Response from Boal and Rosenzweig to Crystallography and chemistry should always go together: a cautionary tale of protein complexes with cisplatin and carboplatin.

Response from Boal and Rosenzweig to Crystallography and chemistry should always go together: a cautionary tale of protein complexes with cisplatin and carboplatin In this issue of Acta Cryst. D, Shabalin et al. (2015) reinterpret diffraction data for a series of protein complexes with cisplatin (cis-Pt) and related drugs, an attractive target for study because of the possibility of drawing upon the extensive knowledge of the chemical and biological properties of platinum-based anticancer therapeutics (Berners-Price, 2000; Arnesano et al., 2009). cis-Pt and associated drugs bind avidly to duplex DNA inside the cell to form intrastrand crosslinks, driven in large part by the bonding preferences of Pt II , and typically distort the structure of the biomolecule significantly (Johnstone et al., 2015). The clinical importance of cisplatin interaction with proteins, however, is far less well understood. Association of Pt II compounds with these targets can be slow and more non-specific than corresponding interactions with nucleic acids (Peleg-Shulman et al., 2002). These properties are perhaps reflected in the findings of the study here in which low occupancies and heterogeneity in the structure and location of cis-Pt binding sites give rise to unique challenges in crystallographic modeling. The authors examined two structures we solved in 2009 of cis-Pt adducts with a human copper chaperone, Atox1 (Boal & Rosenzweig, 2009b). Of the structures discussed in the paper, these are perhaps the most biologically relevant owing to the implication of human copper transport proteins in clinical platinum resistance (Howell et al., 2010), although it remains unknown whether this correlation is a direct consequence of Pt II-protein association in vivo or simply an indirect effect. Our crystallographic study showed that cis-Pt is capable of binding to a Cys-XX-Cys site normally occupied by copper in these proteins, suggesting they are at least capable of trafficking or inactivating platinum-based drugs. In one of these structures, in which cis-Pt is likely first modified by a phos-phine reductant (TCEP) included to reduce the Cys ligands, Pt II interacts with a single Atox1 monomer and maintains a square planar coordination environment that distorts the structure of the Cys-XX-Cys motif. In a second structure obtained afterwards in which the reductant was removed under anaerobic conditions prior to Pt II exposure, the metal binds at the same location but instead bridges an Atox1 dimer, similar to what is observed upon interaction with the native substrate Cu I (Boal & Rosenzweig, 2009a). Interestingly, Pt …