Zinc's exclusive tetrahedral coordination governed by its electronic structure

Zinc is a critical component of more than 300 proteins including farnesyltransferase, matrix metalloproteinases and endostatin that are involved in the front-line cancer research, and a host of proteins termed zinc fingers that mediate protein-protein and protein-nucleic acid interactions. Despite the growing appreciation of zinc in modern biology, the knowledge of zinc's coordination nature in proteins remains controversial. It is typically assumed that Zn2+ coordinates to four to six ligands, which led to intensive debates about whether the catalysis of some zinc proteins is regulated by zinc's four- or five-coordinate complex. Here we report the inherent uncertainty, due to the experimental resolution, in classifying zinc's five- and six-coordinate complexes in protein crystal structures, and put forward a tetrahedral coordination concept that Zn2+ coordinates to only four ligands mainly because of its electronic structure that accommodates four pairs of electrons in its vacant 4s4p(3) orbitals. Experimental observations of five- and six-coordinate complexes were due to one or two pairs of ambidentate coordinates that exchanged over time and were averaged as bidentate coordinates. This concept advances understanding of zinc's coordination nature in proteins and the means to study zinc proteins to unlock the secrets of Zn2+ in human biology. In particular, according to this concept, it is questionable to study zinc's coordination in proteins with Co2+ as a surrogate of Zn2+ for spectroscopic measurements, since the former is a d(7) unclosed shell divalent cation whereas the latter is a d(10) closed shell divalent cation.