Phosphoenzyme conversion of the sarcoplasmic reticulum Ca2+-ATPase - Molecular interpretation of infrared difference spectra

Time-resolved Fourier transform infrared difference spectra of the phosphoe nzyme conversion and Ca2+ release reaction (Ca2E1-P --> E-2-P) of the sarco plasmic reticulum Ca2+-ATPase were recorded at pH 7 and 1 degrees C in H2O and (H2O)-H-2. In the amide I spectral region, the spectra indicate backbon e conformational changes preserving conformational changes of the preceding phosphorylation step. beta-sheet or turn structures (band at 1685 cm(-1)) and alpha-helical structures (band at 1653 cm(-1)) seem to be involved. Spe ctra of the model compound EDTA for Ca2+ chelation indicate the assignment of bands at 1570, 1554, 1411 and 1399 cm(-1) to Ca2+ chelating Asp and Glu carboxylate groups partially shielded from the aqueous environment. In addi tion, an E-2-P band at 1638 cm(-1) has been tentatively assigned to a carbo xylate group in a special environment. A Tyr residue seems to be involved i n the reaction (band at 1517 cm(-1) in H2O and 1515 cm(-1) in (H2O)-H-2). A band at 1192 cm(-1) was shown by isotopic replacement in the gamma-phospha te of ATP to originate from the E-2-P phosphate group. This is a clear indi cation that the immediate environment of the phosphoenzyme phosphate group changes in the conversion reaction, altering phosphate geometry and/or elec tron distribution.