Carbonyl sulfide: an alternate substrate for but not an activator of ribulose-1,5-bisphosphate carboxylase.

Carbonyl sulfide, a competitive inhibitor of ribulose-bisphosphate carboxylase with respect to CO2 (Laing, W. A., and Christeller, J. T. (1980) Arch. Biochem. Biophys. 202, 592-600), is an alternate substrate. Thiocarboxylation was monitored by mass spectrometry as the stoichiometric consumption of carbonyl sulfide. The product, 1-thio-3-phosphoglycerate, was identified by 13C NMR and UV absorption spectroscopy and measured by enzymic conversion to thiolactate, coupled to the oxidation of NADH. The expected stoichiometry of thiocarboxylation was confirmed. The maximal rates of thiocarboxylation for the spinach and Rhodospirillum rubrum enzymes were close to the maximal rates of carboxylation for these two enzymes. Both enzymes favored CO2 over carbonyl sulfide (with Mg2+ as metal ion) by a factor of about 110. Thiocarboxylation could only be demonstrated with enzymes carbamylated with CO2. Incubation of the carbamylated E.ACO2.Mg complex with excess carbonyl sulfide caused the displacement of the activator carbamate. The thiocarbamylated enzyme was catalytically incompetent and did not form a stable quaternary complex with 2'-carboxyarabinitol bisphosphate. Incubation of the thiocarbamylated enzyme with excess CO2 resulted in the displacement of the thiocarbamate, the re-formation of the carbamylated E.ACO2.Mg complex and the restoration of catalytic competence. Computergraphic simulation of the thiocarbamylated quaternary complex indicated unfavorable van der Waals interactions associated with the thiocarbamate.