The distinctive PE and PPE families of proteins in Mycobacterium tuberculosis (M.tb), the tuberculosis (TB) causing bacteria, have been associated primarily with antigenicity, immune-modulation and virulence. Earlier, using structure-based sequence annotation, we identified a 225 amino acid conserved PE-PPE domain (Pfam: PF08237) commonly present in some PE and PPE proteins which was observed to comprise alpha/beta-serine hydrolase fold. The prediction was supported by experimental validations of PE16 that was shown to exhibit esterase activity. In this study, we undertook the characterization of the probable operonic ORFs Rv0151c (pe1) and Rv0152c (pe2). Here we demonstrated that pe1 and pe2 are operonic in organization and are co-transcribed. Both PE1 and PE2 proteins possess esterase activity and hydrolyze short to medium chain p-nitrophenyl esters with more specific activity for p-nitrophenyl caproate (C6) with the optimal catalytic conditions of 37-38 degrees C and pH 7.0-8.0. The thermal denaturation temperature of PE1 and PE2 proteins were found to be 50 degrees C. The esterase activity of full length PE1, PE2 and their PE-PPE (alpha/beta-serine hydrolase) domains are similar indicating that the function of PE-PPE domain is independent of the rest of the protein. The esterase activity of these proteins was validated by mutagenesis of the active site Ser; using PE1 Ser246Ala and PE2 Ser163Ala mutants. With these experiments, we conclusively show that the co-transcribed pe1 and pe2 genes code for enzymes belonging to the esterase family of proteins.
