ApbA, the ketopantoate reductase enzyme of Salmonella typhimurium is required for the synthesis of thiamine via the alternative pyrimidine biosynthetic pathway

The apbA gene of Salmonella typhimurium was shown to encode ketopantoic acid reductase. ApbA was purified from crude cell-free extracts to greater than 95% homogeneity after two chromatographic steps. N-terminal amino acid sequencing (first 15 amino acids) and Western blot analysis confirmed the isolated protein was ApbA. The functional protein was a monomer with a molecular mass of 31.1 kDa. Optimal reaction conditions for the reduction of ketopantoic acid were established at a pH of 6.25, and a temperature of 42 degrees C. The preferred electron source was NADPH, and the apparent K-m constants of the enzyme for NADPH and ketopantoic acid were determined to be 0.776 +/- 0.09 mM and 0.742 +/- 0.01 mM, respectively. The homogeneous enzyme had a specific activity of 64.3.