Abstract
Glycerol can be converted into more valuable compound dihydroxyacetone by the nicotinamide adenine dinucleotide (NAD+)-dependent glycerol dehydrogenase. However, it is economically prohibitive to produce dihydroxyacetone using purified glycerol dehydrogenase at the expense of a stoichiometric amount of the cofactor NAD+. In this study, Escherichia coli was engineered for dihydroxyacetone production by enhancing its glycerol dehydrogenase activity and introducing NADH oxidase activity. Under optimized conditions, dihydroxyacetone productivity reached 0.13 g/h/g wet cell mass by recombinant E. coli D4 (pET-24b-gldA+nox) cells co-expressinggldA gene from E. coli and nox gene from Enterococcus faecalis. It was interesting to note that exogenous NAD+ greatly improved dihydroxyacetone production for the whole-cell biotransformation process. These results should be useful for the development of advanced bioprocess in terms of glycerol utilization. Key words: Dihydroxyacetone, Glycerol dehydrogenase, NAD+, whole-cell biotransformation, Escherichia coli.
Highlights
As glycerol is a relatively cheap and readily available commodity from the biodiesel industry, it is attractive to convert glycerol into value-added products, such as 1,3propanediol (Willke and Vorlop, 2008), glyceric acid (Habe et al, 2009a), xylulose (Habe et al, 2009b) and dihydroxyacetone (DHA) (Li et al, 2010)
In order to compare the performance between D4 and the strains with enhanced glycerol dehydrogenase (GDH) or NADH oxidase (NOX) activity only, gldA or nox gene was expressed in Bl21(DE3) to form strain D3 and D2, respectively
Two protein bands were evident with a molecular size of approximately 39 and 50 kDa, which were in well agreement with expected molecular weight for GDH and NOX, respectively
Summary
As glycerol is a relatively cheap and readily available commodity from the biodiesel industry, it is attractive to convert glycerol into value-added products, such as 1,3propanediol (Willke and Vorlop, 2008), glyceric acid (Habe et al, 2009a), xylulose (Habe et al, 2009b) and dihydroxyacetone (DHA) (Li et al, 2010). Based on the fact that it is a widely used strain in various biotechnological processes, and can be in high cell density fermentation, it seemed worthwhile testing the possibility for the conversion of glycerol into DHA by E. coli. The NAD+-dependent GDH, encoded by the gldA gene (Accession No CP000948) in E. coli, catalyzes reversible reactions for the interconversion of glycerol and DHA. In the forward reaction, glycerol is oxidized to DHA at the expense of a stoichiometric amount of the cofactor NAD+. Results on engineering of E. coli for DHA production by enhancing its GDH activity and introducing NOX activity were reported. In this system, NAD+ was spontaneously regenerated during DHA production by the action of NOX using air as the terminal oxidant (Figure 1). The effects of initial pH and exogenous NAD+ on DHA production were investigated
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