Wherland Sandbox 2

Introduction

Azurin is a bacterial protein that has been extensively studied by bioinorganic and biophysical chemists as a prototype of a Type 1 or "blue" copper protein. It contains a single copper ion that can be in the Cu⁺ or Cu²⁺ or the Cu state. The intensely blue color is due to a charge transfer transition from the cysteine thiolate ligand to the Cu in the Cu²⁺ state. It functions as an electron transfer mediator. The electron transfer reactivity of azurin has been extensively studied, including studies of its reactivity with natural and artificial partners, and intramolecular electron transfer from intrinsic and covalently attached electron transfer partners. The latter studies have been instrumental in defining and evaluating the factors influencing electron transfer reactivity through proteins. These factors include the electron transfer distance, the structure of the intervening peptide medium, the thermodynamic driving force, and the structure of the donor and acceptor. These studies have been instrumental in the iterative testing and advancing of electron transfer theory. One series of studies, delineated here, involves measurement of the rate constant for electron transfer from a disulfide radical, produced by pulse radiolysis, to the Cu²⁺ ion. This reaction can be made to occur because of particular structural features of azurin, the Cu²⁺ site is relatively buried and at the opposite end of the protein from the only disulfide, which is exposed to solvent and electron transfer reagents.

Structure

Azurin has 128 amino acids and a beta barrel structure, as exemplified by the structure of the Pseudomonas aeruginosa protein (4AZUA, chain A). The is shown colored by the secondary structure assignment (pink for alpha helix, purple for a 310 helix, yellow for beta strands, blue for beta turns, and white for other structures). The copper atom (maroon) is at the top of this standard orientation. The primary ligands to the copper atom are the thiolate of Cys112, and ND His117 and His 46, forming a trigonal planar . In addition there are two weaker ligands, the S of Met121 and the carbonyl O of Gly45, occupying axial positions to give an approximately trigonal bipyramidal . In addition to the Cu site, there is single at the at the "bottom" of the protein

Exposure of the disulfide and the copper site

The studies discussed here involve the intramolecular electron transfer between a one electron reduced disulfide radical and the oxidized copper ion. In order to measure the rate constant for this process, the disulfide radical must be produced rapidly by a strong reductant in a bimolecular reaction. This bimolecular reaction must reduce the disulfide preferentially over the Cu²⁺ site. Azurin shows this preferential reactivity due to the lack of exposure of the copper site, with only part of the edge of the coupled with high exposure of the disulfide . The reducing agent typically used is the CO₂^- radical, an especially strong reducing agent produced by pulse radiolysis of formate containing solutions. Thus the electrostatic interaction with the sites is also relevant. The copper site has no charges near the exposed His 46 but the disulfide site has both a positive residue and a negative one.

References