Extremely thermostable orotidine-5′-monophosphate decarboxylase from the archaeon Sulfolobus acidocaldarius

Orotidine-5′-monophosphate decarboxylase (E.C. 4.1.1.23) was purified from Sulfolobus acidocaldarius, one of the archaea that populates geothermal environments. The enzyme was determined to be a dimer of 22-kDa subunits and was readily separated from the orotate phosphoribosyl transferase of S. acidocaldarius. The purified orotidine-5′-monophosphate decarboxylase demonstrated a broad pH optimum centered about pH 8 and an optimal temperature above 98°C. In the absence of substrate and product, however, a considerable thermal inactivation was observed at 85°C or above. To our knowledge, this is the first orotidine-5′-monophosphate decarboxylase of a hyperthermophile or an archaeon to be purified. The S. acidocaldarius enzyme may offer special advantages for the detailed study of enzyme structure-function relationships at extremely high temperatures, due to the fact that either loss or gain of function can be genetically selected in vivo.