Ascorbate oxidase 3 barrel domain

Other [3-Sheet Proteins. Influenza neuraminidase contains a very unusual [I-sheet propellor-like structure. Ascorbate oxidase (Figure 12. le) is a large / -sheet protein that contains parallel / -sheet domains, and interleukin-1/1 may be described as containing both a barrel and a propeller. 

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. 

A comparison of the different variants of the jS-barrel domain structure in Fig. 1 shows that domain 1 of ascorbate oxidase has the simplest /3-barrel with only two four-stranded /8-sheets. Plastocyanin and azurin are quite similar but between strands 4 (El) and 6 (FI) they have insertions of one strand (plastocyanin) or one strand and an a-helix (azurin). Domain 2 has one additional strand H2 in sheet D next to strand E2 (sheet B and strand El in domain 1) and two additional strands, F2 and G2, in sheet C next to strand 12 (sheet A and strand FI in domain 1). Domain 3 resembles domain 2 except for the insertion of the short a-helices and the addition of the four-stranded /8-sheet at its N terminus. 

Structure. The main structural characteristic of ascorbate oxidase is the /3-strand (Fig. 31). Domain 1 is shaped by two four-stranded /1-meanders forming a /1-sandwich structure. Domain 2 contains a six- and a five-stranded /3-meander, whereas domain 3 is shaped by two five-stranded /3-meanders, forming a -barrel, and a four-stranded /3-meander. In contrast to its 33 /1-strands, ascorbate oxidase only contains 7 a-helices. One occurs in domain 2, another connects domains 2 and 3, and the last five a-helices are situated in domain 3 [36]. 

---