2009_Book_FoodChemistry

208 3 Lipids
Fig. 3.29. Lipoxygenase catalysis
a Proposed mechanism of reaction (according to Veldink, 1977); RH: linoleic acid; LOOH: linoleic acid
hydroperoxide
b Regio- and stereospecificity for linoleic acid oxidation. (1) Lipoxygenase from soybean (LOX 1; cf. Table 3.33);
(2) lipoxygenase from tomato (cf. Table 3.33)
is activated by its product and during activation,
Fe 2+ is oxidized to Fe 3+ . The catalyzed
oxidation pathway is assumed to have the following
reaction steps (cf. Fig. 3.29a): abstraction
of a methylene H-atom from the substrate’s
1,4-pentadiene system and oxidation of the
H-atom to a proton. The pentadienyl radical
bound to the enzyme is then rearranged into
a conjugated diene system, followed by the
uptake of oxygen. The peroxy radical formed is
then reduced by the enzyme and, after attachment
of a proton, the hydroperoxide formed is
released.
In the rate-limiting step of catalysis, the
isoenzyme LOX 1 from soybeans abstracts
the pro-(S)-hydrogen from the n-8 methylene
group a of linoleic acid. Molecular oxygen is
then introduced into the fatty acid present as
a pentadienyl radical from the opposite side at
n-6 with the formation of the 13S-hydroperoxide
(Fig. 3.29b). Another group of LOX, to which
the enzyme from tomatoes belongs, abstracts
the pro-(R)-hydrogen. This results in the formation
of a 9S-hydroperoxide (Fig. 3.29b) if the
oxygen coming from the opposite side docks
onto C-9.
Lipoxygenases from plants mostly exhibit 9- or
13-regiospecificity. A LOX with C-8 specificity
has been found in a mushroom (Table 3.33).
a “n”: the C atoms are counted from the methyl end of
the fatty acid.