2009_Book_FoodChemistry

334 4 Carbohydrates
4.4.5.1.4 α-Dextrin Endo-1,6-α-glucosidase
(Pullulanase)
This enzyme hydrolyzes 1,6-α-D-glucosidic
bonds in polysaccharides, e. g., in amylopectin,
glycogen, and pullulan. Linear amylose fragments
are formed from amylopectin.
4.4.5.2 Pectinolytic Enzymes
Pectins (cf. 4.4.4.13) in plant foods are attacked
by a series of enzymes. A distinction is made between:
• Pectin esterases which occur widely in plants
and microorganisms and demethylate pectin to
pectic acid (Formula 4.166).
• Enzymes which attack the glycosidic
bond in polygalacturonides (Table 4.28).
These include hydrolases and lyases which
catalyze a transelimination reaction (see
Formula 4.167). The double bond formed in
the product of the last mentioned reaction
results in an increase in the absorption at
235 nm.
The second group can be further subdivided according
to the substrate (pectin or pectic acid)
and to the site of attack (endo-/exo-enzyme), as
shown in Table 4.28. The endo-enzymes strongly
depolymerize and rapidly reduce the viscosity of
a pectin solution.
(4.167)
Table 4.28. Enzymes that cleave pectin and pectic acid
Enzyme EC No. Substrate
Polygalacturonase 3.2.1.15
Endo-polymethyl
Pectin
galacturonase
Endo-polygalacturonase
Pectic acid
Exo-polygalacturonase 3.2.1.67
Exo-polymethyl
Pectin
galacturonase
Exo-polygalacturonase
Pectic acid
Pectin lyase 4.2.2.10
Endo-polymethyl
Pectin
galacturonlyase
Pectate lyase 4.2.2.2
Endo-polygalacturonate
Pectic acid
lyase
Exo-polygalacturonate 4.2.2.9 Pectic acid
lyase
Polygalacturonases occur in plants and microorganisms.
They are activated by NaCl and some
by Ca 2+ ions as well.
Pectin and pectate lyases are only produced by
microorganisms. They are activated by Ca 2+ ions
and differ in the pH optimum (pH 8.5–9.5) from
the polygalacturonases (pH 5–6.5).
(4.166)