A vegetative storage protein (VSP2) from the model plant Arabidopsis thaliana (At) was purified as a wild-type (AtVSP2) and a recombinant protein (Nus-AtVSP2). D119E is an aspartic acid to glutamic acid substitution at amino acid position 119. The phosphatase activity was measured under different pH conditions. Required:
a. What scientific conclusions can be made about the adding the Nus recombinant tag to AtVSP2?
b. Propose a reasonable scientific hypothesis for the observed effect of the D119E mutation on the phosphatase activity.