Iron-sulfur clusters have no right angles.

Accurate geometric restraints are vital in the automation of macromolecular crystallographic structure refinement. A set of restraints for the Fe4 S4 cubane-type cluster was created using the Cambridge Structural Database (CSD) and high-resolution structures from the Protein Data Bank. Geometries from each source were compared and pairs of refinements were performed to validate these new restraints. In addition to the restraints internal to the cluster, the CSD was mined to generate bond and angle restraints to be applied to the most common linking motif for Fe4 S4 : coordination of the four Fe atoms to the side-chain sulfurs of four cysteine residues. Furthermore, computational tools were developed to assist researchers when refining Fe4 S4 -containing proteins.