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== Zinc Binding == | == Zinc Binding == | ||
Many zinc-dependent proteins are transcriptional regulators<ref>DOI: 10.1128/MMBR.00015-06</ref>. CzrA fits into this category as an [https://en.wikipedia.org/wiki/Allosteric_regulation allosteric inhibitor] of the czr operon. Two [https://en.wikipedia.org/wiki/Zinc Zn<sup> +2</sup>] ions may bind to the dimer<ref name="critical"/>, at the location of the <scene name='69/694218/Alpha_5_helices/2'> alpha 5 </scene> helix from each monomer. As zinc binds, the alpha 5 helices <scene name='69/694218/2kjc_zinc_bound/1'>unalign</scene> to inhibit the DNA binding residues. Furthermore, CzrA must be in its dimer form for zinc to bind. The <scene name='69/694218/Spacefill_with_zinc_pockets/1'>zinc binding pocket</scene> is formed by two residues from each monomer, so Zn<sup>+2</sup> cannot bind to the monomer. The <scene name='69/ | Many zinc-dependent proteins are transcriptional regulators<ref>DOI: 10.1128/MMBR.00015-06</ref>. CzrA fits into this category as an [https://en.wikipedia.org/wiki/Allosteric_regulation allosteric inhibitor] of the czr operon. Two [https://en.wikipedia.org/wiki/Zinc Zn<sup> +2</sup>] ions may bind to the dimer<ref name="critical"/>, at the location of the <scene name='69/694218/Alpha_5_helices/2'> alpha 5 </scene> helix from each monomer. As zinc binds, the alpha 5 helices <scene name='69/694218/2kjc_zinc_bound/1'>unalign</scene> to inhibit the DNA binding residues. Furthermore, CzrA must be in its dimer form for zinc to bind. The <scene name='69/694218/Spacefill_with_zinc_pockets/1'>zinc binding pocket</scene> is formed by two residues from each monomer, so Zn<sup>+2</sup> cannot bind to the monomer. The <scene name='69/694220/Zinc_binding_residues/5'>zinc binding site</scene> is formed by Asp84 and His86 from one monomer, and His97 and His100 from the other monomer. Zinc ions were not present in the solution NMR crystal structure, so a representation of a zinc ion in the binding pocket can be seen in figure 3. Histidines are a repetitive and commonly found residue in zinc-binding proteins <ref>Miller J, McLachlan AD, Klug A. Repetitive zinc-binding domains in the protein transcription factor IIIA from Xenopus oocytes. EMBO J. 1985 Jun 4;4(6):1609-1614.</ref>. | ||
Zinc<sup>+2</sup> binding is driven by a large [https://en.wikipedia.org/wiki/Entropy entropic] gain <ref>DOI:10.1021/ja906131b</ref>. Water molecules around the metal ion and CzrA protein are displaced, and gain greater freedom. This gain in entropy allows Zn<sup>+2</sup> to bind to CzrA with reasonable affinity and speed in vivo. The zinc<sup>+2</sup> ion forms a tetrahedral complex with the four residues (Figure 3). This allows other metal ions to act as allosteric inhibitors to CzrA. Any metal that may form a tetrahedral complex will have some affinity for CzrA, assuming it is not too large to fit into the pocket. However, the metal binding pocket of CzrA has been optimized to bind Zn<sup>+2</sup> with the highest affinity. As CzrA is a transcriptional repressor, binding of Zn<sup>+2</sup> to the dimer will activate the czr operon. Zn<sup>+2</sup> is preferred as CzrB opens a Zn<sup>+2</sup> channel, allowing the excess zinc ions to export the cell. | Zinc<sup>+2</sup> binding is driven by a large [https://en.wikipedia.org/wiki/Entropy entropic] gain <ref>DOI:10.1021/ja906131b</ref>. Water molecules around the metal ion and CzrA protein are displaced, and gain greater freedom. This gain in entropy allows Zn<sup>+2</sup> to bind to CzrA with reasonable affinity and speed in vivo. The zinc<sup>+2</sup> ion forms a tetrahedral complex with the four residues (Figure 3). This allows other metal ions to act as allosteric inhibitors to CzrA. Any metal that may form a tetrahedral complex will have some affinity for CzrA, assuming it is not too large to fit into the pocket. However, the metal binding pocket of CzrA has been optimized to bind Zn<sup>+2</sup> with the highest affinity. As CzrA is a transcriptional repressor, binding of Zn<sup>+2</sup> to the dimer will activate the czr operon. Zn<sup>+2</sup> is preferred as CzrB opens a Zn<sup>+2</sup> channel, allowing the excess zinc ions to export the cell. | ||