Open AccessOcclusion of Divalent Cations in the Phosphorylated Calcium Pump of Sarcoplasmic Reticulum
Author(s) -
DUPONT Yves
Publication year - 1980
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1980.tb04788.x
Subject(s) - divalent , calcium , chemistry , endoplasmic reticulum , binding site , dissociation constant , phosphorylation , dissociation (chemistry) , biophysics , calcium pump , enzyme , calcium atpase , biochemistry , stereochemistry , receptor , organic chemistry , biology , atpase
The calcium pump of sarcoplasmic reticulum possesses high‐affinity calcium‐binding and ATP‐binding sites. At 0°C pH 6.8 and in the absence of calcium, about 3.5 nmol/mg of high‐affinity ATP‐binding sites are titrated with a dissociation constant, K d of 5 μM. In the presence of Ca 2+ , ATP phosphorylates the enzyme at a much lower concentration: K 1/2 = 100 nM. In the absence of ATP the calcium ions reversibly bind to the high‐affinity calcium sites (6.5 nmol/mg); however the following is shown in this paper. 1. Phosphorylation of the enzyme in the presence of calcium leads to the immediate occlusion of the calcium ions bound to the high‐affinity sites. 2. Two moles of calcium are occluded per mole of phosphoenzyme formed. 3. Occlusion can be reversed by ADP. 4. Transport is a slower process which occurs in the presence of Mg 2+ at the same rate as the spontaneous decay of the phosphoenzyme. Experiments performed in the absence of magnesium reveal another divalent cation binding site which is probably directly involved in ATP and P i binding. The nature of the cation bound to this site determines the stability and ADP‐sensitivity of the phosphoenzyme.