Phosphorylation of the sarcoplasmic reticulum Ca2+-ATPase from ATP and ATP analogs studied by infrared spectroscopy

Abstract

Phosphorylation of the sarcoplasmic reticulum Ca2+-ATPase (SERCA1a) was studied with time-resolved Fourier transform infrared spectroscopy. ATP and ATP analogs (ITP, 2′ and 3′-dATP) were used to study the effect of the adenine ring and the ribose hydroxyl groups on ATPase phosphorylation. All modifications of ATP altered conformational changes and phosphorylation kinetics. The differences compared with ATP increased in the following order: 3prime;-dATP > ITP > 2′-dATP. Enzyme phosphorylation with ITP results in larger absorbance changes in the amide I region, indicating larger conformational changes of the Ca2+-ATPase. The respective absorbance changes obtained with 3′-dATP are significantly different from the others with different band positions and amplitudes in the amide I region, indicating different conformational changes of the protein backbone. ATPase phosphorylation with 3′-dATP is also much (∼30 times) slower than with ATP. Our results indicate that modifications to functional groups of ATP (the ribose 2′- and 3′-OH and the amino group in the adenine ring) affect γ-phosphate transfer to the phosphorylation site of the Ca2+-ATPase by changing the extent of conformational change and the phosphorylation rate. ADP binding to the ADP-sensitive phosphoenzyme (Ca2E1P) stabilizes the closed conformation of Ca2E1P.