard-to-characterize structure of apo- and partially metalated MTs. By using different cysteine specific alkylation reagents, time dependent electrospray ionization mass spectrometry (ESI-MS), and step-wise “snapshot” ESI-MS, we are beginning to understand the dynamics of the conformers of apo-MT and related species. In this review we highlight recent papers that use these and similar techniques for structure elucidation and attempt to explain in a concise manner the data interpretations of these complex methods. We expect increasing resolution in our picture of the structural conformations of metal-free MTs as these techniques are more widely adopted and combined with other promising tools for structural elucidation.

Selective cysteine modification of metal-free human metallothionein 1a and its isolated domain fragments: Solution structural properties revealed via ESI-MS

Human metallothionein 1a, a protein with two cysteine-rich metal-binding domains (α with 11 Cys and β with 9), was analyzed in its metal-free form by selective, covalent Cys modification coupled with ESI-MS. The modification profiles of the isolated β- and α-fragments reacted with p-benzoquinone (Bq), N-ethylmalemide (NEM) and iodoacetamide (IAM) were compared with the full length protein using ESI-mass spectral data to follow the reaction pathway. Under denaturing conditions at low pH, the reaction profile with each modifier followed pathways that resulted in stochastic, Normal distributions of species whose maxima was equal to the mol. eq. of modifier added. Our interpretation of modification at this pH is that reaction with the cysteines is unimpeded when the full protein or those of its isolated domains are denatured. At neutral pH, where the protein is expected to be folded in a more compact structure, there is a difference in the larger Bq and NEM modification, whose reaction profiles indicate a cooperative pattern. The reaction profile with IAM under native conditions follows a similar stochastic distribution as at low pH, suggesting that this modifier is small enough to access the cysteines unimpeded by the compact structure. The data emphasize the utility of residue modification coupled with electrospray ionization mass spectrometry for the study of protein structure.

Whole Genome Pathway Analysis Identifies an Association of Cadmium Response Gene Loss with Copy Number Variation in Mutant p53 Bearing Uterine Endometrial Carcinomas

Background

Massive chromosomal aberrations are a signature of advanced cancer, although the factors promoting the pervasive incidence of these copy number alterations (CNAs) are poorly understood. Gatekeeper mutations, such as p53, contribute to aneuploidy, yet p53 mutant tumors do not always display CNAs. Uterine Corpus Endometrial Carcinoma (UCEC) offers a unique system to begin to evaluate why some cancers acquire high CNAs while others evolve another route to oncogenesis, since about half of p53 mutant UCEC tumors have a relatively flat CNA landscape and half have 20–90% of their genome altered in copy number.

Methods

We extracted copy number information from 68 UCEC genomes mutant in p53 by the GISTIC2 algorithm. GO term pathway analysis, via GOrilla, was used to identify suppressed pathways. Genes within these pathways were mapped for focal or wide distribution. Deletion hotspots were evaluated for temporal incidence.

Results

Multiple pathways contributed to the development of pervasive CNAs, including developmental, metabolic, immunological, cell adhesion and cadmium response pathways. Surprisingly, cadmium response pathway genes are predicted as the earliest loss events within these tumors: in particular, the metallothionein genes involved in heavy metal sequestration. Loss of cadmium response genes were associated with copy number changes and poorer prognosis, contrasting with 'copy number flat' tumors which instead exhibited substantive mutation.

Conclusion

Metallothioneins are lost early in the development of high CNA endometrial cancer, providing a potential mechanism and biological rationale for increased incidence of endometrial cancer with cadmium exposure. Developmental and metabolic pathways are altered later in tumor progression.