2009_Book_FoodChemistry

15.2 Individual Constituents 685
Table 15.16. N-terminal sequences of the protein
groups of wheat gluten
Position
5 10
Fig. 15.7. RP-HPLC of low-molecular (LMW) subunits
of wheat glutelin of the cultivar Rektor on Nucleosil
C 18 (60 ◦ C, aqueous 2-propanol/trifluoroacetic
acid/acetonitrile. After the prolamins, the protein fraction
was extracted from the residue with 70% aqueous
ethanol/0.5% dithioerythritol at pH 7.6 and4 ◦ C;
peaks 2–4, 5–7: ω5-, ω1,2-gliadins. Peaks 8–17: LMW
subunits, peaks 20–25: γ-gliadins or related proteins,
according to Wieser et al., 1990)
HMW
subunits of
glutenin
ω5-Gliadins
ω1,2-Gliadins
LMW-s a :
LMW-m a :
α-Gliadins
E G E A S R Q L Q C
E
G
K
S R L L S P R G K E
Q
−−−−−−−−KELQS
ARQLNPSNKE
S H IP G LE R P S
C
METSHIPGLE
C
VRVPVPQLQP
F
P
γ-Gliadins
NMQVDPSGQV
I
S
A
a The LMW type is named after the first amino acid in
the sequence, serine (s) or methionine (m).
of protein. Recurring sequences with the peptide
unit PQQQF evidently start from position 14.
This represents a clear difference to the ω1,2-
type, which explains the typical differences in the
amino acid composition of the two subgroups.
The ω5-gliadins have a higher mobility than
the ω1,2-gliadins in acidic PAGE and a lower mobility
in SDS-PAGE, M r being between 44,000
and 55,000 (Table 15.14).
In comparison with the ω5-gliadins, ω1,2-
gliadins have lower values for Glx (ca. 43%),
Phe (ca. 7.5%) and Ile (ca. 3%) (Table 15.14).
Most of the other values are higher, especially
the content of Pro (ca. 26%), which is the highest
within the gluten groups of proteins. In the
case of the N-terminal sequences, two basic
variants apparently exist a: ARQLNPSNKELQS;
b: KELQS, which with varying length are
homologous and lead into a recurring sequence.
The variant a was found more frequently in
ω2-gliadins and the variant b more frequently
in ω1-gliadins. The N-terminal sequence of the
ω5-gliadins corresponds to that of variant a in
6 positions. The immediately following recurring
sequence consists of the peptide unit PQQPY,
while the dominating recurring peptide unit
in this protein type seems to be the sequence
PQQPFPQQ.
In acidic PAGE, ω1,2-gliadins have the lowest
mobility. The molecular masses are in the range
M r = 34,000−44,000.
15.2.1.3.3 Low-Molecular Group (α-Gliadins,
γ-Gliadins, LMW Subunits
of Glutenin)
The quantitatively predominant low-molecular
protein group in gluten (Table 15.14) has the
best balanced amino acid composition. Most
of the values lie between those of the high and
intermediate molecular groups. Only the content
of Cys, Val, Met, and Leu is higher (Table 15.15).