4. Conclusions
Present biochemical production by microbial biotransformation has
improved with strain development techniques and process engineering. In
tandem, amino acid-based bioconversion has been used to make
biotransformation more efficient and economical because amino acids such
as l-lysine and l-glutamate can be produced affordably
on a large scale (Georgi, Rittmann, &
Wendisch, 2005; Y.-G. Hong et al., 2018;
H. J. Kim et al., 2015;
H. T. Kim et al., 2019;
J.-H. Kim et al., 2017;
Kimura, 2003;
Shin et al., 2018;
Stansen et al., 2005;
Yang et al., 2019;
Yang et al., 2020). Starting from these
compounds, we can avoid the long, complex pathway from glucose to reach
the final product. In addition, we can avoid individual strain
construction, and we can overcome the low productivity of a precursor or
intermediate from a general strain, without identifying a new pathway.
In practice, high-producing strains are not widely accessible, and there
is a large difference between the high-producing strains and a newly
developed strain. Typically, new pathways or strains will have a lower
titer than expected, and it does not necessarily solve the problems
needed to develop a high-producing strain. As a result, biochemical
production starting from highly accessible chemicals with bioconversion
is in high demand. In addition, we can focus our efforts on essential
processes to improve the pathway and final product. There are several
successful examples of bioconversion including cadaverine production
from l-lysine (Huang et al.,
2020; H. J. Kim et al., 2015;
J.-H. Kim et al., 2016;
J.-H. Kim et al., 2017;
Rui et al., 2020), glutaric acid
production from l-lysine (Y.-G.
Hong et al., 2018; Yang et al., 2019;
Yang et al., 2020), and γ-aminobutyric
acid production from l-glutamate
(Ke et al., 2016;
Park et al., 2013;
Plokhov et al., 2000;
Yuan et al., 2019).
For these reasons, we have continued to develop different bioconversion
systems from readily available chemicals and optimized whole-cell
reactions resulting in the highest titer and increase of productivity.
In addition to changing the starting material, we used easily accessible
tools by comparing accessible plasmids and reinforced whole-cell systems
by incorporating tandem genes. The simplicity of techniques in this
study demonstrates that productivity could be improved in a
straightforward manner. Furthermore, this study details many parameters
to adjust for large production and raises several questions for studies
in the near future.
In our approach, we constructed the l-PA production system with
the reinforced gene and optimized culture conditions including medium
composition, temperature, IPTG concentration, agitation speed, metal ion
and buffer concentration. As a result, we produced 93.5 g/L
l-PA and improved production more than 5-fold over that in
previous studies. Furthermore, productivity from 1 M l-lysine
(monohydrochloride) was 0.78 g/L/h by modifying gene copies and
systematically optimizing reaction parameters. l-lysine
monohydrochloride is much cheaper and more soluble than
l-lysine. Further efforts to stabilize the system for longer
times or to shorten the reaction and studies on l-PA recovery
from reaction solution will continue to improve l-PA
production.
Acknowledgments This study was supported by the National
Research Foundation of Korea (NRF) (NRF-2019R1F1A1058805 and
NRF-2019M3E6A1103979), Research Program to solve social issues of the
National Research Foundation of Korea (NRF) funded by the Ministry of
Science and ICT (2017M3A9E4077234). This work was also supported by R&D
Program of MOTIE/KEIT (10067772) and by R&D Program for Forest Science
Technology (Project No. 2020261C10-2022-AC02) provided by Korea Forest
Service (Korea Forestry Promotion Institute). Consulting service
provided by the Microbial Carbohydrate Resource Bank (MCRB, Seoul,
Korea) is highly appreciated.