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Abstract:
Phosphoserine phosphatase (PSP) utilizes one Mg2+ ion to catalyze the hydrolysis of phospho-l-serine. The displacement of Mg2+ by Ca2+ results in the loss of activity. The reaction mechanisms for the enzyme with both Mg2+ and Ca2+ bound were investigated using hybrid density functional theory. A large quantum chemical model abstracted from the X-ray crystal structure was employed in the calculations. Our calculations shed new insight into the catalytic mechanism of the natural enzyme and its lack of activity by Ca2+ substitution. For the catalytic reaction, our calculations showed that the whole reaction proceeds through two steps, namely dephosphorylation and phosphate hydrolysis. The associated barriers for these two steps are calculated to be 11.9 and 12.0 kcal mol−1, respectively. The Mg-bound Asp11 residue functions as a nucleophile to attack the phosphorus moiety, in concomitant with the departure of the leaving group, which takes a proton from the neutral Asp13 residue. In the subsequent step, the newly formed anionic Asp13 residue activates a water molecule to perform the reverse attack on the phosphoryl intermediate, affording the phosphate product. The substitution of Mg2+ by Ca2+ results in different metal coordination fashion, in which the Asp167 residue changes from bidentate to monodentate and a second water molecule becomes ligated to Ca2+. The calculated barriers for the hydrolysis are ca 8 kcal mol−1 higher than those in the native enzyme, which reconciles with the fact that Ca2+ inhibits the activity of PSP. Several possible reasons are discussed.
