Structural zinc binding sites shaped for greater works: Structure-function relations in classical zinc finger, hook and clasp domains

Metal ions are essential elements present in biological systems able to facilitate many cellular processes including proliferation, signaling, DNA synthesis and repair. Zinc ion (Zn(II)) is an important cofactor for numerous biochemical reactions. Commonly, structural zinc sites demonstrate high Zn(II) affinity and compact architecture required for sequence-specific macromolecule binding. However, how Zn(II)-dependent proteins fold, how their dissociation occurs, and which factors modulate zinc protein affinity as well as stability remains not fully understood. The molecular rules governing precise regulation of zinc proteins function are hidden in the relationship between sequence and structure, and hence require deep understanding of their folding mechanism under metal load, reactivity and metal-to-protein affinity. Even though, this sequence-structure relationship has an impact on zinc proteins function, it has been shown that other biological factors including cellular localization and Zn(II) availability influence overall protein behavior. Taking into account all of the mentioned factors, in this review, we aim to describe the relationship between structure-function-stability of zinc structural sites, found in a zinc finger, zinc hook and zinc clasps, and reach far beyond a structural point of view in order to appreciate the balance between chemistry and biology that govern the protein world.