title: Proton transfer in methylmalonyl-CoA epimerase from Propionibacterium shermanii. The reaction of (2R)-methylmalonyl-CoA in tritiated water.
abstract: The reaction catalysed by methylmalonyl-CoA epimerase from Propionibacterium shermanii was studied in tritiated water, in the direction with (2R)-methylmalonyl-CoA as substrate, under 'irreversible' conditions. After partial reaction, even when most of the substrate had been converted into product (isolated as propionyl-CoA) essentially no solvent tritium appeared in residual (2R)-methylmalonyl-CoA. The product, however, did contain tritium, and the specific radioactivity of the (2S)-epimer was deduced to be 0.33 times that of the solvent. These results provide further support for the mechanism proposed for the epimerase-catalysed reaction in the accompanying paper [Leadlay & Fuller (1983) Biochem. J. 213, 635-642], in which two enzyme bases act respectively as proton donor and acceptor. The observed low discrimination against solvent tritium entering the product can be accounted for by a mechanism in which the release of product is slow, and the re-protonation step on the enzyme is reversible, without leading to isotopic exchange with the solvent.
mesh_terms: D000214:Acyl Coenzyme A / Q000031:analogs & derivatives* / Q000378:metabolism; D055598:Chemical Phenomena; D002621:Chemistry; D002848:Chromatography, DEAE-Cellulose; D007535:Isomerases / Q000378:metabolism*; D008316:Malonyl Coenzyme A / Q000031:analogs & derivatives* / Q000378:metabolism; D011424:Propionibacterium / Q000378:metabolism*; D011522:Protons; D019998:Racemases and Epimerases / Q000378:metabolism*; D014316:Tritium; D014867:Water
chemical_list: D000214:Acyl Coenzyme A; D011522:Protons; D014867:Water; D014316:Tritium; C015357:methylmalonyl-coenzyme A; C009061:propionyl-coenzyme A; D008316:Malonyl Coenzyme A; D007535:Isomerases; D019998:Racemases and Epimerases; C021581:methylmalonyl-coenzyme A racemase