Rv2131c gene product : An unconventional enzyme that is both inositol monophosphatase and fructose-1,6-bisphosphatase

Inositol monophosphatase is an enzyme in the biosynthesis of myo-inostiol, a crucial substrate for the synthesis of phosphatidylinositol, which has been demonstrated to be an essential component of mycobacteria. In this study, the Rv2131c gene from Mycobacterium tuberculosis H37Rv was cloned into the pET28a vector and the recombinant plasmid was transformed into Escherichia coli BL21 (DE3) strain, allowing the expression of the enzyme in fusion with a histidine-rich peptide on the N-terminal. The fusion protein was purified from the soluble fraction of the lysed cells under native conditions by immobilized metal affinity chromatography (IMAC). The purified Rv2131c gene product showed inositol monophosphatase activity but with substrate specificity that was broader than those of several bacterial and eukaryotic inositol monophosphatases, and it also acted as fructose-1,6-bisphosphatase. The dimeric enzyme exhibited dual activities of IMPase and FBPase, with K {sub m} of 0.22 {+-} 0.03 mM for inositol-1-phosphate and K {sub m} of 0.45 {+-} 0.05 mM for fructose-1,6-bisphosphatase. To better understand the relationship between the function and structure of the Rv2131c enzyme, we constructed D40N, L71A, and D94N mutants and purified these corresponding proteins. Mutations of D40N and D94N caused the proteins to almost completely lose both the inositol monophosphatase andmore » fructose-1,6-bisphosphatase activities. However, L71A mutant did not cause loss either of the activities, but the activity toward the inositol was 12-fold more resistant to inhibition by lithium (IC{sub 5} {approx} 60 mM). Based on the substrate specificity and presence of conserved sequence motifs of the M. tuberculosis Rv2131c, we proposed that the enzyme belonged to class IV fructose-1,6-bisphosphatase (FBPase IV)« less