Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Enzymes of both families possess three to five xylopyranose binding sites,
neighboring with the catalytic site. The aromatic side groups of Tyr, and not those
of Trp, play the basic role in binding of the xylopyranose units.
Among all known bacterial and archaeal xylanases, the most heat stable are
enzymes from Pyrococcus furiosus, Pyrodictum abysii, Sulfolobus solfataricus MT-
4, and Thermotoga strains, with optimum activities at 105, 110, 90, and 100°C,
respectively. 30
Produced by fungi and some bacteria, exo-acting β-xylosidases release D-xylose
from short oligosaccharides and xylobiose. 2 Because the latter disaccharide is an
inhibitor of endoxylanases, the exo-acting hydrolases improve the effectiveness of
xylan digestion. The most heat-stable and thermoactive (at 50 to 100°C) β-xylosidases
are synthesized by Thermotoga species, together with α-L-arabinofuranosidases,
which are active against branched arabinoxylans, arabinose-substituted xylooligosaccharides,
and p-nitrophenyl-α-L-arabinofuranoside. 30
10.4.5 FRUCTANS
Table 10.7 presents the enzymes involved in the degradation of inulin and levan,
which are relatively short plant reserve fructans (see Chapter 13). Inulin is hydrolyzed
to fructose by inulinase and fructan β-fructosidase. The latter enzyme is active
against inulin, levan, and sucrose, whereas inulinase is also active against sucrose,
similar to invertase. Levan is also hydrolyzed by levanase, but this enzyme is not
TABLE 10.7
Fructan-Converting Enzymes
Enzyme EC Number Systematic Name Application
Inulinase EC 3.2.1.7 2,1-β-D-Fructan
fructanohydrolase
Fructan β-fructosidase EC 3.2.1.80 β-D-Fructan
fructanohydrolase
Levanase EC 3.2.1.65 2,6-β-D-Fructan
fructohydrolase
Inulinase II EC 2.4.1.93 Inulin D-fructosyl-Dfructosyltransferase(1,2′:2,3′dianhydride
forming)
Inulinase III EC 2.4.1.200 Inulin D-fructosyl-Dfructosyltransferase(1,2′:2′,1dianhydrideforming)
Hydrolysis of inulin to
fructose; active
against sucrose;
some inulinases yield
also
fructooligosaccharid
es with DP of 2–7
Hydrolysis of inulin,
levan, and sucrose
Hydrolysis of levan to
fructose
Inulin conversion to
di-D-fructose
1,2′:2,3′ dianhydride
(DFA III)
Inulin conversion to
1,2′:2′,1-dianhydride