1. Introduction
Primary
diol is a kind of cheap and convenient substance with a wide range of
supply sources from the derivations of
epoxy alkanes or olefins (Maihom et
al., 2008), aldehydes, alkanes and so on(Mormul et al., 2016; Ichikawa
et al., 1995; Liu et al., 2014). Moreover, these diols are also ideal
raw materials for making the platform chemicals of hydroxyl acid, which
is further converted to various fine chemical intermediates and
precursors of biopolymers due to its dual functional modules of hydroxyl
and carboxyl groups(Wolinsky et al., 2007; Dunn et al., 2016; Kam and
Yoong, 2015). Especially, hydroxyl carboxylic acids are currently in the
spotlight of medical polymers industry due to its excellent
biocompatibility
and biodegradability(Rossi et al., 2004; Jiang et al., 2005; Moore et
al., 2005). Moreover, however, the existing hydroxyl acid production
technology is difficult to guarantee a high yield and
environmental-friendly, either by oxidation or reduction with chemical
catalysts and reagents(Zhao et al., 2015; Xia et al., 2018). Moreover,
taking 3-hydroxypropionic acid (3-HPA) as an example, it was synthesized
from acrolein catalyzed by perchloric acid/sulphur oxide/gaseous
hydrochloric acid under high temperature and pressure(Bhattacharyya and
Das, 1969). Alternatively, it is prepared by
catalyzing
3-hydroxypropanal
or 1,3-propanediol in aqueous solution of alkali metal under high
temperature and
acidic
conditions(Pina et al., 2009). In short, these approaches use some toxic
catalysts and harsh conditions, which do not meet the development needs
of green chemistry. In addition, the preparation of hydroxyl acids from
diols mainly involves the oxidation of hydroxyl groups, which makes it
difficult to ensure that only one hydroxyl group is catalyzed to
carboxyl. For example, 5-hydroxyvaleric acid (5-HVA) which is applied as
an intermediate in drug synthesis has been produced by reducing furan
compounds with metal catalysts with the yield less than 75%(Asano et
al., 2019; Sun et al., 2019). In other words, although the preparation
of hydroxyl acid is a feasible industry, the low yield seriously hinders
the development of hydroxyl acid industrial production. Fortunately, the
whole-cell catalysis of Gluconobacter oxydans presents a great
possibility for hydroxyl acid production in term of high chemical
selectivity, moderate reaction and clean process, compared with chemical
methods(Zhao et al., 2015).
G. oxydans , a representative obligate aerobic and gram-negative
bacterium, had been well-known for its superb applications in oxidations
of oxy-compounds such as alcohols, aldehydes and sugars(Gupta et al.,
2001; Deppenmeier et al., 2002). With the incomplete oxidation platform
of G. oxydans , the dehydrogenation of sorbitol(Wang et al.,
2013), glucose, glycerol, ethylene glycol(Xia et al., 2018),
2-methyl-1,3-propanediol(Pyo et al., 2012), 1,2-propanediol(Liu et al.,
2010) and other alcohols has been realized. This bacterium does not
uptakes or metabolite, but catalyzes the above substrates by the
periplasm
membrane-bound enzymes mainly including alcohol dehydrogenase(Asakura
and Hoshino, 1995), aldehyde dehydrogenase(Molinari et al., 1995),
glycerol dehydrogenase(Richter et
al., 2010) , sorbitol dehydrogenase(Zhou et al., 2019) and releases
products into the solutions directly, which greatly improves the
biocatalysis efficiency and yield(Zhou and XL, 2017). Moreover, because
these membrane-bound enzymes only undergo dehydrogenation without
catabolism, the substrates can be oxidized incompletely to corresponding
acids or ketones by G. oxydans , but not CO2 and
H2O which can effectively ensures the yield and purity
of the products(Hua et al., 2020). In summary, these special
characteristics, coupled with the strict regioselectivity and
stereoselectivity of G. oxydans , have made it possible to prepare
a variety of high value-added products in the industry, and continue to
expand the catalytic application field gradually.
As described above, hydroxyl acid are useful chemicals in many important
fields, and G. oxydans can be employed as a potential hydroxyl
acid producing strain. At present, considering that there are more or
less technological bottlenecks in the industrial preparation of hydroxyl
acids, it is an important opportunity to develop efficient and green
hydroxyl acid production pathway by studying the reaction mechanism ofG. oxydans whole-cell catalysis of primary diol. Hence, in this
study, we investigated the straight-chain primary diols (C2-C6) as
substrates for hydroxyl acids production by G. oxydans . Focus on
the chemical selectivity of two hydroxyl groups in order to find a
regulatory model to achieve the full preparation of hydroxyl acids
(C2-C6), as to provide a common reference for the platform chemical
bio-production of hydroxyl acids from polyols.