2009_Book_FoodChemistry

74 1 Amino Acids, Peptides, Proteins
1.4.5 Enzyme-Catalyzed Reactions
1.4.5.1 Foreword
Fig. 1.39. Lysinoalanine (LAL) formation from wheat
gluten (2) and corn gluten (1). Protein contents of the
glutens: 70%; heated as 6.6% suspension at 100 ◦ Cfor
4 h. (according to Sternberg and Kim, 1977)
A great number and variety of enzyme-catalyzed
reactions are known with protein as a substrate.
These include hydrolytic reactions (cleavage of
peptide bonds or other linkages, e. g., the ester
linkage in a phosphoprotein), transfer reactions
(phosphorylation, incorporation of acyl residues,
sugar residues and methyl groups) and redox
reactions (thiol oxidation, disulfide reduction,
amino group oxidation or incorporation of
hydroxyl groups). Table 1.32 is a compilation
of some examples. Some of these reactions are
covered in Section 1.4.6.3 or in the sections
related to individual foodstuffs. Only enzymes
that are involved in hydrolysis of peptide bonds
(proteolytic enzymes, peptidases) will be covered
in the following sections.
1.4.5.2 Proteolytic Enzymes
Fig. 1.40. Lysinoalanine (LAL) formation as influenced
by casein concentration. (1): 5%, (2): 15%, and (3) 20%
all at pH 12.8. (according to Sternberg and Kim, 1977)
action of phenylalanine with OH-radicals. In
hydrolysates, the compound can be detected with
the help of HPLC (fluorescence detection or
electrochemical detection). It is under discussion
as an indicator for food radiation. The amount
formed depends on the irradiated dose and on
the temperature. In samples of chicken and pork,
fish and shrimps, <0.1mg/kg (non-radiated
controls), 0.5–0.8mg/kg (5 kGy, −18 ◦ C) and
0.8–1.2mg/kg (5 kGy, 20 ◦ C) were found.
Processes involving proteolysis play a role in the
production of many foods. Proteolysis can occur
as a result of proteinases in the food itself, e. g.,
autolytic reactions in meat, or due to microbial
proteinases, e. g., the addition of pure cultures of
selected microorganisms during the production of
cheese.
This large group of enzymes is divided up as
shown in Table 1.33. The two subgroups formed
are: peptidases (exopeptidases) that cleave amino
acids or dipeptides stepwise from the terminal
ends of proteins, and proteinases (endopeptidases)
that hydrolyze the linkages within the
peptide chain, not attacking the terminal peptide
bonds. Further division is possible, for example,
by taking into account the presence of a given
amino acid residue in the active site of the
enzyme. The most important types of proteolytic
enzymes are presented in the following
sections.
1.4.5.2.1 Serine Endopeptidases
Enzymes of this group, in which activity is
confined to the pH range of 7−11, are denoted
as alkaline proteinases. Typical representatives