Protonation of Ca2+ -ATPase Residues upon Ca2+ Release : [Abstracts of papers at the Fifty-Ninth Annual Meeting of the Society of General Physiologists]

Abstract

We have studied protonation of the sarcoplasmic reticulum Ca2 -ATPase during Ca2 release and E2P formation (Ca2E1→E2P) using rapid scan Fourier transform infrared spectroscopy. The reaction has been investigated from pH 6.0 to 9.0. Infrared spectra show four signals in the spectral region of protonated carboxyl groups at pH 6.0–7.5 and only two at pH 8.0–9.0. The results show that at least two of the protonated carboxyl groups of E2P have a pK of 7.7. We have concluded that these carboxyl groups participate in H countertransport. To identify these carboxyl groups and to assign the IR bands, multiconformation continuum electrostatic calculations (MCCE) have been performed to calculate the residues’ ionization, at various pH, for the calcium-free and the calcium-occluded structure, respectively. The combination of infrared measurements and MCCE calculations clearly indicates that Asp800 is involved in the proton countertransport whereas Glu309 is not. The second carboxyl group involved in the countertransport might be Glu908. Our results also indicate a pH-dependent conformational change in a -sheet or turn structure of E2P. Additionally, we have tentatively assigned a band to the C O bond of the phosphorylated Asp 351. Based on infrared data, we concluded that the bond strength is essentially unchanged but is slightly reduced in E2P compared with Ca2E1P. This reduction is larger when Mg2 is bound to the aspartyl phosphate.