Reduction potentials blue copper proteins

The Cu(II)/Cu(I) reduction occurs at E° = +0.47 V vs. Ag/AgCl (E0> = +0.67 V vs. NHE), the highest potential found in blue copper proteins. Attempts have been made to correlate such a potential value with the presence of the abnormally weak axial Cu-S bond. Nevertheless, more complicated structural factors seem to be responsible.68... 

Simple thermodynamic considerations state that the reduction process is favoured (i.e. more positive cu(ii)/cu(p potential values are obtained) if the electron transfer is exothermic (AH° negative) and if the molecular disorder increases (AS° positive). It is therefore evident that the positive potential value for the reduction of azurin (as well as that of the most blue copper proteins) is favoured by the enthalpic factor. This means that the metal-to-ligand interactions inside the first coordination sphere (which favour the stability of the reduced form over the oxidized form) prevail over the metal complex-to-solvent interactions inside the second... 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

This complex catalyzes the reaction through the Q cycle (Section 18.3.4). In the first half of the Q cycle, plastoquinol is oxidized to plastoquinone, one electron at a time. The electrons from plastoquinol flow through the Fe-S protein to convert oxidized plastocyanin into its reduced form. Plastocyanin is a small, soluble protein with a single copper ion bound by a cysteine residue, two histidine residues, and a methionine residue in a distorted tetrahedral arrangement (Figure 19.17). This geometry facilitates the interconversion between the Cu2+ and the Cu+ states and sets the reduction potential at an appropriate value relative to that of plastoquinol. Plastocyanin is intensely blue in color in its oxidized form, marking it as a member of the "blue copper protein," or type I copper protein family. 

The reduction potential is central for the function of electron-transfer proteins, since it determines the driving force of the reaction. In particular, it must be poised between the reduction potentials of the donor and acceptor species. Therefore, electron-transfer proteins normally have to modulate the reduction potential of the redox-active group. This is very evident for the blue copper proteins, which show reduction potentials ranging from 184 mV for stellacyanin to 1000 mV for the type 1 copper site in domain 2 of ceruloplasmin [1,110,111]. 

Second, quantum chemical calculations of the potential energy surface of the Cu-SMet bond shows that it costs less than 10 kJ/mole to change the Cu-Smci bond length by 100 pm around its optimum value (see Figure 10), a range larger than the natural variation in this bond [14,54]. Thus, even if the proteins could constrain this bond, it would affect the electronic part of the reduction potential by less than 10 kJ/mole, or 100 mV, i.e. much less than the variation found among the blue copper proteins. Moreover, a constrained Cu(I)-SMet bond would... 

However, there are other contributions to the reduction potential than the electronic part, most prominently the solvation energy of the active site caused by the surrounding protein and solvent. We have therefore studied the reduction potential of the blue copper proteins using various methods to include the solvation effects. The results have shown that constraints in the Cu-Smci bond length can affect the reduction potential by less than 70 mV (c.f Figure 10) [35]. 

During electron transfer, the Cua site alternates between the fully reduced and the mixed-valence (Cu +Cu ) forms. Interestingly, the unpaired electron in the mixed-valence form seems to be delocalised between the two copper ions. Several theoretical investigations of the electronic structure and spectrum of the Cua dimer have been published [138-144]. In similarity to the blue copper proteins, it has been suggested that the structure and the properties of the Cua site is determined by protein strain. More precisely, it has been proposed [136] that Cua in its natural state is similar to an inorganic model studied by Tolman and coworkers [145]. This complex has a long Cu-Cu bond (293 pm) and short axial interactions (-212 pm). The protein is said to enforce weaker axial interactions, which is conpensated by shorter bonds to the other ligands and the formation of a Cu-Cu bond. This should allow the protein to modulate the reduction potential of the site [136,146]. 

M.H.M. Olsson U. Ryde (2000) A theoretical study of the reduction potential of blue copper proteins , manuscript in preparation. 

Blue copper proteins have a single Cu atom at the active site, and three characteristic properties (1) an intense blue color at —600 nm, with absorption coefficients of 2000-6000 M cm arising from S(Cys) ()u(II) charge transfer (b) an unusually narrow hyperfine coupling (A values of 0.0035-0.0063 cm ) in the EPR spectrum of the Cu(II) protein due to asymmetry at the metal and (3) high reduction potentials (range 184-680 mV) as compared to the aqua Cu(II/I)... 

Rusticyanin has a high reduction potential (680 mV), which is similar to that for the Type 1 Cu center in fungal as opposed to tree laccase (785 mV) (73). This trend is so far unexplained. From the sequence and EXAFS studies, His-Cys-His-Met coordination is a reasonable possibility for rusticyanin (55). It may well be that the reduction potential is determined by effects of a polypeptide backbone on Cu—S(Cys) and Cu—S(Met) bond distances and the Cu ligand field (74). If this is the case, however, rusticyanin would be expected to have one or both Cu—S distances shorter than in other blue copper proteins, which is not borne out by information from EXAFS (Table IV). A further possibility that the Cu(I) form is three-coordinate, as in the case of plasto-cyanin at low pH (Fig. 2), has no strong support at present (75). 

In this section we look at ways in which Nature carries out redox chemistry with reference to blue copper proteins, iron-sulfur proteins and C5fiochromes the redox steps in Photosystem II were outlined in the discussion accompanying equation 21.53. We have already discussed two topics of prime importance to electron transfer in Nature. The first is the way in which the reduction potential of a metal redox couple such as Fe /Fe + can be tuned by... 

---