Biomedical Engineering Reference
In-Depth Information
Fructose
R=O
GSSG
O
2
, H
2
O
2
NADH+H
+
NADH+H
+
NADH+H
+
NADH+H
+
1
2
3
4
NAD
+
NAD
+
NAD
+
NAD
+
Mannitol
R-OH
2 GSH
H
2
O
2
, H
2
O
Fig. 7.3
Examples of some reactions that allow NADH + H
+
co-factor reoxidation (Adapted from
[
3
] ).
1
Mannitol dehydrogenase,
2
alcohol dehydrogenase,
3
glutathione dehydrogenase,
4
NADH
oxidase.
GSSG
, oxidized glutathione;
GSH
reduced glutathione;
R=O
, aldehyde (e.g., hexanal);
R-OH
, corresponding alcohol (e.g., hexanol)
fermentation quotient (see Chap.
4
) which positively influences the sensory and
shelf-life characteristics of sourdough baked goods [
2
] . Additional ATP is syn-
thesized when acetyl-phosphate is employed for acetate synthesis through ace-
tate kinase (Fig.
7.2
). The synthesis of acetate and ATP as alternative metabolites
from acetyl-phosphate requires the availability of co-substrates to oxidize
NADH that were generated in the upper branch of the 6-PG/PK pathway. When
external acceptors of electrons are available, the recycling of NADH is achieved
without the need to synthesize ethanol from acetyl-phosphate. Most heterofer-
mentative lactic acid bacteria are capable of fructose reduction to mannitol to
achieve co-factor regeneration (Fig.
7.3
). Fructose is quantitatively converted to
mannitol by most heterofermentative lactic acid bacteria under acidic condi-
tions [
3,
14
]. When maltose-negative and maltose-positive sourdough lactic
acid bacteria are associated, fructose conversion may have a further role [
2
] .
Most strains of
Weissella
spp. differ from other heterofermentative lactic acid
bacteria because they do not convert fructose to mannitol with concomitant
acetate formation [
15
]. The activity of the mannitol dehydrogenase of
L. san-
franciscensis
LTH2581 is optimal at 35°C and pH 5.8-8.0 [
16
] . Once synthe-
sized, mannitol could be further used as an energy source by strains of
L. plantarum
. This occurs under anaerobiosis and in the presence of ketoacids
(e.g., pyruvate) as electron acceptors [
17
] .
Oxygen is also used as an external electron acceptor and is reduced to H
2
O with
H
2
O
2
as an intermediate ([
2,
18
] ; Fig.
7.3
). Aerobiosis also induced the expression
of a 12.5-kDa superoxide dismutase (SOD), probably Mn
2+
dependent [
18
] . Overall,
lactic acid bacteria possess various enzymes that are involved in the detoxification
of oxygen radicals. NADH-peroxidases and the system involved in the transport of
L-cysteine are specifically used by sourdough lactobacilli to detoxify H
2
O
2
[
19
] .
The latency phase of growth and cell yield of
L. sanfranciscensis
CB1 are positively
influenced by traces of oxygen and Mn
2+
[
18
]. When aldehydes are available in the
environment,
L. sanfranciscensis
showed R-specific activity by NADH-dependent
alcohol dehydrogenase [
20,
21
] (Fig.
7.3
). For instance, the reduction of hexanal to
hexanol activates the acetate kinase pathway and the synthesis of acetic acid. Also
the reduction of oxidized glutathione (GSSG) into reduced glutathione (GSH), via
glutathione dehydrogenase, protects against oxidative stress, and allows the
synthesis of acetic acid [
22
] (Fig.
7.3
).
