Chemical and Functional Properties of Food Saccharides

peroxidase, or hexokinase and highly specific glucose-6-phosphate dehydrogenase.
Oxidoreductases such as glucose dehydrogenase may also be used for large-scale
production of sugar alcohols such as sorbitol and aldonic acids such as gluconic
acid, applied in food, pharmaceutical, and chemical industries. 15 One of the principal
industrial conversions of sorbitol is its selective oxidation by D-sorbitol dehydrogenase
to L-sorbose, which is a chiral precursor of ascorbic acid. Similarly, as in many
other processes, the latter reaction is run with whole microbial cells (Gluconobacter
species), and not with purified enzymes. Other oxidoreductases useful in production
of nonnatural carbohydrates and related compounds (Table 10.3) are pyranose 2oxidase
(P2O), aldose (xylose) reductase (ALR), 16 and cellobiose dehydrogenase,
which oxidize soluble cellodextrins, mannodextrins, and lactose to their lactones.
Cellobiose dehydrogenase seems to be applicable for conversion of lactose to lactobionic
acid.
Xylose isomerase was the first technically applied preparation of sugar
isomerase. It catalyzes the reversible isomerization of D-xylose to D-xylulose as
well as conversion of glucose to fructose. The latter specificity has been industrially
explored for production of high-fructose syrups. Also, sucrose isomerase is very
useful in conversion of sucrose to isomaltulose, trehalulose, isomaltose, and
isomelezitose. Products of the sucrose isomerization have attracted attention as
potential acariogenic sweeteners (see Chapter 24).
Polysaccharide lyases and esterases are involved in conversion of pectin (see
Chapter 12), xylan (see Chapter 13), and starch. Lyases cleave chains of polymers
via a β-elimination mechanism that leads to the formation of a double bond at the
newly formed nonreducing end. The most commercially important are pectate and
pectin lyases, participating in depolymerization of pectins (Table 10.6). Another
group of lyases that has gained increasing attention is aldolases, harnessed for
synthesis of monosaccharides and their analogs in glycobiotechnology (see Section
10.2.6.1).
TABLE 10.3
Products of Conversion of Mono-and Disaccharides by Means of P2O
and ALR
Substrate Product of Oxidation with P2O a
D-Glucose
D-Galactose
Lactose
Gentiobiose
Allolactose
Isomaltose
Melibiose
© 2004 by CRC Press LLC
2-Keto-D-glucose (D-glucosone)
2-Keto-D-galactose (D-galactosone)
Lactosone
Gentiobiosone
Allolactosone
Isomaltosone
Melibiosone
Product of Further
Reduction with ALRb D-Fructose
D-Tagatose
Lactulose
Gentiobiulose
Allolactulose
Isomaltulose
Melibiulose
a H2O2 formed as a by-product is degraded by catalase (to H2O and O2). b NAD + formed as a by-product is reduced by formate dehydrogenase, which catalyzes the reaction
of formate oxidation to CO2 (HCOO – + NAD + = CO2 + NADH + H + ).