BLBS102-c34 BLBS102-Simpson March 21, 2012 14:7 Trim: 276mm X 219mm Printer Name: Yet to Come
662 Part 5: Fruits, Vegetables, and Cerealsglutamine/glutamate and contain lower levels of proline than
the other secalin groups (Field et al. 1982, Tatham and Shewry
1991). The conformation of HMW secalins has not been re-
ported in detail, but based on studies with HMW glutenin sub-
units (HMW-GS) of wheat, it was suggested that they contain
central repetitive sequences with a rodlike shape, flanked by
nonrepetitive domains having a globular conformation rich in
α-helical structure. The nonrepetitive domains contain most of
the cysteine residues involved in intermolecular cross-linking
(Shewry et al. 1995). The HMW secalins are present only in
an aggregated state stabilized by disulphide bonds (Field et al.
1983, Gellrich et al. 2003, Shewry et al. 1983). They are homol-
ogous to the HMW-GS of wheat and the D hordeins of barley
(Field et al. 1982, 1983; Gellrich et al. 2003; Shewry et al. 1982,
1988; Fig. 34.1).Glutelins In contrast to the wheat glutelins, which are called
“glutenins,” no alternative name has been given to the rye glutelin
fraction. The glutelins are not soluble in aqueous alcohols. They
form HMW polymers stabilized by interchain disulphide bonds.
However, the individual subunits of the polymers obtained in the
presence of a reducing agent are soluble in alcohol media and
rich in proline and glutamine (Field et al. 1982, 1983; Gellrich
et al. 2003; Kipp et al. 1996; Shewry et al. 1983). The building
blocks of the glutelins are therefore considered to be secalins.
The glutelins soluble in alcohol after reduction contain mainly
HMW-secalins (26%) and 75kγ-secalins (52%) and small levels
of 40kγ-secalins (12%) (Gellrich et al. 2003; Fig. 34.1). When
the distribution of the 75kγ-, 40kγ-,ω- and HMW secalins over
the storage proteins (secalins+glutelins) is considered, it can be
concluded that the major portion of 75kγ-secalins is present in
the prolamin fraction (40%) and only 7% in the glutelin fraction.
The 40kγ-secalins (23%) andω-secalins (16%) appear mainly
in the prolamin fraction due to their monomeric state. A larger
portion of HMW secalins is present in the prolamin (4%) than
in the glutelin fraction (3%) (Gellrich et al. 2003).Gluten Formation The wheat gliadin and glutenin fractions
can form a polymeric gluten network by intermolecular noncova-
lent interactions and disulphide bonds. In contrast to wheat, rye
storage proteins do not form gluten. Despite partial homology,
storage proteins of rye differ significantly from those of wheat
with respect to quantitative and structural parameters that are
important for the formation and properties of gluten. Typical for
rye are the low content of storage proteins and the high ratio of
alcohol-soluble proteins to insoluble proteins (Chen and Bushuk
1970, Gellrich et al. 2003, Preston and Woodbury 1975). The
75kγ-secalins and HMW secalins differ in aggregation behavior
from the LMW-GS and HMW-GS of wheat, respectively, due
to the absence (Gellrich et al. 2004b) or presence (Kohler and ̈
Wieser 2000) of cysteine residues in theC-terminal domain in
positions favorable for formation of intramolecular disulphide
bonds and unfavorable for formation of intermolecular disul-
phide bonds. It is likely that these structural differences con-
tribute to the lack of ability of rye to form gluten. Another factorhampering the formation of a protein network from rye secalin
and glutelin fractions may be a higher degree of glycosylation
in rye subunits than in wheat subunits (Kipp et al. 1996).Functional ProteinsAmong the many enzymes and structural proteins in cereals in
general, the most important enzymes present in rye grain that
break down its major constituents (starch, arabinoxylan, and
protein) are theα-amylases, endoxylanases, and proteases, re-
spectively. Such enzymes can be of major importance in rye
processing. Rye also contains specific proteins with inhibition
activity against these enzymes. Presumably, these enzyme in-
hibitors contribute to plant defense mechanisms and/or possibly
intervene in the complex regulation of plant metabolic processes.Starch-Degrading Enzymes and Their Inhibitors α-
amylases or, in full, 1,4-α-d-glucan glucanohydrolases (E.C.
3.2.1.1) catalyze the hydrolysis of the internal 1,4-α-d-glucan
linkages in starch components.
Rye synthesizes two groups ofα-amylases: low pI (≤pI
5.5) and high pI (≤pI 5.8). The low-pIα-amylases are syn-
thesized during grain development, particularly in the pericarp
(Dedio et al. 1975). Their activity decreases during the later
stages of grain development and is low at maturity. The high-pI
α-amylases are synthesized during germination. In germinated
rye, the high-pI groups represent a high proportion of the total
amylase activity (MacGregor et al. 1988). Gabor et al. (1991)
isolated a high-pIα-amylase from germinated rye, also called
“germination-specific”α-amylase, which showed optimal activ-
ity at pH 5.5 and is stable at pH 6.0–10.0 after storage at 4◦C.
The temperature optimum over a 3 minute incubation period was
65 ◦C, but 40% of the activity was lost when the enzyme was
incubated without substrate for 4 hours at 55◦C. Taufel et al. ̈
(1991) also isolated a germination-specificα-amylase from rye
and reported a pH optimum of 5.0, pH stability between pH 5.0
and 7.0 and thermal stability up to 55◦C.
Variation inα-amylase levels exists between different rye lines
(Masojc and Larssonraznikiewicz 1991a). Masojc and Larsson-
raznikiewicz (1991b) showed the existence of low and high
α-amylase genotypes. In contrast, Hansen et al. (2004) stated
that theα-amylase activity is mostly influenced by harvest year,
attributing only a small effect to genotype.
α-Amylase inhibitors can reduce the activity of one or more
α-amylases and are mainly found in two major families, that
is, the “cereal trypsin/α-amylase inhibitor” and the “Kunitz” or
“α-amylase/subtilisin inhibitor” families (Garcia-Olmedo et al.
1987). The cereal trypsin/α-amylase inhibitor family represents
the major part of the albumin and globulin fraction of the
endosperm and consists of monofunctional inhibitors, active
against either trypsin or exogenousα-amylase. The Kunitz-type
inhibitors are bifunctional inhibitors of about 20 k, containing
two intramolecular disulphide bridges. They can simultaneously
inhibit both subtilisin and endogenousα-amylases, in particular
the germination specific, high-pIα-amylase.