Plastocyanin topology

Fig. 3. (a) Copper site in plastocyanin. (b) Ribbon drawing of the plastocyanin backbone, (c and d) Schematic of plastocyanin topology. 

Cucumber basic blue protein (Cbp) is a protein without known function, also known as cusacyanin or plantacyanin. Its structure (Guss et al., 1988) completes the repertoire of cupredoxins with known structures. The topology of its folding is similar (Fig. 5) to those of plastocyanin and azurin, as might have been expected from sequence similarities and... 

The subunits are arranged in the crystals as homotetramers with D2 symmetry. The structure of a subunit is shown schematically in Fig. 1 (87). Each subunit of 552 amino acid residues has a globular shape with dimensions of 49 x 53 x 65 A and is built up of three domains arranged sequentially on the polypeptide chain, tightly associated in space. The folding of all three domains is of a similar jS-barrel type. It is distantly related to the small blue copper proteins, for example, plastocyanin or azurin. Domain 1 is made up of two four-stranded jS-sheets (Fig. lb), which form a jS-sandwich structure. Domain 2 consists of a six-stranded and a five-stranded jS-sheet. Finally, domain 3 is built up of two five-stranded jS-sheets that form the jS-barrel structure and a four-stranded j8-sheet that is an extension at the N-terminal part of this domain. A topology diagram of ascorbate oxidase for all three domains and of the related structures of plastocyanin and azurin is shown in Fig. 2. Ascorbate oxidase contains seven helices. Domain 2 has a short a-helix (aj) between strands A2 and B2. Domain 3 exhibits five short a-helices that are located between strands D3 and E3 (a ), 13 and J3 (a ), and M3 and N3 (a ) as well as at the C terminus (ag and a ). Helix 2 connects domain 2 and domain 3. 

Fig. 2. A topology/packing diagram of the domains of the ascorbate oxidase monomer compared with plastocyanin and azurin. Each /3-strand is represented by a triangle whose apex points up or down depending on whether the strand is viewed from the C or N terminus, a-helices are represented by circles.

The structural studies on amicyanin from P. versutus show that the protein consists of nine B-strands that form into two antiparallel /3-sheets, giving the molecule an overall structural motif known as a /3-sandwich (see Fig. 4). This overall topology is very similar to that of other structurally characterized cupredoxins, with the homology to plastocyanin being greatest (87). The only notable difference between amicyanin and most other cupredoxin structures is the presence of a 21-residue N-tereminal extension that forms an extra /8-strand in the structure. The recently published structural studies (95, 96) on the type 1 blue copper protein rusticyanin show that it, too, possesses an... 

Cytochrome Cg and plastocyanin are different in their basic structures. Their redox centers are heme and copper, respectively. The 5-sheet structure of plastocyanin is absent in cytochrome Cg, which has several a-helices instead. However, there is analogous surface topology in these two mobile proteins. Both proteins feature a north hydrophobic pole around the redox center (Site 1) and an east face with charged groups (Site 2). The north pole is important in electron transfer, whereas the east face is important for orientation during the interaction of these proteins with Photosystem I but is not directly involved in electron transfer. As expected from their equivalent roles, the east face of plastocyanin and that of cytochrome Cg are both acidic or both basic. 

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