BLBS102-c34 BLBS102-Simpson March 21, 2012 14:7 Trim: 276mm X 219mm Printer Name: Yet to Come
34 Rye Constituents and Their Impact on Rye Processing 661- Secalins • Glutelins
- Gliadins
aLMW-GS show strong similarities with gliadins, differing in one very important characteristic, i.e., the occurrence of
interchain disulphide bonds in LMW-GS.- Glutenins
- 40k γ-, 75k γ-secalins
- HMW secalins
- 40k γ-, 75k γ-secalins
- HMW secalins
- ω-, secalins
S-rich prolaminsRye proteinsWheat proteinsProlamins(Mainly monomeric, some smaller polymers)HMW prolaminsS-poor prolamins→→←←→- α-, β-, γ-gliadins - B and C LMW-GSa
- HMW-GS
- ω-gliadins
S-rich prolaminsHMW prolaminsS-poor prolamins - D LMW-GS*→→←←→←Glutelins(Polymeric)Figure 34.1.Schematic representation of the classification of rye and wheat proteins. LMW, low molecular weight; HMW, high molecular
weight.and Shewry 1991), consistent with a higher proportion of repet-
itive sequences in the former group (Shewry et al. 1982).
The twoγ-secalin types differ in aggregation behavior. The
40kγ-secalins predominantly occur as monomers containing
intramolecular disulphide bonds, while the 75kγ-secalins are
present predominantly as disulphide-linked aggregates (Field
et al. 1983, Gellrich et al. 2003, Shewry et al. 1983). A possible
explanation for this different aggregation behavior is the pres-
ence or absence of cysteine residues in positions that are favor-
able or unfavorable for formation of intermolecular disulphide
bonds. Alternatively, it might result from the specific action
of enzymes responsible for the formation of such bonds (Field
et al. 1983, Shewry et al. 1983). Gellrich et al. (2001, 2004b)
showed that the eight-cysteine residues of theC-terminal do-
main of 75kγ-secalin are linked by intramolecular disulphide
bonds and that the cysteine residue located in position 12 of the
N-terminal domain of the 75kγ-secalin forms an intermolecu-
lar disulphide bond with 75kγ-secalins. The cysteine residue
in theN-terminal domain is unique to the 75kγ-secalins and
is likely to be responsible for their aggregative nature (Gellrich
et al. 2003, 2004b).
The 40kγ-secalins are homologous to the wheatγ-gliadins
and to the B hordeins of barley (Field et al. 1983; Gellrich et al.
2001, 2003; Shewry et al. 1982), whereas the 75kγ-secalins
differ from them due to the presence of additional repetitivesequences rich in glutamine and proline and their occurrence
as aggregates. The latter are suggested to be homologous to
the LMW glutenin subunits (LMW-GS) of wheat (Shewry et al.
1982) (Fig. 34.1).
The S-poor orω-secalins are quantitatively minor compo-
nents, accounting for about 19% of the total secalin fraction
(Gellrich et al. 2003). They have SDS-PAGE molecular weights
of about 48 k to 53 k (Field et al. 1983, Gellrich et al. 2003,
Shewry et al. 1983). The amino acid compositions of theω-
secalins are characterized by high levels of glutamine/glutamate,
proline, and phenylalanine and the absence of cysteine and me-
thionine (Tatham and Shewry 1991). Theω-secalins consist al-
most entirely of repetitive sequences (Tatham and Shewry 1991)
forming a stiff, wormlike coil (Tatham and Shewry 1995). They
are present predominantly as monomers (Field et al. 1983, Gell-
rich et al. 2003) and are homologous to theω-gliadins of wheat
and to the C hordeins of barley (Field et al. 1983, Gellrich et al.
2003, Tatham and Shewry 1991) (Fig. 34.1).
HMW secalins are also quantitatively minor components, ac-
counting for about 5% of the total secalin fraction (Gellrich
et al. 2003). SDS-PAGE molecular weights above 100 k were
observed (Field et al. 1982, Gellrich et al. 2003, Kipp et al. 1996,
Shewry et al. 1983), whereas sedimentation equilibrium ultra-
centrifugation showed a molecular weight of 68 k (Field et al.
1982). The HMW secalins have a high content of glycine and