Biomedical Engineering Reference
In-Depth Information
Because HMW-GS do not occur as monomers, it is generally assumed that they
form interchain disulfide bonds. The x-type subunits, except subunit 1Dx5, have
three cysteine residues in domain A and one in domain C (Fig.
2.1
). Cysteines C
a
and C
b
were found to be linked by an intrachain bond, thus, the others (C
d
, C
z
) are
available for interchain bonds. Subunit 1Dx5 has an additional cysteine residue at
the beginning of domain B, and it has been suggested that this might form another
interchain bond. Recently, a so-called head-to-tail disulfide bond between HMW-GS
has been identi fi ed [
71
]. The y-type subunits have five cysteine residues in domain
A and one in each of domains B and C. At present, interchain linkages have only
been found for adjacent cysteine residues of domain A (C
c1
, C
c2
), which are con-
nected in parallel with the corresponding residues of another y-type subunit, and for
cysteine C
y
of domain B, which is linked to C
x
of section IV of LMW-GS. Thus,
HMW- and LMW-GS fulfill the requirement that at least two cysteines forming
interchain disulfide bonds are necessary to participate in a growing polymer; they
act as “chain extenders.” The most recent glutenin model suggests a backbone
formed by HMW-GS linked by end-to-end, probably head-to-tail interchain disulfide
bonds [
65
]. LMW-GS form also linear polymers via cysteine residues of sections I
and IV; they are linked to domain B of y-type HMW-GS. y-Type HMW-secalins of
rye have a second cysteine in domain C, which opens the possibility that an intrachain
disulfide bond within domain C is formed inhibiting an interchain bond for polym-
erization [
72
]. As far as is known, D-hordeins possess ten (!) cysteine residues [
63
] ;
the formation of a regular polymer backbone appears to be impossible.
2.3.2.3
Molecular Weight Distribution
Most information on the quantitative MW distribution (MWD) of native storage
(gluten) proteins is available for wheat, because MWD of gluten proteins has been
recognized as one of the main determinants of the rheological properties of wheat
dough. Native gluten proteins consist of monomeric a / b- and g-gliadins with MW
around 30,000 and monomeric w5- and w1,2-gliadins with MW between 40,000 and
55,000. They are alcohol-soluble and amount to ~50% of gluten proteins (Fig.
2.3
).
Besides monomers the alcohol-soluble fraction contains oligomers with MW
roughly ranging between 60,000 and 600,000. They are formed by modified gliadins
with an odd number of cysteine residues and LMW-GS via interchain disulfide
bonds and account for ~15% of gluten proteins. Composition and quantity of the
oligomeric fraction are strongly determined by the conditions of alcohol extraction,
for example by temperature and duration. The remaining proteins (~35%) are alco-
hol-insoluble and mainly composed of LMW-GS and HMW-GS linked by disulfide
bonds. Their MW ranges approximately from 600,000 to more than 10 million. The
largest polymers termed “glutenin macropolymers” (GMP) are insoluble in SDS
solutions and have MW well into the multimillions indicating that they may belong
to the largest proteins in nature [
73,
74
]. Their amounts in flour (20-40 mg/g) are
strongly correlated with dough strength and bread volume. GMP is characterized
by higher ratios of HMW-GS to LMW-GS and x-type to y-type HMW-GS in
