Biomedical Engineering Reference
In-Depth Information
natto being responsible for its synthesis. Ever since Bovarnick [6] reported the free secretion
of γ-PGA in the growth medium of
B. subtilis
, several researchers have explored the
extracellular synthesis of γ-PGA from a variety of
Bacillus
species [5]. Although most studies
on γ-PGA was carried out between 1950s and 1970s [7-13], an increasing attention was
drawn in recent years mainly because of its biodegradable and biocompatible properties.
The γ-PGA is primarily composed of repetitive glutamic acid monomer units, which are
connected by γ-amide linkages between α-amino and γ-carboxyl groups and are thus
synthesized in a ribosome-independent manner [5,14,15] (Figure 1). The naturally-produced
γ-PGA contains nearly equal proportion of D- and L-glutamic acid units, yet the ratio of two
optical isomers can be partially controlled by technological methods, yielding γ-PGA with
different degree of stereoselectivity [5,14-16]. In addition, γ-PGA can be synthesized in
different salt forms (Na, K, Ca, Mg and NH
4
) and varying molecular weights (10,000 to 2
million Daltons) [5,14,15]. Owing to its non-toxic, polyanionic and multifunctional
characteristics, γ-PGA finds potential application in a wide range of fields such as food,
agriculture, cosmetics, medicine and environment [5,14,15]. Some specific applications of γ-
PGA include its use as a health food, thickener, humectant, bitterness-relieving agent,
osteoporosis-preventing agent, cryoprotectant, drug carrier, sustained release material, curable
biological adhesive, biodegradable fibers, hydrogel (super water absorbent), moisturizer,
tissue engineering material, biodegradable packing material, dispersant, flocculant, adsorbent
of toxic cations and chemical mutagens, animal feed additive, enzyme immobilizing material,
liquid display and conductive display materials [17-32]. This topic chapter intends to review
the reported studies on the application of γ-PGA as a bioflocculant, and an adsorbent for
scavenging cationic dyes and chemical mutagens.
2. Biosynthesis and Physico-chemical Properties of
γ
-PGA
2.1. Biosynthesis
Several
Bacillus
species produce γ-PGA as a capsular component or an extracellular
viscous material outside the cell body for subsequent release into the fermentation broth. To
enhance the productivity of γ-PGA, researchers have investigated the nutrient requirements
[5], which varied according to the type of
Bacillus
strain. Based on the nutrient requirements,
γ-PGA-producing bacteria are divided into two groups: one requires the addition of L-
glutamic acid (
de nova
method) to the medium to stimulate both cell growth and γ-PGA
synthesis, while the other does not (salvage bioconversion method) [5,33-35]. Table 1
summarizes the nutrient requirements, cultivation condition, production yield, D/L glutamic
acid ratio and molecular weight of γ-PGA synthesized from various
Bacillus
strains [7-9,36-
40]. Besides carbon and nitrogen sources, factors such as ionic strength, aeration and medium
pH, all of which can affect the productivity, molecular weight, stereochemical composition
and quality of γ-PGA [5,41]. For instance, the molecular weight of γ-PGA synthesized from
B. licheniformis
ATCC 9945A increased by a factor of approximately 1.8 (1.2 to 2.2 million
g/mol) for a concentration rise of NaCl from 0 to 4% [5,42]. The schematic pathway proposed
for synthesis of γ-PGA from
B. subtilis
IFO 3335 is depicted in Figure 2 [42]. A more
