Catalyzes the enolization of 2,3-diketo-5-methylthiopentyl-1-phosphate (DK-MTP-1-P) into 2-hydroxy-3-keto-5-methylthiopentenyl-1-phosphate (HK-MTPenyl-1-P). 5-methylsulfanyl-2,3-dioxopentyl phosphate = 2-hydroxy-5-methylsulfanyl-3-oxopent-1-enyl phosphate Binds 1 Mg(2+) ion per subunit. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 3/6. Homodimer. Has no RuBP-carboxylation activity. Belongs to the RuBisCO large chain family. Type IV subfamily. Although this resembles mtnW of B.subtilis, its genomic context is different, it is too short and it is missing a Mg-binding residue. Thus it may not be a functional ortholog.