The Binding of Calcium and Magnesium to Sarcoplasmic Reticulum Vesicles as Studied by Manganese Electron Paramagnetic Resonance

Abstract

The binding pattern of the biologically relevant ions calcium and magnesium has been investigated via the binding of the ion-analogue manganese. The binding parameters of manganese are obtained conveniently by standard electron paramagnetic resonance techniques, the binding of calcium and magnesium is inferred from competition experiments.The quantitative analysis was carried out with a computer program which was able to treat the competition of three kinds of ions for three classes of independent and one class of cooperative binding sites.It was possible to correlate specific binding classes with biological functions of the sarcoplasmic reticulum membrane. The magnesium and manganese specific binding class of medium affinity is related to the catalytic function of these ions in the ATP-splitting. For the first time a manganese and calcium specific clash of cooperative binding sites has been observed and it can be related to the inhibition of the calcium transport at high concentrations of manganese or calcium.From the binding results a model can be developed which allows a full description of the role of the divalent ions and their specific binding sites during calcium transport.

Effect of Ca2+ on the dimeric structure of scallop sarcoplasmic reticulum.

“…Thus, the low affinity sites detected in addition to the high affinity sites by 45Ca2+ binding to scallop SR do not represent the specific low affinity luminal sites of the Ca 2÷-ATPase protein as such, but are accounted for by the binding of Ca 2÷ to the phospholipid component of the membrane. A similar picture has been suggested for the low affinity binding of Ca 2÷ observed in addition to the high affinity binding with rabbit SR (Kalbitzer et al, 1978).…”

Mechanism of Free Energy Coupling in Active Transport

“…The res ults show unambiguously that the number of binding sites is equal to the number of ions translocated in each reaction cycle (86,87). The res ults show unambiguously that the number of binding sites is equal to the number of ions translocated in each reaction cycle (86,87).…”

Mechanism of Ca2+Transport by Sarcoplasmic Reticulum

“…Four sets of Mg 2+-binding sites were derived from competition by M~+ for the binding of Mn 2+ to SR (266). The association constants (K, in M-I) and the number of sites (n) in each group are as follows: II, K = 2.3 X 10 2, n = 2; 12, K = 6 X 10 3 , n = 1; 13, K = 8.5 X 10 2, n = 23; C, K = 5.8 X 10 2, n = 2.…”