Dephosphorylates 2-hydroxy-3-keto-5-methylthiopentenyl-1-phosphate (HK-MTPenyl-1-P) yielding 1,2-dihydroxy-3-keto-5-methylthiopentene (DHK-MTPene). 2-hydroxy-5-methylsulfanyl-3-oxopent-1-enyl phosphate + H2O = 1,2-dihydroxy-5-(methylsulfanyl)pent-1-en-3-one + phosphate Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 4/6. Belongs to the HAD-like hydrolase superfamily. MtnX family.