How Outer Coordination Sphere Modifications Can Impact Metal Structures in Proteins: A Crystallographic Evaluation

摘要

A challenging objective of de Novo metalloprotein design is to control of the outer coordination spheres of an active site to fine tune metal properties. The well-defined three stranded coiled coils, TRI and CoilSer peptides, are used to address this question. Substitution of Cys for Leu yields a thiophilic site within the core. Metals such as Hg(II), Pb(II) and As(III) result in trigonal planar or trigonal pyramidal geometries; however, spectroscopic studies showed Cd(II) formed 3-, 4- or 5-coordinate Cd(II)S3(OH2)x (where x=0–2) when the outer coordination spheres were perturbed. Unfortunately, there has been little crystallographic examination of these proteins to explain the observations. Herein, we compare the high-resolution x-ray structures of apo- and mercurated proteins to explain the modifications that lead to metal coordination number and geometry variation.It reveals that Ala substitution for Leu opens a cavity above the Cys site allowing for water excess, facilitating Cd(II)S3(OH2).Replacement of Cys by Pen restricts thiol rotation, causing a shift in the metal binding plane that displaces water, forming Cd(II)S3.D-Leu, above the Cys site, reorients the side chain towards the Cys layer diminishing the space for water accommodation yielding Cd(II)S3, while D-Leu below opens more space, allowing for equal Cd(II)S3(OH2) and Cd(II)S3(OH2)2.These studies provide insights on how to control desired metal geometries in metalloproteins using coded and non-coded amino acids.