Engineering O-Succinyl-L-Homoserine Mercaptotransferase for Efficient L-Methionine Biosynthesis by Fermentation-Enzymatic Coupling Route

L-Methionine is the unique sulfur-containing amino acid essential for humans and animals, which is widely used in pharmaceutical, food and feed industries. Its green and efficient production has attracted a great deal of attention. Fermentation-enzymatic coupling route is regarded to be promising for L-methionine biosynthesis, while O- succinyl-L-homoserine mercaptotransferase (MetZ) is the key biocatalyst. Exploring and engineering of MetZ with ideal catalytic properties is highly desired. Herein, the MetZ from Chromobacterium violaceum ( Cv MetZ) was screened and through in silico analyses, potential beneficial amino acid residues both at the access channel and around the oxygen anion holes were anchored for site-saturation mutagenesis. The positive mutants were obtained with improved activity for L-methionine biosynthesis using O -succinyl-L-homoserine (OSH) and methyl mercaptan as substrate. The best mutant was further applied for L-methionine production and supply of methyl mercaptan was optimized in a fed-batch approach. The conversion of 100 g/L OSH reached 92% with L-methionine yield of 62.6 g/L, which was well coupled with the OSH fermentation level.