Chitinases as Food Allergens

Abstract

Food allergies originate from adverse immune reactions to some food components. Ingestion of food allergens can cause effects of varying severity, from mild itching to severe anaphylaxis reactions. Currently there are no clues to predict the allergenic potency of a molecule, nor are cures for food allergies available. Cutting-edge research on allergens is aimed at increasing information on their diffusion and understanding structure-allergenicity relationships. In this context, purified recombinant allergens are valuable tools for advances in the diagnostic and immunotherapeutic fields. Chitinases are a group of allergens often found in plant fruits, but also identified in edible insects. They are classified into different families and classes for which structural analyses and identification of epitopes have been only partially carried out. Moreover, also their presence in common allergen databases is not complete. In this review we provide a summary of the identified food allergenic chitinases, their main structural characteristics, and a clear division in the different classes.

Keywords: allergen; chitinase; epitope mapping; food allergy.

Figure 1

Epitope mapping on hevein and class I chitinases. (A) Epitope mapping on hevein 3D structure. The reported structure was obtained from the 3D structure of the protein (PDB 4MPI) by restriction to Chain A and by selection of the amino acids 12–31, corresponding to the consensus sequence C₁₂XXXXCCSXAφXφCGXΩXAcYC₃₁. Colour code: Blue, C12 and C31; cyan, S19; red, W23; yellow, G25; magenta, Y30. The molecule is shown from three different points of view, corresponding to clockwise and counterclockwise rotations of 90°. (B) Epitope mapping on the 3D model of the chestnut chitinase. The model was obtained by supplying the sequence of the chestnut chitin binding module to the Phyre2 program [39] using the hevein structure (4MPI) as template. Coloured amino acids are as for the hevein structure. All images were prepared by using PyMol [40] resources.

Figure 2

Epitope mapping on the predicted 3D structure of a class II rice chitinase. The reported 3D structure was obtained for the rice chitinase protein (UniProt O24007) by Phyre2 prediction. The program predicted a structure with 100% confidence on the template 2DKV, a rice chitinase with 62% identity with the query sequence. Coloured regions correspond to predicted linear epitope described by Mishra et al. [28]. Colour code: red, epitope E₁₂TTGGTRGSSDQFQ₂₅; green, epitope K₃₁EEINKATSPPYYGR₄₅; cyan, epitope N₁₄₀DANVDRIGYYKRYCDMLGTGYGSNLD₁₆₆. The molecule is shown from two different points of view, corresponding to a rotation of 180°.

Figure 3

Epitope mapping on the 3D structure of the class III pomegranate chitinase (PDB 4TOQ). The region P₂₂₁AAPEAAGSG₂₃₀ of the pomegranate chitinase, resulting 80% identical to an epitope described in velvet grass [42], was coloured and shown in the protein 3D structure. The selected region was coloured green, except for the terminal amino acids: P₂₂₁ (red) and G₂₃₀ (cyan).

Figure 4

Epitope mapping on the 3D structure of the class IV maize chitinase (PDB 4MCK). The regions I₁₅₁–R₁₇₉ and N₂₅₂–I₂₇₉ of the maize chitinase corresponding to the epitopes K₃₁EEINKATSPPYYGR₄₅ and N₁₄₀DANVDRIGYYKRYCDMLGTGYGSNLD₁₆₆ of the rice class II chitinase [28] were selected and coloured in cyan. In addition, the amino acids of the two epitopes that are conserved between the rice and maize chitinases were coloured in red (for the first epitope) and green (for the second epitope). The molecule is shown from two different points of view, corresponding to a rotation of 90°.