Optical spectra, blue copper

Electronic spectra of metalloproteins find their origins in (i) internal ligand absorption bands, such as n->n electronic transitions in porphyrins (ii) transitions associated entirely with metal orbitals (d-d transitions) (iii) charge-transfer bands between the ligand and the metal, such as the S ->Fe(II) and S ->Cu(II) charge-transfer bands seen in the optical spectra of Fe-S proteins and blue copper proteins, respectively. Figure 6.3a presents the characteristic spectrum of cytochrome c, one of the electron-transport haemoproteins of the mitochondrial... 

Spectral studies of the native enzyme are difficult to interpret due to simultaneous contributions from all three types of copper. The EPR spectrum (Fig. 38) of native laccase98) shows features attributable to the type 2 (g = 2.237, gx = 2.053, A = 206 x 10-4 cm-1) and type 1 (gz = 2.300, gy = 2.055, g = 2.030, Az = 43 x 10-4 cm-1) coppers. The type 3 site is EPR-non-detectable and diamagnetic (-2 J >550 cm-1)56,57). This site should be compared to the met derivative of hemocyanin and tyrosinase only in the sense that it appears to contain two anti-ferromagnetically coupled copper(II) s lacking an EPR signal. A characteristic Blue band of the type 1 copper is seen in the optical spectrum at 614 nm (c = 5700 M-1 cm-1). The shoulder observed at 330 nm (e 2800 M-1 cm-1) was originally associated with the type 3 copper site, as it reduces with two electrons at... 

Type I Cu(II) is the blue copper responsible for the unusually intense absorption at 614 nm in laccase. Its EPR spectrum is unusual with low g values and very small hyperfine splittings. Type II Cu(II) shows a more normal EPR spectrum and is thus responsible for the so called low field line in the EPR spectrum of laccase (in spite of considerable overlap with the type I spectrum). There are no optical absorptions which strongly depend on the oxidation state of the type II copper in laccase and it is thus considered transparent. It is well established89 that anions such as F- and N3 strongly bind to type II Cu(II) in laccase. In the case of F- binding91, the low field EPR line was split by the 19F nucleus, the enzyme was inactivated, and a very high binding constant measured. 

Type 2 Cu2+. This form of Cu is present in all the blue multi-copper oxidases. It is characterized by lacking sufficient optical absorption to be observed above that of the other Cu-chromophores in these molecules. Consequently it is sometimes referred to as the colorless Cu. Further, its EPR spectrum is similar to those exhibited by most small Cu2+ complexes. However, its presence is essential to the functioning of the multi-copper oxidases, and it has very unique chemical properties which distinguish it from Cu + bound to the non-blue Cu proteins. The T5q)e 2 designation should therefore be reserved for classification of the types of Cu2+ sites observed in the blue multi-copper oxidases, and it should not be used to classify the binding sites of non-blue copper proteins which have distinctly different chemical behavior. Thus, for example, any purported analogies between... 

Reaction with O2 (and CO) is shown to be reversible in the solid state although in solution (MeNOz) oxygenation of the bimetallic complex is more complicated. A report on the reaction of blue oxidases with Og indicates that the optical and e.p.r. absorptions correlate well. The reaction time with fully reduced fungal laccase is 1 s and for the Rhus enzyme it is 10 s. Since only one line of the e.p.r. spectrum may be observed because of overlapping copper signals, identification of the oxygen species involved is diflBcult but the most likely candidate at present is the ion 0. ... 

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