Biomedical Engineering Reference
In-Depth Information
A range of models has been developed to explain the structure and functionality
of glutenins. Most recently, the experimental findings on disulfide bonds were trans-
formed into a two-dimensional model [
65
] (Fig.
2.4
). HMW-GS and LMW-GS
polymerize separately, both forming linear backbone polymers. Both polymers are
cross-linked by a disulfide bond between section IV of LMW-GS and section B of
y-type HMW-GS. The backbone of HMW-GS is established by end-to-end, probably
head-to-tail linkages. LMW-GS polymers are linked between two sections I and
between sections I and IV. The polymerization of HMW-GS and LMW-GS is termi-
nated by chain terminators, either by modified gliadins or LMW thiol compounds.
2.3.3
Storage Proteins of Corn, Millet, Sorghum, and Rice
Overall, the storage proteins of corn, sorghum, millet, and rice are, in part, related
and differ significantly from those of wheat, rye, barley, and oats. According to the
amino acid composition they contain less glutamine and proline and more hydro-
phobic amino acids such as leucine [
56
]. Corn storage proteins, called zeins, can be
subgrouped into alcohol-soluble monomeric zeins and cross-linked zeins alcohol-
soluble only on heating or after reduction of disulfide bonds. With respect to differ-
ent structures zeins have been divided into four different subclasses [
95
] . a -Zeins
are the major subclass (71-85% of total zeins), followed by g- (10-20%), b - (1-5%)
and d-zeins (1-5%), respectively [
96
] . a-Zeins are monomeric proteins with appar-
ent MW of 19,000 and 22,000 determined by SDS-PAGE. Their amino acid
sequences contain up to ten tandem repeats [
97
]. Proteins of the other subclasses are
cross-linked by disulfide bonds and their subunits have apparent MW of 18,000 and
27,000 (g-zein), 18,000 (b-zein), and 10,000 (d -zein).
In many ways the storage proteins of sorghum and millet called kafirins are
similar to zeins. Sorghum kafirins have also been subdivided into a , b -, g - and
d-subclasses based on solubility, MW, and structure [
98
] . a -Ka fi rins are monomeric
proteins and represent the major subclass accounting for around 65-85% of total
kafirins. Proteins of the other subclasses are highly cross-linked and alcohol-soluble
only after reduction of disulfide bonds. On average, each of them accounts for less
than 10% of total kafirins [
99,
100
]. Within the numerous millet species and variet-
ies the proteins of foxtail millet were studied in detail [
101
] . SDS-PAGE of unre-
duced kafirins revealed bonds with apparent MW ranging from 11,000 to 150,000.
After the reduction of disulfide bonds two major bands with MW of 11,000 (subunit
A) and 16,000 (subunit B) were obtained. Unreduced proteins with higher MW
were formed by cross-links of A and/or B subunits. The storage proteins of rice are
characterized by the highly unbalanced ratio of prolamins to glutelins (~1:30) [
102
] .
Both fractions show the lowest proline content (~5 mol-%) amongst cereal storage
proteins [
56
]. SDS-PAGE patterns of rice prolamins (oryzins) showed a major band
with MW 17,000 and a minor band with MW 23,000 [
103
] . The apparent MW of
glutelin subunits was in a range from 20,000 to 38,000.
