Phosphorylation of the sarcoplasmic calcium-activated adenosinetriphosphatase as studied by 31P nuclear magnetic resonance

A reinvestigation of a study in which the 31P nuclear magnetic resonance (NMR) signal of the phosphointermediate of the sarcoplasmic (Ca2+, Mg2+)-ATPase has been identified shows that the signal described most probably originates from free Mg x ATP but not from the phosphoenzyme itself. It was possible to detect the 31P NMR signal of the phosphoenzyme in peptic fragments of sarcoplasmic ATPase phosphorylated either by ATP or by inorganic phosphate. The two products exhibit the same spectral characteristics in 31P NMR, implying that most probably both reaction pathways yield the same chemical product. Chemical shifts at low pH (-6.5 ppm) and high pH (-1.4 ppm) of the phosphoryl group are indicative of a β-phosphoaspartyl moiety, thus confirming independently the results from chemical analysis. The relatively low pK value of 4.3 of the phosphoryl group suggests an interaction with a positively charged group of the enzyme