Agriculture Reference
In-Depth Information
HPLC, but this methodology does not provide
fractionation up into the very large size range of
the glutenin polymers. Field-fl ow fractionation
(FFF) has no upper limit to its separation poten-
tial. In the examples of FFF profi les in Fig. 21.2,
the upper limit of size distribution is not indicated
precisely, because of diffi culties in this set of
experiments in obtaining suffi ciently large pro-
teins for calibration purposes.
There is still uncertainty about the structure of
the polymeric glutenins. As Fig. 21.2 indicates,
they cover a wide continuous molecular-weight
range. There is also uncertainty about how the
glutenin polypeptides are disulfi de cross-linked
together. A linear backbone of high-molecular-
weight (HMW) subunits is likely, forming an
elastic structure, possibly via the beta-spirals of
their central domains (Wieser et al., 2006). No
doubt, the very long polymers become entangled,
thereby contributing to the combined effects of
resistance to extension and viscous drag. These
various attributes have earned the glutenin poly-
mers the reputation of being “amongst the most
complex aggregates in nature” (Wieser et al.,
2006), as well as being among nature's largest
proteins.
x- and y-type genes. The frequencies of gluten-
protein alleles in wheat worldwide have been
reviewed by Bekes et al. (2007).
Allelic variation at the
Glu-1
loci (HMW sub-
units, Table 21.3) correlates with differences in
the genetic potential for rheological properties
(especially dough strength as R
max
, the height of
the extensigraph curve) and thus breadmaking
quality (Payne et al., 1987; Cornish et al., 2001,
2006; Vawser et al. 2002; Eagles et al., 2006).
Table 21.3 combines the results of these and other
recent publications on the rankings of HMW sub-
units. This list goes beyond the maximum score
of 4 allocated by Payne (1987) for HMW subunits
5 + 10, due to the great contribution of the over-
expressed 7 subunit. The addition of a zero score
should not imply that alleles on the bottom line
contribute nothing to dough strength, but rather
that they are the least contributors to R
max
overall.
Some alleles (Glu-A1p, Glu-B1b, and Glu-B1a)
are not included in Table 21.3 as they are rare,
and reliable estimates of their ranking is diffi cult
because there is limited data about them. Fur-
thermore, they are represented to a very small
extent in current cultivars around the world. For
example, normal subunit 7 was originally identi-
fi ed (with subunit 8) in the reference cultivar
Chinese Spring. We now know that the band 7 of
Chinese Spring's 7 + 8 pair differs slightly from
what is more commonly found; thus the com-
monly encountered pair of subunits is designated
Dough quality and polypeptide
composition
A few decades of intense study of the subunits of
glutenin (reviewed by Shewry et al., 2003) have
elucidated much of their contributions, as com-
ponent polypeptides, to the dough properties of
glutenin. The HMW subunits of glutenin (70,000
to 100,000 Da) have received more attention than
the LMW subunits, historically, because the
greater size range of the HMW subunits holds
them at the top of SDS-gel electrophoresis pat-
terns, without interference from other fl our pro-
teins. By contrast, initial extraction of gliadins
is necessary for the LMW subunits (20,000-
50,000 Da) to be seen clearly in SDS-electropho-
retic patterns. The HMW and LMW subunits
are coded by genes at distinct loci (Table 21.2) on
the long and short arms, respectively, of the
homoeologous group-1 chromosomes. The
Glu-1
locus is complex, involving pairs of paralogous
Table 21.3
Dough strength rankings and allele designations
(as lowercase letters) for HMW subunits of glutenin (
Glu-1
).
Dough-Strength
Score
Glu-A1
Glu-B1
Glu-D1
5
al
(
7
+
8)
4
d
(5
+
10)
3
a
(1)
f
(13
+
16)
b
(2*)
2
u
(7*
+
8)
i (17
+
18)
1
c
(null)
c
(7
+
9)
a
(2
+
12)
c
(4
+
12)
0
d
(6
+
8)
b
(3
+
12)
e
(20x
+
20y)
f
(2.2
+
12)
Note:
Subunit
7
is so indicated as it is overexpressed for
this allele.
