Agriculture Reference
In-Depth Information
to
Gli-1
(Pogna et al.
1993
),
Gli-6
and
Gli-7
on short arm of chromosome 1A and
1D (Metacovsky et al.
1996
; Hassani et al.
2006
).
Amino Acid Composition and Structure
Among the four gliadin groups, ω-gliagins have high level of glutamione and pro-
line while low level of sulfurous amino acids (Gianibelli et al.
2002
). Compara-
tively, they had few amino acids and high phenylalanine levels as compared to other
gliadin groups (Kasarda et al.
1983
; Tatham and Shewry
1995
). Difference does
exist among gliadin groups for surface hydrophobicity and ω-gliagins are lower
hydrophobic than that of the α- and γ-type gliadins. Popineau and Pineau (
1987
)
identified the gliadins as first peptides elute from the reverse phase-HPLC column.
Among all the gluten protein fractions, gliadins are highest hydrophilic with refer-
ence to amino acid composition with only a few residues with charged side chains
(Dupont et al.
2000
). Three different types of ω-gliagins have been observed on
the basis of the N-terminal sequences. The nomenclature of ω-gliadins is followed
from the first three amino acids in their N-terminal sequences (Kasarda et al.
1983
).
Therefore, these three groups are called ARQ-, KEL-, and SRL-types based on the
aforesaid nomenclature system (Tatham and Shewry
1995
).
Similar to the ω-gliadins, α-, β- and γ-gliadins are also rich in glutamine and
proline. In these groups, about 90 % of the glutamic and aspartic acid residues are
amidated (Bietz et al.
1977
; Kasarda et al.
1983
). They are also characterized by
low levels of basic amino acids and high leucine. In α/β- and γ-gliadins cysteine
residues are 6 and 8, respectively and both all types are rich in sulfur. Müller and
Wieser (
1995
, 1997) confirmed that 3-4 disulfide bonds between molecules are
formed. The α/β-gliadins are represented by a very small sequence of five amino
acid residues (VRVPV) on the basis of N-terminal sequences (Bietz et al.
1977
). In
α/β-gliadin, the pentapeptide motifs (PQQQP and PQQPY) are always present in a
repetitive region that follows the N-terminal region of the proteins (Shewry et al.
1986
). Contrastingly, in γ-gliadins the N-terminal region is consisted twelve amino
acid residues (NMQVDPSGQVQW) followed by several repeats of consensus mo-
tif PQQPFPQ (Kasarda et al.
1983
; Shewry and Tatham
1990
).
Recently, Qi et al. (
2009
) analyzed 170 γ-gliadin genes isolated from common
wheat and its closely related species, among which 138 sequences are putatively
functional. The ORF lengths of these sequences range from 678 to 1089 bp, and the
repetitive region is mainly responsible for the size heterogeneity of γ-gliadins. The
repeat motif
P
(Q/L/S/T/I/V/R/A)
F
(S/Y/V/Q/I/C/L)
P
(R/L/S/T/H/C/Y)
Q
1-2
(P
(S/L/T/A/F/
H)
QQ)
1-2
is repeated from 7 to 22 times. They found a wide range of amino acid composition
in γ-gliadins, and those γ-gliadins from subgroup SG-10 and SG-12 and γ-gliadins
with a short repetitive domain are more nutritional.
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