A single protein redox ruler

Redox reactions are the heart of function in biological systems. Using a deceptively simple toolbox, consisting largely of first row transition metals and the 20 canonical amino acids, nature has evolved proteins that contain cofactors with reduction potentials (E°′) between +1 V and −1 V. Chemists have long known that ligands to transition metals markedly alter E°′ of the metal, often in a systematic way. A set of such trends is much less clear in metalloproteins. In PNAS, Hosseinzadeh et al. (1) used azurin, a blue copper protein from Pseudomonas aeruginosa, as a framework that can be rationally modified to shift E°′ of the embedded metal site between +0.97 V and −0.95 V [all potentials are with respect to the normal hydrogen electrode (NHE)]. Tuning E°′ over such a large range was made possible using five (or fewer) nature-inspired mutations and substitution with CuII or NiII. Copper sites carry out many redox roles in biological systems. Functions of type 1 sites include single electron transfer (ET) reactions (e.g., plastocyanin) and substrate oxidation coupled to dioxygen reduction (e.g., laccases) (2). The unique properties of type 1 sites have been recognized since the 1950s, and those properties have been manipulated in a great many ways (3). In addition to its striking blue color, type 1 Cu centers display E°′ values that are generally higher than for CuII/I in water (E°′ = 0.16 V), ranging from 0.18 V (stellacyanin) to almost 0.8 V for fungal laccase (4), and ≥1 V estimated for ceruloplasmin (5). This large range of E°′ is even more remarkable given the structural similarities of the proteins and the ligand sets (two His, one Cys, and a more weakly bound Met; Fig. 1). Mutations to Cu-ligating residues have major effects on spectroscopic and redox properties …