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The biological inorganic chemistry of zinc ions ☆

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ABSTRACT

The solution and complexation chemistry of zinc ions is the basis for zinc biology. In living organisms, zinc is redox-inert and has only one valence state: Zn(II). Its coordination environment in proteins is limited by oxygen, nitrogen, and sulfur donors from the side chains of a few amino acids. In an estimated 10% of all human proteins, zinc has a catalytic or structural function and remains bound during the lifetime of the protein. However, in other proteins zinc ions bind reversibly with dissociation and association rates commensurate with the requirements in regulation, transport, transfer, sensing, signalling, and storage. In contrast to the extensive knowledge about zinc proteins, the coordination chemistry of the “mobile” zinc ions in these processes, i.e. when not bound to proteins, is virtually unexplored and the mechanisms of ligand exchange are poorly understood. Knowledge of the biological inorganic chemistry of zinc ions is essential for understanding its cellular biology and for designing complexes that deliver zinc to proteins and chelating agents that remove zinc from proteins, for detecting zinc ion species by qualitative and quantitative analysis, and for proper planning and execution of experiments involving zinc ions and nanoparticles such as zinc oxide (ZnO). In most investigations, reference is made to zinc or Zn2+ without full appreciation of how biological zinc ions are buffered and how the d-block cation Zn2+ differs from s-block cations such as Ca2+ with regard to significantly higher affinity for ligands, preference for the donor atoms of ligands, and coordination dynamics. Zinc needs to be tightly controlled. The interaction with low molecular weight ligands such as water and inorganic and organic anions is highly relevant to its biology but in contrast to its coordination in proteins has not been discussed in the biochemical literature. From the discussion in this article, it is becoming evident that zinc ion speciation is important in zinc biochemistry and for biological recognition as a variety of low molecular weight zinc complexes have already been implicated in biological processes, e.g. with ATP, glutathione, citrate, ethylenediaminedisuccinic acid, nicotianamine, or bacillithiol.

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The pH dependence of five zinc aqua-hydroxo complexes in water solution. a) Molar fraction distribution of particular zinc species as a function of pH. b) Logarithmic plot of the concentration of particular species [17]. Black, red, green, blue and magenta color lines correspond to [Zn(H2O)x]2+, [Zn(OH)(H2O)x-1]+, [Zn(OH)2(H2O)x-2](aq), [Zn(OH)3(H2O)x-3]-, and [Zn(OH)4]2-, respectively.
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fig2: The pH dependence of five zinc aqua-hydroxo complexes in water solution. a) Molar fraction distribution of particular zinc species as a function of pH. b) Logarithmic plot of the concentration of particular species [17]. Black, red, green, blue and magenta color lines correspond to [Zn(H2O)x]2+, [Zn(OH)(H2O)x-1]+, [Zn(OH)2(H2O)x-2](aq), [Zn(OH)3(H2O)x-3]-, and [Zn(OH)4]2-, respectively.

Mentions: The pH dependence of the aquo-hydroxo complexes is based on these equilibria (Fig. 2). The deprotonation of coordinated water molecules begins at slightly alkaline pH, above 7.8 (5% of [Zn(OH)(H2O)x-1]+), and terminates with the fully deprotonated species above pH 11.8 (95% of [Zn(OH)4]2-). The concentration of the uncharged complex, [Zn(OH)2(H2O)x-2](aq), is highest at pH 9.9 and decreases above that value.


The biological inorganic chemistry of zinc ions ☆
The pH dependence of five zinc aqua-hydroxo complexes in water solution. a) Molar fraction distribution of particular zinc species as a function of pH. b) Logarithmic plot of the concentration of particular species [17]. Black, red, green, blue and magenta color lines correspond to [Zn(H2O)x]2+, [Zn(OH)(H2O)x-1]+, [Zn(OH)2(H2O)x-2](aq), [Zn(OH)3(H2O)x-3]-, and [Zn(OH)4]2-, respectively.
© Copyright Policy - CC BY
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120989&req=5

fig2: The pH dependence of five zinc aqua-hydroxo complexes in water solution. a) Molar fraction distribution of particular zinc species as a function of pH. b) Logarithmic plot of the concentration of particular species [17]. Black, red, green, blue and magenta color lines correspond to [Zn(H2O)x]2+, [Zn(OH)(H2O)x-1]+, [Zn(OH)2(H2O)x-2](aq), [Zn(OH)3(H2O)x-3]-, and [Zn(OH)4]2-, respectively.
Mentions: The pH dependence of the aquo-hydroxo complexes is based on these equilibria (Fig. 2). The deprotonation of coordinated water molecules begins at slightly alkaline pH, above 7.8 (5% of [Zn(OH)(H2O)x-1]+), and terminates with the fully deprotonated species above pH 11.8 (95% of [Zn(OH)4]2-). The concentration of the uncharged complex, [Zn(OH)2(H2O)x-2](aq), is highest at pH 9.9 and decreases above that value.

View Article: PubMed Central - PubMed

ABSTRACT

The solution and complexation chemistry of zinc ions is the basis for zinc biology. In living organisms, zinc is redox-inert and has only one valence state: Zn(II). Its coordination environment in proteins is limited by oxygen, nitrogen, and sulfur donors from the side chains of a few amino acids. In an estimated 10% of all human proteins, zinc has a catalytic or structural function and remains bound during the lifetime of the protein. However, in other proteins zinc ions bind reversibly with dissociation and association rates commensurate with the requirements in regulation, transport, transfer, sensing, signalling, and storage. In contrast to the extensive knowledge about zinc proteins, the coordination chemistry of the “mobile” zinc ions in these processes, i.e. when not bound to proteins, is virtually unexplored and the mechanisms of ligand exchange are poorly understood. Knowledge of the biological inorganic chemistry of zinc ions is essential for understanding its cellular biology and for designing complexes that deliver zinc to proteins and chelating agents that remove zinc from proteins, for detecting zinc ion species by qualitative and quantitative analysis, and for proper planning and execution of experiments involving zinc ions and nanoparticles such as zinc oxide (ZnO). In most investigations, reference is made to zinc or Zn2+ without full appreciation of how biological zinc ions are buffered and how the d-block cation Zn2+ differs from s-block cations such as Ca2+ with regard to significantly higher affinity for ligands, preference for the donor atoms of ligands, and coordination dynamics. Zinc needs to be tightly controlled. The interaction with low molecular weight ligands such as water and inorganic and organic anions is highly relevant to its biology but in contrast to its coordination in proteins has not been discussed in the biochemical literature. From the discussion in this article, it is becoming evident that zinc ion speciation is important in zinc biochemistry and for biological recognition as a variety of low molecular weight zinc complexes have already been implicated in biological processes, e.g. with ATP, glutathione, citrate, ethylenediaminedisuccinic acid, nicotianamine, or bacillithiol.

No MeSH data available.


Related in: MedlinePlus