Biomedical Engineering Reference
In-Depth Information
The most important proteolytic enzymes are acidic peptidases. They exist in multiple
forms having pH-optima between 4.2 and 5.5 and include both endo- and exotypes.
On the basis of their catalytic mechanism they can be classified as serine, metallo-,
aspartic, and serine peptidases. According to their biological function to provide the
embryo with amino acids, their activity is highest during the germination of grains.
Other Hydrolyzing Enzymes
Lipases are the most important enzymes that hydrolyze ester bonds. They attack
triacylglycerols yielding mono- and diacylglycerols and free fatty acids. Lipase
activity is important, because free fatty acids are more susceptible to oxidative ran-
cidity than fatty acids bound in triacylglycerols. The activity varies widely among
cereals with oats and millet having the highest activity. Exogenous lipases are in use
to improve the baking performance of wheat flour.
Phytase is an esterase that hydrolyzes phytic acid to inositol and free phosphoric
acid. Even partial hydrolysis of phytic acid by phytase is desirable from a nutritional
point of view, because the strong complexation of cations such as zinc, calcium, and
magnesium ions by phytic acid is significantly reduced.
2.3.4.2
Oxidizing Enzymes
Lipoxygenase is present in high levels in the germ. It catalyzes the peroxidation of
certain polyunsaturated fatty acids by molecular oxygen. Its typical substrate is
linoleic acid containing a methylene-interrupted, doubly unsaturated carbon chain
with double bonds in the
cis
-con fi guration.
Polyphenoloxidases preferably occur in the outer layers of the grains. They catalyze
the oxidation of phenols, such as catechol, pyrogallol, and gallic acid, to quinons by
molecular oxygen. Peroxidase and catalase may be classified as hydroperoxidases
catalyzing the oxidation of a number of aromatic amines and phenols, for example
ferulic acid in arabinoxylans, by hydrogen peroxide. Other oxidizing enzymes are
ascorbic acid oxidase and glutathione dehydrogenase.
2.3.4.3
Enzyme Inhibitors
Many investigators have isolated and characterized enzyme inhibitors from germ
and endosperm. Most important inhibitors are targeted on hydrolyzing enzymes to
prevent the extensive degradation of starch and storage proteins during grain
development and to defend plant tissues from animal (insect) or microbial enzymes.
Predominant classes are amylase and protease inhibitors concentrated in the albu-
min/globulin fractions. Amylase inhibitors can be directed towards both cereal and
noncereal amylases and protease inhibitors towards proteases from both cereals and
