Biomedical Engineering Reference
In-Depth Information
-sucrose
- fructose, glucose
- maltose
- other acceptor sugars
(pentoses)
Enzyme properties:
-
pH and temperature optimum
-
polymer length, linkage type,
and degree of branching
Strain properties:
-
general growth parameters
-
hydrolysis to transferase ratio
-
glucansucrase /
fructansucrase expression
Substrate:
-
-
formation of lactate and
acetate (utilization of
fructose as electron
acceptor)
buffering capacity (pH profile of
fermentation)
-
concentration of acceptor
carbohydrates and release during
fermentation.
III Hydrolysis
-
other metabolic properties
relevant for bread quality
-
presence of other structure forming
polymers (e.g. pentosans, gluten)
II Oligosaccharide
formation
Process conditions:
-
lactate,
acetate,
ethanol, CO
2
(mannitol)
time, temperature of fermentation
-
Level and type of sucrose addition
(batch / fed-batch)
I Exopolysaccharide formation
Influence on bread quality: texture, shelf life, and content of dietary fibre
Fig. 7.12
Factors influencing yield of exopolysaccharides and the influence of exopolysaccha-
rides on bread quality. Glucansucrases catalyze three alternative reactions: (I) polymerization of
glucose or fructose to exopolysaccharides, (II) glycosyl transfer to acceptor carbohydrates to form
oligosaccharides, (III) sucrose hydrolysis to glucose and fructose. EPS properties and yield are
additionally dependent on the concentration of substrate-derived acceptor carbohydrates, and the
ambient pH and temperature
phosphor-glycosyltransferases or glycosyltransferases (EpsE, EpsF, EpsG, and EpsI
in
S. thermophilus
Sfi6). EPS is synthesized by glycosyltransferases with sugar
nucleotides derived from glucose-6-phosphate as substrates. The repeating unit of
the polysaccharide, consisting of two to eight monosaccharides, is assembled by
sequential addition of monosaccharides to the membrane-bound lipid carrier undec-
adeprenyl pyrophosphate. The repeating unit is exported and polymerized extracel-
lularly. HePS biosynthesis requires energy-rich sugar nucleotides as precursors, and
the assembly and export of the repeating units competes with the peptidoglycan
biosynthesis; therefore, HePS yields are relatively low, typically well below 1 g/L.
HoPS synthesis is mediated by extracellular cell-wall associated or soluble glu-
cansucrases or fructansucrases [
80,
81
]. Both groups of enzymes are classified as
retaining glycosyl hydrolases and use sucrose as substrate. Fructansucrases but not
glucansucrases also employ raffinose, stachyose, or verbascose as substrate [
40
] .
Catalysis by glucansucrases and fructansucrases proceeds through a covalently
linked glucosyl-enzyme or fructosyl-enzyme intermediate, respectively, with subse-
quent transfer of the glycosyl moiety to a growing polymer chain, a suitable acceptor
carbohydrate, or water (Fig.
7.12
). Sucrose hydrolysis or oligosaccharide formation
are thus alternative reactions to polysaccharide synthesis [
80,
95-
97
] . The ratio of
sucrose hydrolysis to oligo- or polysaccharide synthesis depends on the enzyme
structure as well as the substrate concentrations [
98,
99
]. Because the catalytic mech-
anism retains the chemical energy of the glycosidic bond of the substrate, HoPS
synthesis does not require energy-rich substrates or co-factors. Several recent reviews
