L-Homoserine

Noncanonical D-amino acids are involved in peptidoglycan and biofilm metabolism in bacteria. Previously, we identified amino acid racemases with broad substrate specificity, including YgeA from Escherichia coli, which strongly prefers homoserine as a substrate. In this study, we investigated the functions of this enzyme in vivo. When wild-type and ygeA-deficient E. coli strains were cultured in minimal medium containing D-homoserine, the D-homoserine level was significantly higher in the ygeA-deficient strain than in the wild-type strain, in which it was almost undetectable. Additionally, D-homoserine was detected in YgeA-expressed E. coli cells cultured in minimal medium containing L-homoserine. The growth of the ygeA-deficient strain was significantly impaired in minimal medium with or without supplemental D-homoserine, while L-methionine, L-threonine or L-isoleucine, which are produced via L-homoserine, restored the growth impairment. Furthermore, the wild-type strain formed biofilms significantly more efficiently than the ygeA-deficient strain. Addition of L- or D-homoserine significantly suppressed biofilm formation in the wild-type strain, whereas this addition had no significant effect in the ygeA-deficient strain. Together, these data suggest that YgeA acts as an amino acid racemase and plays a role in L- and D-homoserine metabolism in E. coli.

Background: O-Acetyl-L-homoserine (OAH) is an important potential platform chemical. However, low levels of production of OAH are greatly limiting its industrial application. Furthermore, as a common and safe amino acid-producing strain, Corynebacterium glutamicum has not yet achieved efficient production of OAH. Results: First, exogenous L-homoserine acetyltransferase was introduced into an L-homoserine-producing strain, resulting in the accumulation of 0.98 g/L of OAH. Second, by comparing different acetyl-CoA biosynthesis pathways and adding several feedstocks (acetate, citrate, and pantothenate), the OAH titer increased 2.3-fold to 3.2 g/L. Then, the OAH titer further increased by 62.5% when the expression of L-homoserine dehydrogenase and L-homoserine acetyltransferase was strengthened via strong promoters. Finally, the engineered strain produced 17.4 g/L of OAH in 96 h with acetate as the supplementary feedstock in a 5-L bioreactor. Conclusions: This is the first report on the efficient production of OAH with C. glutamicum as the chassis, which would provide a good foundation for industrial production of OAH.

L-homoserine and its derivatives (O-succinyl-L-homoserine and O-acetyl-L-homoserine) are precursors for the biosynthesis of L-methionine, and various C4 compounds (isobutanol, γ-butyrolactone, 1, 4-butanediol, 2, 4-dihydroxybutyric acid) and L-phosphinothricin. Therefore, the fermentative production of L-homoserine and its derivatives became the research hotspot in recent years. However, the low fermentation yield and conversion rate, and the unclear regulation mechanism for the biosynthesis of L-homoserine and its derivatives, hamper the development of an efficient production process for L-homoserine and its derivatives. This review summarized the advances in the biosynthesis of L-homoserine and its derivatives by metabolic engineering of Escherichia coli from the aspects of substrate uptake, redirection of carbon flow at the key nodes, recycle of NADPH and export of target products. This review may facilitate subsequent metabolic engineering and biotechnological production of L-homoserine and its derivatives.