Positional isotope exchange analysis of the pantothenate synthetase reaction

Pantothenate synthetase from Mycobacterium tuberculosis catalyzes the formation of pantothenate from ATP, D-pantoate, and β-alanine. The formation of a kinetically competent pantoyl-adenylate intermediate was established by the observation of a positional isotope exchange (PIX) reaction within ¹⁸O-labeled ATP in the presence of D-pantoate. When [βγ-¹⁸O₆]-ATP was incubated with pantothenate synthetase in the presence of D-pantoate, an ¹⁸O label gradually appeared in the αβ-bridge position from both the β- and the γ-nonbridge positions. The rates of these two PIX reactions were followed by ³¹P NMR spectroscopy and found to be identical. These results are consistent with the formation of enzymebound pantoyl-adenylate and pyrophosphate upon the mixing of ATP, D-pantoate, and enzyme. In addition, these results require the complete torsional scrambling of the two phosphoryl groups of the labeled pyrophosphate product. The rate of the PIX reaction increased as the D-pantoate concentration was elevated and then decreased to zero at saturating levels of D-pantoate. These inhibition results support the ordered binding of ATP and D-pantoate to the enzyme active site. The PIX reaction was abolished with the addition of pyrophosphatase; thus, PP_i must be free to dissociate from the active site upon formation of the pantoyladenylate intermediate. The PIX reaction rate diminished when the concentrations of ATP and D-pantoate were held constant and the concentration of the third substrate, β-alanine, was increased. This observation is consistent with a kinetic mechanism that requires the binding of β-alanine after the release of pyrophosphate from the active site of pantothenate synthetase. Positional isotope exchange reactions have therefore demonstrated that pantothenate synthetase catalyzes the formation of a pantoyl-adenylate intermediate upon the ordered addition of ATP and pantoate.