Biomedical Engineering Reference
In-Depth Information
the gluten protein fraction in wheat flour. Gluten is the water-insoluble viscoelastic
protein mass left after soluble proteins, starch, and other nonprotein materials have
been washed out from the flour. Gluten is composed mainly of hydrated forms of
the two major wheat protein fractions, namely, gliadins (wheat prolamins) and
glutenins (wheat glutelins).
63,75
These proteins contribute in different ways to the
properties of the flour during processing and in the final product. Thus, gliadins
provide viscosity and extensibility to bread dough, whereas glutenins provide the
elasticity that is all-important in dough stability and in the structure and texture of
bread. Differences in proportions and properties of these two protein fractions
determine whether a particular wheat variety has good bread-making properties or
is more suitable for the production of other products, such as pasta or biscuits. A
molecular basis for these properties has been proposed, based on the characteristics
of the purified components, their amino acid composition and the primary structures
of certain wheat proteins.
75
The components responsible for good bread-making
quality have been tentatively identified and ascribed to the glutenin fraction, notably
to the high molecular weight (HMW) glutenin proteins (95,000 to 150,000 apparent
MW).
75,76
Payne et al. (cited by Reference 63) have shown a positive correlation
between the molecular weight of native glutenin and the amount of HMW glutenin
subunits, and have suggested that interactions of these subunits with other polypep-
tides are important in stabilizing the glutenin structure. Furthermore, it has been
concluded that allelic variation does correlate with good or poor baking quality,
although other factors, possibly other wheat proteins, may be involved.
63
Certain of
the gliadin polypeptides have also been implicated in baking quality and dough
strength. The positive identification and cloning of the genes encoding polypeptides
that contribute to good bread-making quality could potentially allow the transfer of
this trait to poorer-quality wheats carrying other desirable attributes.
63
Additionally,
it might be possible to manipulate the functional properties of the gluten for purposes
other than bread making by transferring multiple copies of certain genes, thereby
altering the proportion of the HMW glutenins.
77
There are 12 genes for HMW glutenin proteins in hexaploid bread wheat, four
coming from each of the three progenitor species although two genes are inactive
in all varieties.
75
The genes are of two types,
Glu-1-1
and
Glu-1-2
, which give rise
to X and Y HMW glutenin subunits, respectively.
Glu-1-1
and
Glu-1-2
loci are very
closely linked on the long arms of the chromosomes of the homologous group 1.
Thus, there are six pairs of loci each of which carries an X and Y gene.
78
Series of
genotypes have been assayed, possessing new combinations of X and Y subunit
genes at the
Glu-1D-1
and
Glu-1D-2
loci.
79
The comparisons have enabled the
separate contributions of subunits associated with poor bread-making quality [2(X)
and 12(Y)] and subunits associated with good bread-making quality [5(X) and
10(Y)]. Flavell et al.
75
have showed that in these seeds the major variation was
contributed by the Y subunits, with subunit 12 conferring poorer dough quality than
subunit 10. They compared the amino acid sequences of these closely related proteins
and found that differ in their central regions, which consist of an array of repeating
hexamers and nonamer amino acid units; HMW glutenin 10 has a higher proportion
of repeats of the consensus type than glutenin 12 and they postulated that this
produces a more regular pattern of repetitive
β
turns in the protein, contributing to
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