25 Rye Constituents and Their Impact on Rye Processing 579components. Enzymic hydrolysis also depends on
the substrate selectivity of the endoxylanases. Cer-
tain endoxylanases preferentially release high molec-
ular weight arabinoxylans from water-unextractable
arabinoxylans and slowly degrade water-extractable
arabinoxylans; other endoxylanases solubilize arabi-
noxylans of low molecular weight and/or extensively
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 (Hen-
rissat 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 fami-
lies 10 and 11 (Coutinho and Henrissat 1999).
Arabinofuranosidases (-L-arabinofuranosidases,
E.C. 3.2.1.55) hydrolyze the linkage between arabi-
nose residues and the xylan backbone, rendering the
latter less branched and more accessible to depoly-
merization 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--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 endoxylanas-
es can be expected, it is not always observed
(Figueroa-Espinoza et al. 2002, 2004).
Endogenous xylanolytic activity, although low,
has been reported to be present in ungerminated rye
(Rasmussen et al. 2001). The rye endogenous en-
doxylanase showed optimal activity 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 germination 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 the
Secale cerealeL. xylanase inhibitor (SCXI) (Goes-
aert et al. 2002), representing a family of isoinhib-
itors (SCXI I–IV) with similar structures and speci-
ficities. These inhibitors are basic proteins with
isoelectric points of at least 9.0 and have highly
homologous N-terminal amino acid sequences.
They occur in two molecular forms, that is, a mon-
omeric 40 k protein with at least one intramolecular
disulphide bridge and, presumably following prote-
olytic cleaving of this form, a heterodimer
consisting of two disulphide linked subunits (30 and
10 k). They specifically inhibit family 11 endoxy-
lanases, while fungal family 10 endoxylanase is
not affected (Goesaert et al. 2002). These inhibi-
tors are homologous with wheat Triticum aestivum
L. xylanase inhibitor I (TAXI I) (Gebruers et al.
2001).
Another family of isoinhibitors in rye, is the XIP-
type (xylanase inhibiting protein) endoxylanase
inhibitor family (Elliott et al. 2003, Goesaert et al.
2003). These inhibitors are basic, monomeric pro-
teins with a molecular weight of 30 k with pI values
of at least 8.5. They show inhibitory activity against
fungal glycosyl hydrolase family 10 and 11 endoxy-
lanases (Goesaert et al. 2003). Structural character-
istics 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 Inhib-
itors Literature on proteins from different botani-
cal sources (e.g., soy, wheat, rice) reveals that their
enzymic hydrolysis can significantly alter their pro-
perties (Caldéron 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 pro-
tein.
Proteases hydrolyze the peptide bonds between
the amino end of one amino acid residue and the
carboxyl end of the adjacent amino acid residue in a
protein. Endoproteases hydrolyze peptide bonds
somewhere along the protein chain, whereas exo-
proteases attack the ends of the protein chain and
remove one amino acid at a time. The latter are
called carboxypeptidases when acting from the car-
boxy terminus or aminopeptidases when acting from
the amino terminus. Based on the chemistry of their
catalytic mechanism, proteases have been classified
into four groups: serine (E.C. 3.4.21), metallo- (E.C.
3.4.24), aspartic (E.C. 3.4.23), and cysteine (E.C.
3.4.22) proteases.
Nowak and Mierzwinska (1978) reported the
presence of endopeptidase, carboxypeptidase, and
aminopeptidase activities in the embryo and en-
dosperm of rye seeds. Endoproteolytic, exoprote-
olytic, carboxypeptidase, aminopeptidase, and N--
benzoylarginine-p-nitroanilide (BAPA) hydrolyzing