2009_Book_FoodChemistry

682 15 Cereals and Cereal Products
Fig. 15.5. RP-HPLC of the gliadin fractions of various wheat cultivars a on Synchro Pac C 18 (50 ◦ C, aqueous
2-propanol/trifluoroacetic acid/acetonitrile; 22–34 min: ω-gliadins, 33–51 min: α-gliadins, 52–72 min: γ-gliadins;
according to Wieser et al., 1987)
a CWRS (Canadian Western Red Spring) is a mark of origin.
contain no recurring sequences and are characterized
by the occurrence of Cys and amino acids
with charged side chains. The middle segment
consists of recurring sequences with the peptide
unit QQPGQG as the backbone and insertions
with the sequences YYPTSP, QQG, and QPG.
It largely determines the unusual amino acid
composition (high Gly and Tyr content). The
individual HMW subunits differ mainly in the
substitution of individual amino acid residues
and in the number and arrangement of recurring
peptide units.
The relative molecular masses (M r ) calculated
from the known total sequences are 67,000–
88,000, while the molecular masses derived from
SDS-PAGE are 35–40% higher (Table 15.14).
Based on typical differences in the N-terminals
and middle sequence segments, the
HMW subunits can be assigned to two subgroups
(x-type, M r = 83,000−88,000; y-type,
M r = 67,000−74,000) (Tables 15.14 and 15.17).
These proteins result from the fact that two
genes are localized on each of the chromosomes
of group 1 (1A, 1B, 1C; cf. 15.1.2). These
genes code for the HMW subunits of type x
and type y, e. g., ID contains the genes 1Dx and
1Dy. However, not all the genes are expressed
in wheat cultivars. The allele pairs 1Dx2 and
1Dy12 as well as 1Dx5 and 1Dy10 are common.
The pairs 1Bx6 and 1By8 as well as 1Bx7 and
1By9 are also frequently found. The occurrence
of the corresponding HMW subunits 2, 5, 6–10,