Chemistry Reference
In-Depth Information
Nevertheless, the enzyme has been commercialized since it was claimed that there was a
positive synergism with glucose oxidase
162
and it has been available for a number of years.
Nowadays it has been withdrawn from the market for baking enzymes.
Amino acid oxidase (
EC 1.10.3.3
) and Dehydroascorbic acid dehydrogenase (
EC 1.8.5.1
)
are both mentioned in relation to the oxidation and reduction of ascorbic acid (AA) in wheat
dough and the concomitant oxidation of glutathion. The improving effect of AA is mediated
through its oxidation in dehydroascorbic acid (DHA). The latter compound is able to oxidize
two thiol groups into a disulphide bridge.
163
Although AA oxidase has been characterized in
wheat flour by many researchers,
164-166
the possible oxidation of AA by other enzymes
167
or by non-enzymatic reactions
165, 168
cannot be excluded.
Reduction of DHA into AA with the concomitant formation of disulphide bridges from
thiol groups is an enzymatic reaction, especially when the thiol groups are from glutathion.
134, 169
Glutathion DHA reductases are present in wheat flour and wheat bran.
170
This enzyme
is specific for glutathion and inactive against cysteine and cysteine-containing peptides.
In spite of the clear effects these enzymes have on the oxidation reduction systems in
wheat dough, none of them has been commercialized.
Concluding remarks on oxidases may be that in spite of extensive research done globally
on oxidases, the commercial success is so far rather limited. One of the reasons that has
frequently been given is the requirement of oxidases for molecular oxygen. Since the amount
of oxygen in a dough is limited and also taken up by yeast, this could explain the limited
success of oxidases. However, incidentally, very positive results have been obtained with
oxidases.
171
This, in combination with the lack of complete mechanistic understanding of
the oxidation processes occurring in dough, leads to the conclusion that positive oxidase
effects must be possible. Most likely the right oxidases still need to be discovered and
commercialized.
6.6
PROTEASES
Proteolytic enzymes, also referred to as proteases, proteinases and peptidases, catalyze the
hydrolysis of peptide bonds in proteins. A wide variety of proteases exist in nature; in plant
material, animal tissue and in many different microorganisms. Commercially, proteases are
by far the largest group of enzymes sold for a wide variety of applications.
Commercial proteases can be of cereal (or other plant materials), animal, fungal or
bacterial origin. In contrast to, for example, amylases, proteases do not differ much in terms
of heat stability. They differ widely, however, in terms of pH dependence and even more in
terms of catalytic specificity.
6.6.1 Classification
Proteases can be divided into two subclasses: endoproteases and exoproteases. The first
group hydrolyzes peptide bonds of proteins in the interior of the polypeptide chain, thereby
generating smaller peptides and sometimes even free amino acids. Endoproteases can be
further subdivided into four groups:
Serine proteases (
EC 3.4.21
...
)
Cysteine proteases (
EC 3.4.22
...
)
Aspartic proteases (
EC 3.4.23
...
)
