4. Discussion
The homology modeled Hev b 13, comprising 230 amino acid residues, exhibits the typical esterase fold architecture which consists of a central bundle of parallel β-strands connected by extended loops to an external crown of α-helices (Fig. 3A and B). This molecular organization confers a stocky vase shape to the molecule. However, our three-dimensional model is incomplete and lacks both the N- (55 amino acid residues) and C-terminal (84 amino acid residues) regions of the polypeptide chain. Two putative N-glycosylation sites occur at positions Asn160 (Asn-Leu-Thr) and Asn167 (Asn-Ala-Thr) but, according to the GlyProt server, none of both sites are actually glycosylated due to unfavourable geometric conditions. This is rather surprising for the second Asn167-Ala-Thr site which is apparently well exposed on an extended loop located at the top of the molecule. Another 3rd N-glycosylation site occurs at Asn292 (Asn-Phe-Ser) but is lacking on the three-dimensional model built for Hev b 13 which ends six residues before, at Gly281. This 3rd site should be actually glycosylated since Hev b 13 has been unambiguously characterized as a glycoprotein ( Arif et al., 2004). In addition, this presumably glycosylated site should belong to the surface-exposed epitope #8 (274--281) which is unusually short and is likely to include the glycosylation site. This assumption is in agreement with the exposed character of the site-containing amino acid sequence stretch predicted from both the hydropathic profiles and HCA plot (result not shown).