ATP-induced phosphorylation of the sarcoplasmic reticulum Ca2+ ATPase: Molecular interpretation of infrared difference spectra

Abstract

Time-resolved infrared difference spectra of the ATP-induced phosphorylation of the sarcoplasmic reticulum Ca2+-ATPase have been recorded in H2O and 2H2O at pH 7.0 and 1°C. The reaction was induced by ATP release from P3-1-(2-nitro)phenylethyladenosine 5'-triphosphate (caged ATP) and from [γ-18O3]caged ATP. A band at 1546 cm-1, not observed with the deuterated enzyme, can be assigned to the amide II mode of the protein backbone and indicates that a conformational change associated with ATPase phosphorylation takes place after ATP binding. This is also indicated between 1700 and 1610 cm-1, where bandshifts of up to 10 cm-1 observed upon protein deuteration suggest that amide I modes of the protein backbone dominate the difference spectrum. From the band positions it is deduced that α-helical, β-sheet, and probably β-turn structures are affected in the phosphorylation reaction. Model spectra of acetyl phosphate, acetate, ATP, and ADP suggest the tentative assignment of some of the bands of the phosphorylation spectrum to the molecular groups of ATP and Asp351, which participate directly in the phosphate transfer reaction: a positive band at 1719 cm-1 to the C=O group of aspartyl phosphate, a negative band at 1239 cm-1 to the ν(as)(PO2/-) modes of the bound ATP molecule, and a positive band at 1131 cm-1 to the ν(as)(PO3/2-) mode of the phosphoenzyme phosphate group, the latter assignment being supported by the band's sensitivity toward isotopic substitution in the γ-phosphate of ATP. Band positions and shapes of these bands indicate that the α- and/or β- phosphate(s) of the bound ATP molecule become partly dehydrated when ATP binds to the ATPase, that the phosphoenzyme phosphate group is unprotonated at pH 7.0, and that the C=O group of aspartyl phosphate does not interact with bulk water. The Ca2+ binding sites seem to be largely undisturbed by the phosphorylation reaction, and a functional role of the side chains of Asn, Gln, and Arg residues was not detected.