Blue copper proteins solution structures

A. The Diamagnetic Copper(I) State Spectroscopic Studies and Solution Structures of Blue Copper Proteins... 

Some of the first protein systems where pulse radiolysis was used to help determine mechanism were those of blue copper proteins. These are proteins that are blue in solution and contain what are known as type (I) and type (2) copper centers. Two of the most well-known and well-characterized examples of these are azurin and cytochrome c. It was the studies of these systems that opened up the field of long-distance electron transfer in proteins and, by using the protein structure as a framework for electron transfer through space and through bonds, allowed for the development of a broad theoretical basis and many fascinating experiments on long-range electron transfer. Here, I will limit the discussion to electron transfer studies in azurin as illuminated by pulse radiolysis studies. ... 

Another problem with small models is that molecules from the solution (e.g. water) may come in and stabilise tetragonal structures and higher coordination numbers [224]. It is illustrative that very few inorganic con5)lexes reproduce the properties of the blue copper proteins [66,67], whereas typical blue-copper sites have been constructed in several proteins and peptides by metal substitution, e.g. insulin, alcohol dehydrogenase, and superoxide dismutase [66]. This shows that the problem is more related to protection from water and dimer formation than to strain. 

The first crystal structure information on a blue copper protein, for poplar plastocyanin in the Cu(II) state, was published in 1978 (2, 3). Since then, the Cu(I) state and related apo and Hg(II) substituted forms (5, 6), the green algal plastocyanin from Enteromorpha prolifera [Cu(II)] (7), azurin from Alcaligenes denitrificans [Cu(II) and Cu(D] (8, 9), azurin from Pseudomonas aeruginosa [Cu(II)] (10, 11), as well as pseudoazurin from Alcaligenes faecalis S-6 (12), and the cucumber basic protein, both in the Cu(II) state, have been published (13), making this one of the best-documented class of proteins. In addition, information as to three-dimensional structure in solution has been obtained from two-dimensional NMR studies on French bean and Scenedesmus obliquus plastocyanins (14,15). This review is concerned in the main with the active site chemistry. Other recent reviews are listed (16-20). 

Rusticyanin is an abundant, highly stable periplasmic blue copper protein (e.g., Cobley and Haddock 1975 Jedlicki et al. 1986 Ronk et al. 1991 Nunzi et al. 1993 Blake et al. 1993). Studies of rusticyanin include kinetic competence in the iron oxidation reaction (Blake and Shute 1994), identification of a His ligand to the copper center (Casimiro et al. 1995), a solution NMR structure (Botuyan et al. 1996), and a high-resolution X-ray structure (Walter et al. 1996). Rusticyanin forms a complex with new c-type heme cytochrome in iht A. ferrooxidans electron transport chain (Giudici-Orticoni et al. 2000). 

Binding of a paramagnetic, redox-inactive [Cr(CN)6]3- anion to specific sites of a blue copper protein, amicyanin, has been used in NMR-spectroscopic studies of the protein structure in solutions.285Ab initio calculations of the ligand-field spectra of [Cr(CN)6]3 have been performed and the results compared with those for cyano complexes of the other first-row transition metals.286 The role of Cr—C—N bending vibrations in the phosphorescence spectra... 

Other functions of the protein are also conceivable. Evidently, the protein may change almost freely between the different type 1 structures owing to the extremely flat potential surface. Furthermore, the protein forms a protected environment that prohibits the contact between two copper-thiolate units (thereby inhibiting the homeolysis to Cu(I) and disulfide) and it tunes the reduction potential. However, this is not a special property of the blue copper proteins, but instead an inevitable effect of any protein, presenting an ordered array of charges and dipoles and a dielectric milieu widely different from water solution. 

Fig. 16 a, b. Folding pattern of the small blue protein rusticyanin. The protein consists of thirteen /1-strands a ribbon model - the copper ion is represented by the blue circle [112] b disposition of ligands and spatially close side-chains in the copper site of one of the solution structures of Cu(I) rusticyanin. [112a] with permission... 

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