Rack‐induced bonding in blue‐copper proteins

Abstract

The unique spectroscopic properties of blue-copper centers, i.e. the strong charge-transfer band at approximately 600 nm and the narrow hyperfine coupling in the EPR spectrum, are reviewed. The concept of rack-induced bonding is summarized. The tertiary structure of the protein creates a preformed chelating site with very little flexibility, the geometry of which is in conflict with that preferred by Cuz+. The structure of the metal site in azurin is discussed. It is shown that the three strong ligands, one thiolate S and two imidazole N, are in a configuration intermediate between those preferred by Cu2+ and Cu'. It is emphasized that cysteine is an obligatory component of a blue site, whereas the weak interaction with a methionine S is not necessary. The minimum rack energy is estimated to be 70 kJ . mol-'. It is pointed out that the high reduction potentials of bluecopper centers are a result of the protein-forced ligand-field-destabilized site structure. It is suggested that the potentials are tuned by variations in n back bonding, and this is supported by a linear increase in ALF (ligand field) with decreasing electron-transfer enthalpy. Site-directed mutagenesis has shown that large hydrophobic residues in the site increase the potential, whereas negative groups or water decrease it. It is also shown that the fine-tuning of the properties of the metal site by rackinduced bonding can alter the electron-transfer reorganization energy. Kinetic results with azurin mutants support a through-bond tunneling mechanism for intramolecular electron transfer in proteins. Finally, it is pointed out that the concept of rack-induced bonding is a universal principle of macromolecular structure/function relationships, which should be applied also to other systems.

Spectroscopic and Kinetic Studies on the Oxygen-centered Radical Formed during the Four-electron Reduction Process of Dioxygen byRhus vernicifera Laccase

“…The reason that the electron transfer from type I copper takes place before that from type II copper could be caused by that the redox potential of each copper site (38,39) changes under turnover conditions. To support this, the redox potentials of the trinucelar copper center easily change by acting the exogenous ligands such as N 3 Ϫ and F Ϫ (12) and by the mutations for ligand groups (13,40,41). Alternatively, the redox potential of the type II copper in T1Hg might shift toward a more positive potential than that in the native laccase.…”

Geometric and Electronic Structure Contributions to Function in Bioinorganic Chemistry: Active Sites in Non-Heme Iron Enzymes

“…The differences between normal and blue Cu(I1) centers have been used to support the existence of a "rack" or "entatic" state in bioinorganic chemistry. (7)(8)(9) In an entatidrack state the protein imposes an unusual geometry on the active site which enhances its reactivity. (7)(8)(9) In blue Cu proteins, the protein rack (or entatic state) can be thought of as opposing the Jahn-Teller distortion, thus effecting little geometric change upon oxidation facilitating rapid electron transfer.…”

Backbone Dynamics of Plastocyanin in Both Oxidation States

“…In plastocyanins, the Cu(II)/Cu(I) ion resides in the so-called type I copper center, characterized by the presence of one cysteine and two histidine residues strongly bound to copper in a trigonal plane (5)(6)(7). A weakly bound methionine sulfur atom completes the distorted coordination geometry of the metal ion (8).…”