In enzymology, a S-methyl-5-thioribose-1-phosphate isomerase (EC 5.3.1.23) is an enzyme that catalyzes the chemical reaction

S-methyl-5-thio-alpha-D-ribose 1-phosphate S-methyl-5-thio-D-ribulose 1-phosphate

Hence, this enzyme has one substrate, S-methyl-5-thio-alpha-D-ribose 1-phosphate, and one product, S-methyl-5-thio-D-ribulose 1-phosphate.

This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting aldoses and ketoses. The systematic name of this enzyme class is S-methyl-5-thio-alpha-D-ribose-1-phosphate aldose-ketose-isomerase. Other names in common use include methylthioribose 1-phosphate isomerase, 1-PMTR isomerase, 5-methylthio-5-deoxy-D-ribose-1-phosphate ketol-isomerase, S-methyl-5-thio-5-deoxy-D-ribose-1-phosphate ketol-isomerase, S-methyl-5-thio-5-deoxy-D-ribose-1-phosphate, aldose-ketose-isomerase, 1-phospho-5'-S-methylthioribose isomerase, and S-methyl-5-thio-D-ribose-1-phosphate aldose-ketose-isomerase. This enzyme participates in methionine metabolism.

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes 1T9K and 1W2W.

• RH; Savarese, TM; Dexter, DL; Parks Jr, RE; Trackman, PC; Abeles, RH (1984). "Characterization of a defect in the pathway for converting 5'-deoxy-5'-methylthioadenosine to methionine in a subline of a cultured heterogeneous human colon carcinoma". J. Biol. Chem. 259 (11): 6715–9. doi:10.1016/S0021-9258(17)39787-9. PMID 6725268.
• Trackman PC, Abeles RH (1983). "Methionine synthesis from 5'-S-Methylthioadenosine. Resolution of enzyme activities identification of 1-phospho-5-S methylthioribulose". J. Biol. Chem. 258 (11): 6717–20. doi:10.1016/S0021-9258(18)32277-4. PMID 6853500.
• Furfine ES, Abeles RH (1988). "Intermediates in the conversion of 5'-S-methylthioadenosine to methionine in Klebsiella pneumoniae". J. Biol. Chem. 263 (20): 9598–606. doi:10.1016/S0021-9258(19)81558-2. PMID 2838472.