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

Man Liu ^*, Maria Krasteva  and Andreas Barth 

^* Institut für Biophysik, Johann Wolfgang Goethe-Universität, Frankfurt am Main, Germany; and Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden

Correspondence: Address reprint requests to Andreas Barth, Dept. of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, S-106 91 Stockholm, Sweden. Tel: 46-8-162452; Fax: 46-8-155597; E-mail: Andreas.Barth{at}dbb.su.se.

Phosphate binding to the sarcoplasmic reticulum Ca²⁺-ATPase was studied by time-resolved Fourier transform infrared spectroscopy with ATP and isotopically labeled ATP ([ß-¹⁸O₂, ß-¹⁸O]ATP and [-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 ß-phosphate, bands likely near 1154, and 1098–1089 cm^–1 are affected by -phosphate labeling. The findings indicate that the strength of interactions of ß- and - 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.

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