Inner- and outer-sphere metal coordination in blue copper proteins

Abstract

Blue copper proteins (BCPs) comprise classic cases of Nature's profound control over the electronic structures and chemical reactivity of transition metal ions. Early studies of BCPs focused on their inner coordination spheres, that is, residues that directly coordinate Cu. Equally important are the electronic and geometric perturbations to these ligands provided by the outer coordination sphere. In this tribute to Hans Freeman, we review investigations that have advanced the understanding of how inner-sphere and outer-sphere coordination affects biological Cu properties.

Figure 1

Copper coordination in representative blue copper proteins in their oxidized forms. (A) P. aeruginosa azurin; (B) T. ferrooxidans rusticyanin; (C) P. nigra plastocyanin; (D) P. denitrificans amicyanin; (E) C. sativus cumber basic protein; (F) C. sativus stellacyanin. Spheres indicate carbon (green), oxygen (red), nitrogen (blue) and copper (tan). The protein tertiary structure is shown as a partially transparent grey ribbon.

Figure 2

Comparison of Met121Glu (A) and Met121Ala (B) with the structure of WT P. aeruginosa azurin. The WT structure is shown in white and is partially transparent. Spheres indicate carbon (green), oxygen (red), nitrogen (blue) and copper (tan). The protein tertiary structure is shown as a partially transparent grey or green ribbon.

Figure 3

X-ray structure of Cys112Asp/Met121Leu (type zero) azurin [77] (PDB ID 3FPY). Spheres indicate carbon (green), oxygen (red), nitrogen (blue) and copper (tan). The protein tertiary structure is shown as a partially transparent green ribbon. Cu-ligand distances (Å) are: His46-N3 (1.94); His117-N3 (2.04); Asp112-O (1.92); Gly45-O (2.35) and Leu121-C (3.84).

Figure 4

Outer-sphere coordination in representative blue copper proteins in their oxidized forms. (A) P. aeruginosa azurin; (B) T. ferrooxidans rusticyanin; (C) P. nigra plastocyanin; (D) P. denitrificans amicyanin; (E) C. sativus cumber basic protein; (F) C. sativus stellacyanin. Cu-ligating residues are the same as in Figure 1.

Figure 5

View of the H-bond network surrounding Cu in P. aeruginosa azurin. Cu-ligating residues are the same as in Figure 1.

Figure 6

Structural changes in Cys112Asp/Met121Glu P. aeruginosa azurin at pH 10 (green, solid) and pH 7 (cyan, partially transparent). The PDB IDs are 3OQR and 3NP3, respectively. Spheres indicate carbon (green/cyan), oxygen (red), nitrogen (blue) and copper (tan). The protein tertiary structure is shown as a partially transparent grey ribbon.