Ascorbate oxidase topology

Fig. 10. (a) Copper site in domain 1 of ascorbate oxidase, (b) Ribbon drawing of tbe ascorbate oxidase domain 1 backbone, (c and d) Schematic of ascorbate oxidase domain 1 topology. 

Fig. 11. (a) Ribbon drawing of the ascorbate oxidase domain 2 backbone, (b and c) Schematic of ascorbate oxidase domain 2 topology. 

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.

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