Biomedical Engineering Reference
In-Depth Information
benzaldehyde
3
1
2
phenylalanine
phenylpyruvate
phenyllactate
4
4
5
6
7
phenylacetate
phenylacetaldehyde
phenylethanol
Fig. 7.10
Phenylalanine catabolism in
Lactobacillus plantarum
and
Lactobacillus sanfranciscen-
sis
. The most abundant metabolic products are
underlined
, those responsible for the aroma com-
pounds are in
bold
(Adapted from [
3
] ).
1
Transaminase,
2
dehydrogenase,
3
chemical oxidation,
4
multienzyme complex,
5
decarboxylase,
6
and
7
, dehydrogenase
conversion of amino acids. Glutamate dehydrogenase activity is variable depending
on the biotype of lactic acid bacteria [
73
]. This enzyme is also dependent on NADH
+ H
+
, which links the catabolism of glutamate of
L. sanfranciscensis
DSM20451 to
the regeneration of NADH during the central metabolism of carbohydrates (Fig.
7.9
)
[
74
]. Glutamate decarboxylase (GAD) activity was also found in sourdough lactic
acid bacteria [
75,
76
]. GAD converts L-glutamate to GABA, through a single-step
a-decarboxylation. GABA, a four-carbon nonprotein amino acid, has several
functional activities. Sourdough fermentations with
L. plantarum
, and
Lc. lactis
o r
L. reuteri
allowed the synthesis of GABA in concentrations of up to 9 g/kg, compa-
rable to those found in functional preparations [
75,
76
] . Glutamate decarboxylase-
mediated acid resistance was shown to contribute to the persistence of
L. reuteri
in
type II sourdough fermentations [
64
] .
The catabolism of phenylalanine by
L. sanfranciscensis
and
L. plantarum
is shown
in Fig.
7.10
. Leucine, isoleucine, and valine undergo a similar mechanism. Alternative
pathways for leucine metabolism were proposed but remain to be validated by enzy-
matic and genetic analyses [
77
]. Phenyl-lactate or phenyl-acetate are the main end
products of the catabolism of phenylalanine. For
L. plantarum
TMW1.468, a gluta-
mate-dehydrogenase negative strain, the synthesis of phenyl-lactate is increased
through addition of aKG but not with glutamate and citrate added. Conversely, for
L. sanfranciscensis
DSM20451, a glutamate-dehydrogenase positive strain, the
synthesis of phenyl-lactate is increased through addition of aKG, glutamate and pep-
tides containing glutamate. However, aKG and glutamate favor the conversion of
phenylalanine only when fructose or citrate are also present, probably owing to the
co-factor dependence of glutamate dehydrogenase [
74
] . During sourdough fermenta-
tion,
L. plantarum
TMW1.468 and
L. sanfranciscensis
DSM20451 synthesize ca.
0.35-1.1 mM of phenyl-lactate; the concentration of 1.5 mM inhibits the growth of
L. sanfranciscensis
DSM20451 during growth in mMRS at pH 5.3 [
3
] .
Cystathionine lyase activities are also diffuse within sourdough lactic acid bac-
teria (Fig.
7.11
) [
78
]. A homotetrameric 160-kDa cystathionine g -lyase was puri fi ed
and characterized from
L. reuteri
DSM 20016 [
79
]. The enzyme is active towards a
large number of amino acids and amino acid derivatives, including methionine, and
allows the synthesis of ammonia, a-ketobutyrate, and low-molecular-weight volatile
sulfur compounds.
