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 663T ̈aufel et al. (1991, 1997) purified two proteinaceous in-
hibitors ofα-amylase from rye: a regulation or R-type in-
hibitor, which only inhibits the endogenous germination-specific
α-amylase, and a defense or D-type inhibitor, which only in-
hibits exogenousα-amylases of animal and human origin. The
inhibitors are present in different tissues of the rye kernel (Taufel ̈
et al. 1997). The R-type inhibitor predominantly occurs in the
bran, whereas the D-type inhibitor is enriched in the germ and
the endosperm. The high D-type inhibitor activity in these ker-
nel sections is consistent with its assumed defense function, as
it can protect the starch against exogenousα-amylases from in-
sects and rodents. The levels of both types of inhibitor vary with
growing conditions (T ̈aufel et al. 1997). Rye varieties cultivated
under drought conditions show high D-type inhibitor activities
and low R-type inhibitor activities, whereas the opposite is true
when they are cultivated under wet conditions. Small variations
inα-amylase inhibitor activities exist between sprout-stable and
sprout-sensitive rye genotypes (T ̈aufel et al. 1997). Whereas
α-amylase activity increases rapidly after 24 hours of germina-
tion, the R- and D-type inhibitor activities are stable during the
72 hours of germination (Taufel et al. 1997). The ̈ α-amylase
inhibitors obviously have no importance at the early stages of
growth.Arabinoxylan-Degrading Enzymes and Their Inhibitors
Arabinoxylan structural and physicochemical properties can be
impacted by enzymes such as arabinofuranosidases, xylosidases,
esterases, and endoxylanases, with the latter being by far the
most relevant in cereal processing.
Endoxylanases or, in full, endo-1,4-β-d-xylan xylanohydro-
lases(E.C. 3.2.1.8) hydrolyze internal 1,4 linkages betweenβ-d-
xylopyranose residues of (arabino-) xylan molecules, generat-
ing (arabino-) xylan fragments with lower molecular weight and
(un)substituted xylooligosaccharides. Their action can thus lead
to (partial) solubilization of water-unextractable arabinoxylans
and to a decrease in molecular weight of water-extractable and/or
solubilized arabinoxylans. The susceptibility of arabinoxylans
to endoxylanase attack probably depends on their substitution
degree and their substitution pattern as well as on their linkages
to other cell wall components. Enzymic hydrolysis also depends
on the substrate selectivity of the endoxylanases. Certain en-
doxylanases preferentially release high molecular weight ara-
binoxylans from water-unextractable arabinoxylans and slowly
degrade water-extractable arabinoxylans; other endoxylanases
solubilize arabinoxylans of low molecular weight and/or exten-
sively degrade water-extractable arabinoxylans.
Based on their primary sequence and structure, endoxylanases
are classified into two main groups: glycosyl hydrolase family
10 and family 11 (Henrissat 1991). Plant endoxylanases to date
have been exclusively classified in glycosyl hydrolase family
10 in contrast to fungal and bacterial endoxylanases, which are
present in both glycosyl hydrolase families 10 and 11 (Coutinho
and Henrissat 1999).
Arabinofuranosidases (α-l-arabinofuranosidases, E.C.
3.2.1.55) hydrolyze the linkage between arabinose residues andthe xylan backbone, rendering the latter less branched and more
accessible to depolymerization by endoxylanases. Ferulic acid
esterase (feruloyl esterase, E.C. 3.1.1.73) can hydrolyze the
ester linkage between ferulic acid and arabinose. Xylosidases
(exo-1,4-β-xylosidase, E.C. 3.2.1.37) release single xylose
residues from the nonreducing end of arabinoxylan fragments,
thereby increasing the level of reducing end sugars. While
synergy between these classes of enzymes and endoxylanases
can be expected, it is not always observed (Figueroa-Espinoza
et al. 2002, 2004).
Endogenous xylanolytic activity, although low, has been re-
ported to be present in ungerminated rye (Rasmussen et al.
2001). The rye endogenous endoxylanase showed optimal ac-
tivity at pH 4.5 and 40◦C and was pH stable but heat labile.
Xylosidase and arabinofuranosidase enzymes are also present
in ungerminated rye grain (Rasmussen et al. 2001). During ger-
mination of rye, endoxylanase activity increases significantly
after the first day (Autio et al. 2001).
Endoxylanase inhibitors can affect the enzymic hydrolysis of
arabinoxylans by endoxylanases. Two different types of such
inhibitors have been found to be present in rye (Goesaert et al.
2002, 2003).
The first type of inhibitor found in rye is theSecale cerealeL.
xylanase inhibitor (SCXI) (Goesaert et al. 2002), representing a
family of isoinhibitors (SCXI I–IV) with similar structures and
specificities. These inhibitors are basic proteins with isoelectric
points of at least 9.0 and have highly homologousN-terminal
amino acid sequences. They occur in two molecular forms, that
is, a monomeric 40 k protein with at least one intramolecular
disulphide bridge and, presumably following proteolytic cleav-
ing of this form, a heterodimer consisting of two disulphide
linked subunits (30 and 10 k). They specifically inhibit family
11 endoxylanases, while fungal family 10 endoxylanase is not
affected (Goesaert et al. 2002). These inhibitors are homologous
with wheatTriticum aestivumL. xylanase inhibitor I (TAXI I)
(Gebruers et al. 2001).
Another family of isoinhibitors in rye, is the XIP-type (xy-
lanase inhibiting protein) endoxylanase inhibitor family (Elliott
et al. 2003, Goesaert et al. 2003). These inhibitors are basic,
monomeric proteins with a molecular weight of 30 k with pI val-
ues of at least 8.5. They show inhibitory activity against fungal
glycosyl hydrolase family 10 and 11 endoxylanases (Goesaert
et al. 2003). Structural characteristics and inhibition specificities
from the rye XIP-type inhibitors are similar to those of wheat
XIP-type inhibitors (Gebruers et al. 2002).Protein-Degrading Enzymes and Their Inhibitors Litera-
ture on proteins from different botanical sources (e.g., soy,
wheat, rice) reveals that their enzymic hydrolysis can signifi-
cantly alter their properties (Calderon de la Barca et al. 2000, ́
Popineau et al. 2002, Shih and Daigle 1997, Wu et al. 1998).
This, however, has not been reported for rye protein.
Proteases hydrolyze the peptide bonds between the amino
end of one amino acid residue and the carboxyl end of the adja-
cent amino acid residue in a protein. Endoproteases hydrolyze