680 15 Cereals and Cereal Products
cereals,Triticeaeprolamins contain significantly
higher levels of glutamic acid and proline. This
suggests that the difference in prolamin compo-
sition, induced by these amino acids, may be re-
sponsible forCeliacdisease.
15.2.1.3 Protein Components
of Wheat Gluten
Wheat protein fractionation by the Osborne
method provides prolamins and glutelins in
a ratio of 1:1. Both fractions, in hydrated form,
have different effects on the rheological char-
acteristics of dough: prolamins are responsible,
preferentially, for viscosity, and glutelins for
dough strength and elasticity.
The genes for the gluten proteins occur at nine
different complex loci in the wheat genome.
The high molecular weight glutenin subunits
are coded by the loci Glu-A1, Glu-B1 and
Glu-D1, which are carried on the long arms
of the chromosomes 1A, 1B and 1D. The
low molecular weight glutenin subunits, the
ω-gliadins and theγ-gliadins are coded by the
loci Gli-A1, Gli-B1 and Gli-D1, which occur
on the short arms of the chromosomes 1A, 1B
and 1D. Theα-gliadins are coded by the loci
Gli-A2, Gli-B2 and Gli-D2 on the short arms of
the group G chromosomes. It is presumed that
the variation seen in different varieties is due
to the presence of allelic genes at each of the
nine storage protein loci. The relative importance
of different alleles for gluten quality seems to
be Glu-1>Gli-1>Gli-2.
A fractionation of gluten proteins is possible by
two-dimensional electrophoresis. Figure 15.3
provides a schematic overview of the position
of the most important protein groups in a two-
dimensional electropherogram. The pattern of
glutenins of two wheat cultivars are shown in
Fig. 15.4.
Gluten proteins can be separated into their com-
ponents on an analytical and micropreparative
scale by using RP-HPLC. In general, this separa-
tion starts with theOsbornefractions or subfrac-
tions.
In this way, theprolaminesof wheat can be sepa-
rated intoω-,α-,γ-gliadins (Fig. 15.5), differ-
ent varieties of wheat giving different patterns,
e. g., the cultivars Clement and Maris Huntsman
Fig. 15.3.Wheat endosperm proteins (cultivar “Chinese
Spring”). Simplified schematic representation of a two-
dimensional electrophoretic separation.
1st. Dimension: isoelectric focussing (IEF) and non-
equilibrium pH gradient electrophoresis (NEPHGE).
The electropherograms obtained by both methods are
put together at the broken line in such a way that a con-
tinuous pH gradient is formed.
2nd. Dimension: polyacrylamide gel electrophoresis in
the presence of sodium dodecyl sulfate and mercap-
toethanol (SDS-PAGE).
The following protein fractions can be recognized:
high-molecular glutenin subunits (1); basic (2) and
acidic (3) low-molecular glutenin subunits;γ-(4)and
ω-gliadins (5); subunits of the triplet band (6, 10); high-
molecular albumins (7); globulins (8) and nonreserve
proteins (9). (according toPayneet al., 1985)known to produce sticky dough have a character-
istically highω-gliadin content.
The prolamin patterns of other cereals (Fig. 15.6)
differ greatly from that of wheat. In rye,the
hydrophilicω-secalins are followed by the hy-
drophobicγ-secalins. And unlike wheat (α-glia-
dins), the area of moderate hydrophobicity is
not occupied. Inbarley, a hydrophilic fraction is
missing: the C-hordeins eluted in the middle area
are followed by the hydrophobic B-hordeins. The
chromatogram ofoatsis characterized by two
hydrophobic fractions that are close to each other.
Thelow-molecular subunits of wheat glutelins
also give a chromatogram rich in components