Interactions of Phosphate Groups of ATP and Aspartyl Phosphate with the Sarcoplasmic Reticulum Ca2+-ATPase: An FTIR Study

Phosphate binding to the sarcoplasmic reticulum Ca2+-ATPase was studied by time-resolved Fourier transform infrared spectroscopy with ATP and isotopically labeled ATP ([β-18O2, βγ-18O]ATP and [γ-18O3]ATP). Isotopic substitution identified several bands that can be assigned to phosphate groups of bound ATP: bands at 1260, 1207, 1145, 1110, and 1085 cm−1 are affected by labeling of the β-phosphate, bands likely near 1154, and 1098–1089 cm−1 are affected by γ-phosphate labeling. The findings indicate that the strength of interactions of β- and γ- phosphate with the protein are similar to those in aqueous solution. Two bands, at 1175 and 1113 cm−1, were identified for the phosphate group of the ADP-sensitive phosphoenzyme Ca2E1P. They indicate terminal and bridging P-O bond strengths that are intermediate between those of ADP-insensitive phosphoenzyme E2P and the model compound acetyl phosphate in water. The bridging bond of Ca2E1P is weaker than for acetyl phosphate, which will facilitate phosphate transfer to ADP, but is stronger than for E2P, which will make the Ca2E1P phosphate less susceptible to attack by water.