It is being proposed from the homology study that the alpha/beta-gliadins would have similar tertiary structures but different from the gamma-gliadins (Table 4.2). Three of the gamma-gliadins (GDB2, GDBB and GDBX) would also have similar structures but different to the fourth gamma-gliadin (GDB1) which would have a structure similar to the LMW glutenins (Table 4.5). The motif search was based on the principle that if the structure and function of sequences in other proteins are known, and the same sequences are found in gliadin, then there is the possibility that those sequences in gliadin may have the same or similar structure and function. In section 4.6.2 can be found a list of protein sequences which were also found in gliadin. There wasn’t enough time to analyse those proteins individually. The secondary structure for alpha/beta-gliadin was predicted as 34% alpha-helix, 23% beta-sheet and the remainder 43% whereas, those for deamidated gliadin were 69% alpha-helix, 2% beta-sheet and the remainder 29% (Figures 4.12 and 4.13). The supersecondary structure predicted for lysozyme matched its x-ray crystallographic structure (Table 4.6 and Figure 7.3) so the method was used to predict the supersecondary structure of gliadin and deamidated gliadin (Tables 4.7 and 4.8). The results of the prediction was given as irregular structure. Metal binding proteins like the metallothionines have irregular structure, therefore, a study was carried out to find if SWP binds metals (chapter 7).
The gluten proteins are responsible for coeliac disease in some children and adults causing weight loss, abdominal pain and malabsorption of nutrients. Various theories have been put forward to explain the effect of gluten in coeliac disease, i.e. enzyme theory, lectin theory, permeability theory and immunological theory. Gluten is the protein fraction of wheat flour which is insoluble in water. It is a heterogenous material and attempts have been made to isolate the toxic component. Gluten subfractions were found to be cytotoxic to several types of cells in tissue culture (Hudson et al., 1976). The toxic component was implicated to be gliadin (Howdle et al., 1984). Because gliadin is rich in proline and glutamine, Jos et al., (1982) and Cornell and Maxwell (1982), suggested that the toxicity resides in fractions containing these residues. It has been reported that 43% of gliadin is composed of glutamine and that a bound form of glutamine may be responsible for causing toxicity (Van de Kamer and Weijers, 1955). None of the theories of gluten toxicity is completely proven, and elements of each theory may provide part of the explanation. Part of the difficulty is the lack of understanding of the tertiary structure of gliadin. As structure is related to function, there is a strong correlation that understanding of the structure of gliadin could lead to an understanding of its mechanism of toxicity in coeliac disease and its effects on other biomolecules. Cornell et al., (1992), collected fraction 9, which is a peptic-tryptic-pancreatinic digest of wheat gliadin, known to be toxic to individuals with coeliac disease and found a fraction with the sequence RPQQPYPQPQPQ amongst other fractions. This fragment can be found underlined in the sequence below.
Since deamidation changes the glutamine residues to glutamic acid, it is possible that deamidated gluten as found in SWP will not cause coeliac disease.