Chemical Modification of the Ca2+-dependent ATPase of Sarcoplasmic Reticulum from Skeletal Muscle. I. Binding of N-Ethylmaleimide to Sarcoplasmic Reticulum: Evidence for Sulfhydryl Groups in the Active Site of ATPase and for Conformational Changes Induced by Adenosine Tri- and Diphosphate1

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Abstract
The time course of binding of N-ethylmaleimide (NEM) to the SR was measured at pH 7.5 in the presence or absence of ATP or ADP. The following results were obtained. 1. Both in the presence and absence of nucleotide, the ATPase [EC 3.6.1.3] activity decreased linearly with increase in the amount of NEM bound to the fragmented sarcoplasmic reticulum (SR), and was inhibited almost completely by the binding of 2 moles of NEM per 105 g of the SR protein. 2. The amount of NEM incorporated into the ATPase (M.W. = 105, 000) was measured by SDS disc-gel electrophoresis. It was shown that the ATPase activity was inhibited almost completely by the binding of 2 moles of NEM per mole of ATPase. 3. The rate of binding of NEM to SR decreased by 30–40% in the presence of either ATP or ADP. The concentrations of both ATP and ADP for half-saturation were 0.1–0.2 mM. 4. The effect of nucleotide on the rate of binding of NEM was not changed by the presence of Ca2+ and Mg1+ ions. Similar effects were also observed even when the SR membranes were solubilized with Triton X-100. It is suggested from these results that one or two SH groups are located in the active site of the SR ATPase, and that conformational changes are induced by the addition of ATP and ADP.

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