FTIR studies on the bond properties of the aspartyl phosphate moiety of the Ca2+-ATPase

As part of our work to determine the bond properties of the aspartyl phosphate moiety of the Ca2+-ATPase (SERCA1a) phosphoenzymes, we analyzed Morse potentials of the bridging P-O bond as well as C=O bond strengths for the model compound acetyl phosphate and the two phosphoenzyme intermediates Ca(2)E1P and E2P. Reaction-induced infrared difference spectroscopy was used and a carbonyl band of E2P at 1708 cm(-1) in the presence of MM Mg2+ was tentatively assigned to the carbonyl group of phosphorylated Asp(351) because of its sensitivity to divalent cations. This band is found at 1716 cm(-1) with mM Ca2+, for, Ca2EIP at 1717 cm(-1) with Mg2+, and at 1719 cm(-1) with Ca2+ and at 1718 cm(-1) for acetyl phosphate in the absence of divalent cations. The similar band positions indicate similar strengths of interaction of the carbonyl oxygen in acetyl phosphate and the two phosphoenzymes. Together with information on the P-O bond strengths, this implies that the bridging oxygen exerts stronger interactions in the phosphoenzymes than in acetyl phosphate. (c) 2005 Wiley Periodicals, Inc.