Catalyzes the interconversion of methylthioribose-1-phosphate (MTR-1-P) into methylthioribulose-1-phosphate (MTRu-1-P) (PubMed:31950558). Also catalyzes the interconversion of 5-deoxyribose 1-phosphate and 5-deoxyribulose 1-phosphate (PubMed:31950558). Part of a bifunctional DHAP-shunt salvage pathway for SAM by-products (PubMed:31950558). S-methyl-5-thio-alpha-D-ribose 1-phosphate = S-methyl-5-thio-D-ribulose 1-phosphate 5-deoxy-alpha-D-ribose 1-phosphate = 5-deoxy-D-ribulose 1-phosphate kcat is 0.49 sec(-1) with 5-methylthioribose-1-phosphate as substrate. kcat is 2.02 sec(-1) with 5-deoxyribose 1-phosphate as substrate. kcat is 0.67 sec(-1) with ribose-1-phosphate as substrate. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 1/6. Inactivation of the gene precludes growth of the strain with 5-deoxyribose. Belongs to the eIF-2B alpha/beta/delta subunits family. MtnA subfamily.