Assay and properties of acetone carboxylase, a novel enzyme involved in acetone-dependent growth and CO2 fixation in Rhodobacter capsulatus and other photosynthetic and denitrifying bacteria

The photosynthetic bacterium Rhodobacter capsulatus is able to grow, in the presence of carbon dioxide, under anaerobic (photosynthetic) conditions with the solvents acetone or butanone as carbon source. The carboxylation of acetone to form acetoacetate is the most likely initial step in acetone metabolism. This paper describes an assay for acetone carboxylation, in which fixation of radiolabelled carbon dioxide from (NaHCO3)-C-14 is measured in the presence of acetone, ATP, magnesium ions and acetyl-CoA. Acetone carboxylase activity was specifically induced by growth of R. capsulatus on acetone or butanone and was associated with a high-molecular-mass protein complex containing two major polypeptides, of 70 and 85 kDa. Partial purification of the activity was achieved by FPLC ion-exchange chromatography, which confirmed that the 70 and 85 kDa proteins were subunits of the enzyme and suggested that at least one additional protein (60 kDa) may be associated with carboxylase activity. N-terminal sequences of the two major subunits were not significantly similar to any other carboxylases in the databases and neither contained covalently bound biotin, indicating that the enzyme represents a novel type of carboxylase. Acetone carboxylase activity was also demonstrated in cell-free extracts of acetone-grown Rhodomicrobium vannielii and the denitrifying bacterium Thiosphaera pantotropha.