CopC Protein from Pseudomonas syringae: Intermolecular Transfer of Copper from Both the Copper(I) and Copper(II) Sites

The CopC protein from Pseudomonas syringae pathovar tomato is expressed as one of four proteins encoded by the operon CopABCD that is responsible for copper resistance. It is a small soluble molecule (10.5 kDa) with a (beta)-barrel structure and features two distinct copper binding sites, which are highly specific for CuI (KD =>10^-13) and CuII (KD (almost equal)10^-15). These dissociation constants were estimated via ligand competition experiments monitored by electronic spectral and fluorescence probes. The chemistries of the two copper sites are interdependent. When the CuII site is empty, the CuI ion is oxidized by air, but when both sites are occupied, the molecule is stable in air. The availability of an unoccupied site of higher affinity induces intermolecular transfer of either CuI or CuII while maintaining free copper ion concentrations in solution at sub-picomolar levels. This intriguing copper chemistry is consistent with the proposed role of CopC as a copper carrier in the oxidizing periplasmic space. These properties would allow it to exchange either CuI or CuII with its putative partners CopA, CopB, and CopD, contrasting with the role of the CuI (only) chaperones found in the reducing cytoplasm.