Spectroscopic characterizations of bridging cysteine ligand variants of an engineered Cu₂(S_Cys)₂ Cu_A azurin

Bridging cysteine ligands of the Cu_A center in an engineered CUA azurin were replaced with serine, and the variants (Cys116Ser and Cys112Ser CU_A azurin) were characterized by mass spectrometry, as well as UV-vis and electron paramagnetic resonance (EPR) spectroscopic techniques. The replacements resulted in dramatically perturbed spectroscopic properties, indicating that the cysteines play a critical role in maintaining the structural integrity of the Cu center. The replacements at different cysteine residues resulted in different perturbations, even though the two cysteines are geometrically symmetrical in the primary coordination sphere with respect to the two copper ions. The Cys112Ser variant contains two distinct type 2 copper centers, while the Cys116Ser variant has one type 1 copper center with slight tetragonal distortion. Both the UV-vis and EPR spectra of the Cys116Ser variant change with pH, and the pK_a of the transition is 6.0. A type 1 copper EPR spectrum with A_∥ = 26 G was obtained at pH 7.0, while a type 2 copper EPR spectrum with A_∥ = 140 G was found at pH 5.0. Interestingly, lowering the temperature from 290 to 85 K resulted in conversion of the Cys116Ser variant from a type 1 copper center to a type 2 copper center, suggesting rearrangement of the ligand around the copper or binding of an exogenous ligand at low temperature. This difference in mutation effects at different cysteines may be due to different constraints exerted on the two cysteines by hydrogen-bonding patterns in the ligand loop.