The ADP- and Mg2+-reactive calcium complex of the phosphoenzyme in skeletal sarcoplasmic reticulum Ca2+-ATPase |
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Authors: | Jun Nakamura |
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Affiliation: | Biological Institute, Faculty of Science, Tohoku University, Sendai, Miyagi 980 Japan |
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Abstract: | The effects of ATP on Ca2+ binding in the absence of added Mg2+ to the purified sarcoplasmic reticulum Ca2+-ATPase were studied at pH 7.0 and 0°C. ATP increased the number of Ca2+-binding sites of the enzyme from 2 to 3 mol per mol of phosphorylatable enzyme. The association constant for the ATP-induced Ca2+ binding was 4·105 M?1, which was not significantly different from that obtained in the absence of ATP. AdoP[CH2]PP has little effect on the Ca2+-binding process. The amount of phosphoenzyme formed was equivalent to the level of ATP-induced Ca2+ binding. ADP decreased the level of ATP-induced Ca2+ binding and phosphoenzyme by the same amount. These results suggest that ATP-induced Ca2+ binding exists in the form of an ADP-reactive phosphoenzyme·Ca complex. In addition, the Ca2+ bound to the enzyme in the presence of ATP was released on the addition of 1 mM MgCl2; after the release of Ca2+, the phosphoenzyme decayed. These observations suggest that Mg2+, added after the ATP-induced Ca2+-binding process, may replace the Ca2+ on the phosphoenzyme and initiate phosphoenzyme decomposition. |
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Keywords: | Enzyme phosphorylation (Sarcoplasmic reticulum) EGTA adenosine 5′-[α,β-methylene]triphosphate |
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