The Ala-161→Thr Substitution in Escherichia coliAlkaline Phosphatase Does Not Result in Loss of Enzymatic Activity Although the Homologous Mutation in Humans Causes Hypophosphatasia

The correlation between sequence homology and catalytic importance of a specific amino acid in the E. coliand Liver/Kidney/Bone (L/K/B) human alkaline phosphatase was investigated. For this reason Ala-l6l in the E. colienzyme was substituted with a threonine residue via site-specific mutagenesis. The homologous amino acid in the L/K/B alkaline phosphatase sequence has been shown to cause inactivation of the enzyme. In E. colialkaline phosphatase the Ala-l61→Thr substitution results in a mutant enzyme with virtually unchanged catalytic properties when compared to the wild-type enzyme. Our results show that Ala-161 in the E. colialkaline phosphatase does not have an important catalytic role. The results suggest that the three-dimensional topology of the L/K/B alkaline phosphatase may be different from that observed for the enzyme from E. coli.