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.