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.