Enzyme of the methionine cycle that catalyzes the irreversible cleavage of the glycosidic bond in 5'-methylthioadenosine (MTA) and S-adenosylhomocysteine (SAH/AdoHcy) to a lesser extent, to adenine and the corresponding thioribose, 5'-methylthioribose and S-ribosylhomocysteine, respectively (PubMed:18342331, PubMed:19249293). Contributes to the maintenance of AdoMet homeostasis and is required to sustain high rates of ethylene synthesis. H2O + S-methyl-5'-thioadenosine = 5-(methylsulfanyl)-D-ribose + adenine H2O + S-adenosyl-L-homocysteine = adenine + S-(5-deoxy-D-ribos-5-yl)-L-homocysteine 5'-deoxyadenosine + H2O = 5-deoxy-D-ribose + adenine kcat is 2.0 sec(-1) with MTA as substrate. Optimum pH is 6.0. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; S-methyl-5-thio-alpha-D-ribose 1-phosphate from S-methyl-5'-thioadenosine (hydrolase route): step 1/2. Homodimer. No visible phenotype. Belongs to the PNP/UDP phosphorylase family. MtnN subfamily.