Distinction of the roles of the two high-affinity calcium sites in the functional activities of the Ca2+-ATPase of sarcoplasmic reticulum.

The effects of trypsin digestion and low temperature on Ca2+ binding and on Ca2+ activation of ATP hydrolysis by the high-affinity transport sites of the Ca2+, Mg2+-ATPase of sarcoplasmic reticulum were examined. Sarcoplasmic reticulum vesicles contain 0.7-1.1 high-affinity Ca2+ sites per 10(5) g sarcoplasmic reticulum with K = 3-5 X 10(5) M-1, as well as sites of lower affinity. The first cleavage of the ATPase with trypsin (TD1) has no effect on the binding properties of the high affinity sites. The second tryptic cleavage (TD2) decreases the affinity of the high sites to K = 3 X 10(4) M-1 with conservation of the total number of sites. The purified ATPase contains 1.6-2.0 high affinity Ca2+ sites per 10(5) g protein when measured at 23 degrees C, while at 0-4 degrees C there is approximately equal to 1 high-affinity (K = 5 - 10 X 10(5) M-1) affinity site and approximately equal to 1 intermediate-affinity (K = 3 X 10(4) M-1) site per 10(5) g. Trypsin digestion to the point of TD1 has no effect on either the number or the binding constants of the high-affinity sites. Upon TD2 cleavage, one of the sites is converted to the intermediate-affinity state, while the other remains at high affinity. After TD2 modification of the enzyme both of the sites are in the intermediate affinity state at 4 degrees C. On the basis of the binding data, several models for the roles of the Ca2+ sites in the activation of ATP hydrolysis are derived. The results are summarized by a scheme in which the two high-affinity Ca2+ sites are heterogeneous with respect to sensitivity to temperature and to TD2 modification. The results of this and a previous study [Scott, T. L. and Shamoo, A. E. (1982) J. Membr. Biol. 64, 137-144] indicate that while occupation of either of the two Ca2+ sites can stimulate ATP hydrolysis, the site which is sensitive to TD2 is essential for the coupling of hydrolysis to Ca2+ transport.