Metal Complexes of Two Specific Regions of ZnuA, a Periplasmic Zinc(II) Transporter from Escherichia coli.

Aleksandra Hecel,Daniela Valensin,Henryk Kozlowski,Arian Kola,Magdalena Rowinska-Zyrek

doi:10.1021/acs.inorgchem.9b03298

Aleksandra Hecel, Daniela Valensin + Show 3 more

Open Access

https://doi.org/10.1021/acs.inorgchem.9b03298

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Abstract

The crystal structure of ZnZnuA from Escherichia coli reveals two metal binding sites. (i) The primary binding site, His143, is located close the His-rich loop (residues 116–138) and plays a significant role in Zn(II) acquisition. (ii) The secondary binding site involves His224. In this work, we focus on understanding the interactions of two metal ions, Zn(II) and Cu(II), with two regions of ZnuA, which are possible anchoring sites for Zn(II): Ac-115MKSIHGDDDDHDHAEKSDEDHHHGDFNMHLW145-NH2 (primary metal binding site) and Ac-223GHFTVNPEIQPGAQRLHE240-NH2 (secondary metal binding site). The histidine-rich loop (residues 116–138) has a role in the capture of zinc(II), which is then further delivered into other regions of the protein. For both Zn(II) complexes, histidine residues constitute the main anchoring donors. In the longer, His-rich fragment, a tetrahedral complex with four His residues is formed, while in the second ligand, two imidazole nitrogens are involved in zinc(II) binding. In both cases, so-called loop structures are formed. One consists of a 125HxHxExxxExHxH137 motif with seven amino acid residues in the loop between the two central histidines, while the other is formed by a 224HFTVNPEIQPGAQRLH239 motif with 14 amino acid residues in the loop between the two nearest coordinating histidines. The number of available imidazoles also strongly affects the structure of copper(II) complexes; the more histidines in the studied region, the higher the pH in which amide nitrogens will participate in Cu(II) binding.

Highlights

ZnuABC is a Zn(II)-specific uptake system that is responsible for the distribution and excretion of zinc(II) ions
First indentified in E. coli, the ZnuABC system has been shown to be necessary in the capture and transport of zinc(II) ions
The crystal structure of ZnuA from E. coli reveals two metal binding sites: (i) the primary binding site, His[143], located close the Hisrich loop, which plays a significant role in Zn(II) acquisition, and (ii) the secondary binding site, encompassing His[224]

Summary

■ INTRODUCTION

ZnuABC is a Zn(II)-specific uptake system that is responsible for the distribution and excretion of zinc(II) ions. The HuPrP76−114 (Ac-PHGGGWGQPHGGGWGQGGGTHSQWNKPSKPKTNMKHMAG-NH2) ligand binds Cu(II) ions with higher affinity, because it forms a very effective macrochelate complex with a set of up to four imidazole nitrogens This analysis is in good agreement with spectroscopic studies indicating that only two histidine residues are involved in copper binding in the Ac-115MKSIHGDDDDHDHAEKSDEDHHHGDFNMHLW145-NH2 system. It is due to the fact that above pH 9, additional amides are involved in the copper(II) coordination sphere, forming a square-planar complex [1Nim, 3N−] in the case of the Ac-223GHFTVNPEIQPGAQRLHE240-NH2 ligand This finding is in good agreement with our previous findings that show that the more histidines are present in the peptide sequence, the more likely it is for amides to participate in Cu(II) binding at a higher pH than usual.[30,33,34,38]. With what can be expected; the fewer histidines participate in metal binding (two imidazoles for the second ZnuA fragment and four for ZnuD), the less stable the complex

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES