Structures of the Ca-ATPase complexes with ATP, AMPPCP and AMPPNP. An FTIR study

We studied binding of ATP and of the ATP analogs adenosine 5′-(β,γ-methylene)triphosphate (AMPCP) and β,γ-imidoadenosine 5′-triphosphate (AMPPNP) to the Ca²⁺-ATPase of the sarcoplasmic reticulum membrane (SERCA1a) with time-resolved infrared spectroscopy. In our experiments, ATP reacted with ATPase which had AMPPCP or AMPPNP bound. These experiments monitored exchange of ATP analog by ATP and phosphorylation to the first phosphoenzyme intermediate Ca₂E1P. These reactions were triggered by the release of ATP from caged ATP. Only small differences in infrared absorption were observed between the ATP complex and the complexes with AMPPCP and AMPPNP indicating that overall the interactions between nucleotide and ATPase are similar and that all complexes adopt a closed conformation. The spectral differences between ATP and AMPPCP complex were more pronounced at high Ca²⁺ concentration (10 mM). They are likely due to a different position of the γ-phosphate which affects the β-sheet in the P domain.