Agriculture Reference
In-Depth Information
However, in most cases the marker proteins are
polypeptides (part of the proteome), not necessar-
ily functional proteins. This distinction is shown
in Fig. 21.1. Gene action results in the production
of messenger RNA (the transcriptome), and
thence the polypeptide, whose amino-acid
sequence is specifi ed by the respective DNA and
RNA sequences. Thereafter, folding and disul-
fi de bonding leads to the production of the func-
tional protein then capable, for example, of
enzymic activity. Proteome analyses have demon-
strated the presence of about 1,700 polypeptides
in developing wheat endosperm, but even this
large number is only about 30% of the number of
polypeptides that should be present, given the
identifi cation of over 6,000 mRNA species at this
stage of endosperm development (Skylas et al.,
2005).
polypeptide chain. Table 21.2 also lists the gene
designations for the respective groups of gliadin
and glutenin polypeptides.
The gliadin proteins have a molecular-weight
range up to about 100,000 Da, contributing to the
viscosity and extensibility of the dough. However,
genetic assessments of the functional properties
of the
Gli-1
controlled gliadins are confused by
their tight linkage to the
Glu-3
genes controlling
the low-molecular-weight (LMW) subunits of
glutenin (Shewry et al., 2003).
The polymeric glutenin proteins have molecu-
lar sizes ranging up into the tens of millions of
Daltons (Southan and MacRitchie 1999; Wieser
et al., 2006). The great length of these chains is
presumed to confer on wheat its unique dough-
forming properties. Conversely, dough strength
increases as the proportion of very large glutenin
polymers increases. However, excessive dough
strength is inappropriate even for products such
as pan breads and pasta (Table 21.1). A balance
of strength and extensibility is needed. The level
of this balance depends on processing require-
ments, indicated in Table 21.1 as dough
strength.
This balance is apparent in the molecular weight
distribution of the polymeric glutenin proteins,
and also in consideration of the balance of glutenin
to gliadin, which can again be seen as a very broad
molecular-weight distribution (Wrigley et al.,
2006). Analysis of the molecular-weight distribu-
tion has generally been performed by size- exclusion
Dough quality and functional proteins
The glutenin proteins exemplify the important
distinction between polypeptide and functional
protein. Traditionally, they have been examined
as polypeptides (subunits), extracted after the
rupture of all disulfi de bonds. However, it is the
native polymers of the glutenin subunits that
make the contribution to dough strength; the sub-
units alone cannot contribute to dough strength
(Table 21.2). In contrast to the glutenin proteins,
the gliadin polypeptides are synthesized as single
chains, with disulfi de bonds formed within the
Table 21.2
Marker proteins (and thus genes) for specifi c aspects of grain quality.
Locus Designation
Polypeptide
Functional Protein
Quality Attribute
Gli-1
Gliadin proteins
Gliadin proteins
Modest contribution to dough
viscosity and extensibility
a
Gli-2
Gliadin proteins
Gliadin proteins
Glu-1
HMW subunits
of glutenin
Polymeric glutenin
proteins
Major contribution to dough
strength
Glu-3
LMW subunits
of glutenin
Signifi cant contribution to
extensibility
a
Pin-a
Puroindoline a
Puroindoline a
Grain hardness
Pin-b
Puroindoline b
Puroindoline b
Grain hardness
GBSS
b
Wx-1
GBSS
Starch properties
a
Contributions of
Gli-1
,
Gli-2
, and
Glu-3
require further investigation for more precise assessment.
b
Granule-bound starch synthase.
