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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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Related in: MedlinePlus

The pH dependence of the solubility of zinc hydroxide (ε-Zn(OH)2(s)) [20]. The ordinate indicates on a logarithmic scale the sum of all zinc soluble species present at a particular pH (Zn2+(aq)) which include aquo- and hydroxocomplexes presented in Fig. 2. The dashed line indicates the −log[Zn2+(aq)] value at pH 7.4.
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fig3: The pH dependence of the solubility of zinc hydroxide (ε-Zn(OH)2(s)) [20]. The ordinate indicates on a logarithmic scale the sum of all zinc soluble species present at a particular pH (Zn2+(aq)) which include aquo- and hydroxocomplexes presented in Fig. 2. The dashed line indicates the −log[Zn2+(aq)] value at pH 7.4.

Mentions: Eq. (15) also allows for calculating the solubility of Zn(OH)2(s), which is frequently considered as a completely insoluble zinc species. In a presentation of the relationship between pH and the solubility of zinc hydroxide (Fig. 3), the ordinate gives the sum of all soluble zinc species [Zn2+(aq)] present at various molar fractions at a particular pH. For example, −log values of the concentration of all soluble Zn(II) species at pH 7 and 7.4 are 2.47 and 3.26, corresponding to 3.4 and 0.55 mM, respectively. Even at pH ∼10, where the solubility of Zn(OH)2(s) is lowest, the concentration of soluble zinc species is 0.13 μM. These values are much higher than what researchers usually estimate. They have been confirmed experimentally, for example by using atomic absorption spectrophotometry [23]. A relatively high solubility of Zn(OH)2(s) is the reason why addition of ZnSO4 or ZnCl2 at submillimolar concentrations to a HEPES buffer (at a pH close to the pKa of the buffer) does not result in precipitation. This fact turns out to be useful when preparing solutions of these zinc salts for biophysical measurements [24].


The biological inorganic chemistry of zinc ions ☆
The pH dependence of the solubility of zinc hydroxide (ε-Zn(OH)2(s)) [20]. The ordinate indicates on a logarithmic scale the sum of all zinc soluble species present at a particular pH (Zn2+(aq)) which include aquo- and hydroxocomplexes presented in Fig. 2. The dashed line indicates the −log[Zn2+(aq)] value at pH 7.4.
© Copyright Policy - CC BY
Related In: Results  -  Collection

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

fig3: The pH dependence of the solubility of zinc hydroxide (ε-Zn(OH)2(s)) [20]. The ordinate indicates on a logarithmic scale the sum of all zinc soluble species present at a particular pH (Zn2+(aq)) which include aquo- and hydroxocomplexes presented in Fig. 2. The dashed line indicates the −log[Zn2+(aq)] value at pH 7.4.
Mentions: Eq. (15) also allows for calculating the solubility of Zn(OH)2(s), which is frequently considered as a completely insoluble zinc species. In a presentation of the relationship between pH and the solubility of zinc hydroxide (Fig. 3), the ordinate gives the sum of all soluble zinc species [Zn2+(aq)] present at various molar fractions at a particular pH. For example, −log values of the concentration of all soluble Zn(II) species at pH 7 and 7.4 are 2.47 and 3.26, corresponding to 3.4 and 0.55 mM, respectively. Even at pH ∼10, where the solubility of Zn(OH)2(s) is lowest, the concentration of soluble zinc species is 0.13 μM. These values are much higher than what researchers usually estimate. They have been confirmed experimentally, for example by using atomic absorption spectrophotometry [23]. A relatively high solubility of Zn(OH)2(s) is the reason why addition of ZnSO4 or ZnCl2 at submillimolar concentrations to a HEPES buffer (at a pH close to the pKa of the buffer) does not result in precipitation. This fact turns out to be useful when preparing solutions of these zinc salts for biophysical measurements [24].

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