Enhanced O-succinyl-l-homoserine production by recombinant Escherichia coli ΔIJBB*TrcmetL/pTrc-metAfbr -Trc-thrAfbr -yjeH via multilevel fermentation optimization

AIMS

Constructing a strain with high yield of O-succinyl-l-homoserine (OSH) and improving the titre through multilevel fermentation optimization.

METHODS AND RESULTS

OSH high-yielding strain was first constructed by deleting the thrB gene to block the threonine biosynthesis. Single-factor experiment was carried out, where a Plackett-Burman design was used to screen out three factors (glucose, yeast and threonine) from the original 11 factors that affected the titre of OSH. The Box-Behnken response surface method was used to optimize the fermentation conditions. Through gene editing and medium optimization, the titre of OSH increased from 7·20 to 8·70 g l^-1 in 500 ml flask. Furthermore, the fermentation process and fed-batch fermentation conditions including pH, temperature, feeding strategy and feeding medium were investigated and optimized. Under the optimal conditions, the titre of OSH reached 102·5 g l^-1 , which is 5·6 times higher than before (15·6 g l^-1 ).

CONCLUSIONS

O-succinyl-l-homoserine fermentation process was established and the combination of response surface methodology and metabolic pathway analysis effectively improved the titre of OSH.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, the titre of OSH reached the needs for industrial production and the metabolic pathway of OSH was demonstrated for further optimization.

Metabolic engineering of E. coli for the production of O-succinyl-l-homoserine with high yield

O-succinyl-l-homoserine (OSH) is a promising platform chemical for the production of C4 chemicals with huge market potential which can be produced by fermentation from glucose. To construct a strain capable of producing OSH with high yield, the metJ (encodes transcriptional repressor) and metI (encodes a subunit of dl-methionine transporter) were deleted in Escherichia coli W3110 to obtain a strain E. coli ∆JI. Then, overexpression of metL (encodes bifunctional aspartate kinase/homoserine dehydrogenase II) and inactivation of metB (encodes cystathionine γ-synthase) were implemented in one step, and the OSH titer of the resulting strain E. coli ∆JIB^* TrcmetL was dramatically increased to 7.30 g/L. The feedback regulation was further relieved by progressively overexpressing metA^fbr (encodes homoserine O-succinyltransferase), yjeH (encodes l-methionine exporter), and thrA^fbr (encodes bifunctional aspartate kinase/homoserine dehydrogenase I) to increase the metabolic flux from aspartate to OSH. The 100% rationally designed strain E. coli ∆JIB^* TrcmetL/pTrc-metA^fbr -Trc-thrA^fbr -yjeH produced 9.31 g/L OSH from 20 g/L glucose (0.466 g/g glucose) in batch fermentation, which represents the highest OSH yield from glucose reported to date. The culture profiles of the newly constructed strains were recorded to investigate their productive properties. The effects of l-methionine addition on the fermentation process of the optimal strain were also studied. Our results demonstrate that tuning the expression level of metL, inactivation of metB, and attenuation of feedback resistance of the crucial enzymes in the biosynthetic pathway are the key factors that impact the OSH production in E. coli.

A role for glutamate-333 of Saccharomyces cerevisiae cystathionine γ-lyase as a determinant of specificity

Cystathionine γ-lyase (CGL) catalyzes the hydrolysis of l-cystathionine (l-Cth), producing l-cysteine (l-Cys), α-ketobutyrate and ammonia, in the second step of the reverse transsulfuration pathway, which converts l-homocysteine (l-Hcys) to l-Cys. Site-directed variants substituting residues E48 and E333 with alanine, aspartate and glutamine were characterized to probe the roles of these acidic residues, conserved in fungal and mammalian CGL sequences, in the active-site of CGL from Saccharomyces cerevisiae (yCGL). The pH optimum of variants containing the alanine or glutamine substitutions of E333 is increased by 0.4-1.2 pH units, likely due to repositioning of the cofactor and modification of the pKa of the pyridinium nitrogen. The pH profile of yCGL-E48A/E333A resembles that of Escherichia coli cystathionine β-lyase. The effect of substituting E48, E333 or both residues is the 1.3-3, 26-58 and 124-568-fold reduction, respectively, of the catalytic efficiency of l-Cth hydrolysis. The Km(l-Cth) of E333 substitution variants is increased ~17-fold, while Km(l-OAS) is within 2.5-fold of the wild-type enzyme, indicating that residue E333 interacts with the distal amine moiety of l-Cth, which is not present in the alternative substrate O-acetyl-l-serine. The catalytic efficiency of yCGL for α,γ-elimination of O-succinyl-l-homoserine (kcat/Km(l-OSHS)=7±2), which possesses a distal carboxylate, but lacks an amino group, is 300-fold lower than that of the physiological l-Cth substrate (kcat/Km(l-Cth)=2100±100) and 260-fold higher than that of l-Hcys (kcat/Km(l-Hcys)=0.027±0.005), which lacks both distal polar moieties. The results of this study suggest that the glutamate residue at position 333 is a determinant of specificity.