Activating Metal Sites for Biological Electron Transfer

Abstract

This review focuses on the unique spectroscopic features of the blue copper active sites. These reflect a novel electronic structure that activates the site for rapid long-range electron transfer in its biological function. The role of the protein in determining the geometric and electronic structure of this site is defined, as is its contribution to function. This has been referred to as the entatic/rack-induced state. These concepts are then extended to cytochrome c, which is also determined to be in an entatic state.

[Cu(aq)]2+ is structurally plastic and the axially elongated octahedron goes missing

“…In this metalloprotein, the Fe(ii)-S Met and Fe(iii)-S Met bond enthalpies have been estimated as ∆H = −2.6 and −5.5 kcal mol −1 , respectively. 118 The same maximum entropy approach yields estimated equilibrium binding −∆G values of 2.19 kcal mol −1 and 5.09 kcal mol −1 , respectively. The equivalent full occupancy 5 Å statistical model will assign one thioether sulfur to each of the four corners of a square with sides of 4.085 Å, centered over the single open face of heme iron.…”

CuA-based chimeric T1 copper sites allow for independent modulation of reorganization energy and reduction potential

“…UV-vis spectra of mononuclear copper proteins are well established reporters of the geometric distortions of the metal sites. 39 , 40 Canonical axial T1 sites are characterized by an intense absorption at around 600 nm ( ε max ≈ 3000–6000 M –1 cm –1 ) that accounts for the typical blue colour. Rhombic sites, in contrast, present an intense feature around 450 nm ( ε max ≈ 1500–3500 M –1 cm –1 ) responsible for the greenish hue, along with a much weaker 600 nm band compared to axial sites.…”

“…Red letters denote the conserved ligand set. 40,41,[44][45][46][47] Spectra of the 2R2R-Tt-Cu A and 4A3A-Tt-Cu A variants, as well as spectral deconvolutions, are shown in Fig. S1.…”

Hypothesis: entaticversusecstatic states in metalloproteins

“…55 Or should we follow the spectroscopic approach of Solomon's group and focus on calculating the electronic coupling matrix element in the rate equation? 56 In summary, after 50 years we have not been able to agree on what entasis is, on what criteria should be tested to its identification or refutation, and on how this testing in the laboratory or in the computer should be carried out. Developing clear and unequivocal definitions of these matters could perhaps initiate a true renaissance of the entatic state.…”