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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.

No MeSH data available.


Pourbaix diagram for the speciation of zinc. Red and green dashed lines demonstrate two possible cathodic reactions, oxygen reduction (oxygen dissolved in water in equilibrium with water) and hydrogen ion reduction (water in equilibrium with gaseous hydrogen), respectively. The orange arrow shows the range of biological standard reduction potentials at pH 7.0: from ∼820 mV to ∼ −670 mV.
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fig1: Pourbaix diagram for the speciation of zinc. Red and green dashed lines demonstrate two possible cathodic reactions, oxygen reduction (oxygen dissolved in water in equilibrium with water) and hydrogen ion reduction (water in equilibrium with gaseous hydrogen), respectively. The orange arrow shows the range of biological standard reduction potentials at pH 7.0: from ∼820 mV to ∼ −670 mV.

Mentions: The speciation of zinc is illustrated best with a Pourbaix diagram (Fig. 1). Such a diagram shows chemical species as a function of both redox standard potential and pH values, and demonstrates a remarkable property of zinc that is critical for its functions in biology: In the absence of other coordinating ligands, zinc is present as hydrated Zn2+(aq), over the entire ranges of redox potentials and pH values in biology. The orange arrow in Fig. 1 indicates the range of biologically important standard potentials determined at pH 7.4. Even for the lowest half reactions of the reductions of acetate to acetaldehyde (−581 mV) or succinate to α-ketoglutarate (−670 mV) zinc maintains its the +2 oxidation state [15]. In other words, zinc is redox-inert in biology and therefore its redox properties are irrelevant. Therefore, we refer to zinc in the +2 oxidation state (Zn(II)) simply to zinc in this article though this term is reserved to the element in the 0 oxidation state in chemistry. In biology, many ligands are present and the species observed as a function of the concentrations of these ligands and pH are relevant as they modify the behavior of zinc ions. The resulting implications for function are discussed in this article.


The biological inorganic chemistry of zinc ions ☆
Pourbaix diagram for the speciation of zinc. Red and green dashed lines demonstrate two possible cathodic reactions, oxygen reduction (oxygen dissolved in water in equilibrium with water) and hydrogen ion reduction (water in equilibrium with gaseous hydrogen), respectively. The orange arrow shows the range of biological standard reduction potentials at pH 7.0: from ∼820 mV to ∼ −670 mV.
© Copyright Policy - CC BY
Related In: Results  -  Collection

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

fig1: Pourbaix diagram for the speciation of zinc. Red and green dashed lines demonstrate two possible cathodic reactions, oxygen reduction (oxygen dissolved in water in equilibrium with water) and hydrogen ion reduction (water in equilibrium with gaseous hydrogen), respectively. The orange arrow shows the range of biological standard reduction potentials at pH 7.0: from ∼820 mV to ∼ −670 mV.
Mentions: The speciation of zinc is illustrated best with a Pourbaix diagram (Fig. 1). Such a diagram shows chemical species as a function of both redox standard potential and pH values, and demonstrates a remarkable property of zinc that is critical for its functions in biology: In the absence of other coordinating ligands, zinc is present as hydrated Zn2+(aq), over the entire ranges of redox potentials and pH values in biology. The orange arrow in Fig. 1 indicates the range of biologically important standard potentials determined at pH 7.4. Even for the lowest half reactions of the reductions of acetate to acetaldehyde (−581 mV) or succinate to α-ketoglutarate (−670 mV) zinc maintains its the +2 oxidation state [15]. In other words, zinc is redox-inert in biology and therefore its redox properties are irrelevant. Therefore, we refer to zinc in the +2 oxidation state (Zn(II)) simply to zinc in this article though this term is reserved to the element in the 0 oxidation state in chemistry. In biology, many ligands are present and the species observed as a function of the concentrations of these ligands and pH are relevant as they modify the behavior of zinc ions. The resulting implications for function are discussed in this article.

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.