title: Two distinct classes of nucleotide binding sites in sarcoplasmic reticulum Ca-ATPase revealed by 2',3'-O-(2,4,6-trinitrocyclohexadienylidene)-ATP.
abstract: It was previously reported that 2',3'-O-(2,4,6-trinitrocyclohexadienylidene) (TNP)-nucleotides bind with high affinity to the sarcoplasmic reticulum Ca-ATPase (Dupont, Y., Chapron, Y., and Pougeois, R. (1982) Biochem. Biophys. Res. Commun. 106, 1272-1279 and Watanabe, T., and Inesi, G. (1982) J. Biol. Chem. 257, 11510-11516). Here we report a study of the Ca-ATPase nucleotide binding sites using TNP-nucleotides. Competition at equilibrium between TNP-nucleotides and ATP was measured in the absence of calcium; it was found that TNP-nucleotides and ATP competitively bind to two classes of sites of equal concentration (3.5 nmol/mg). The ATP dissociation constants for the two classes of sites were found to be sensitive to H+ and Mg2+ concentrations. In the absence of Mg2+ (independently of pH) or at acid pH (independently of Mg2+ concentration), the nucleotide sites behave like one single family of sites of intermediate affinity (Kd = 20 microM). They split into two classes of sites of high (Kd = 2-4 microM) and low (Kd greater than 1 mM) affinity at pH values higher than neutral and in the presence of Mg2+. The calcium-activated ATP hydrolysis is accelerated by TNP-ATP (or TNP-AMP-PNP) binding on the phosphorylated enzyme. It is concluded 1) that the Ca-ATPase enzyme possesses two classes of ATP binding sites, 2) that the affinity of these two sites and the nature of their interaction is modulated by the H+ and Mg2+ concentrations, and 3) that the hydrolytic activity of the high affinity ATP binding site is activated by ATP or TNP-AMP-PNP (or TNP-ATP) binding in a low affinity ATP binding site.
authors.forename: Y
authors.initials: Y
authors.lastname: Dupont
authors.forename: R
authors.initials: R
authors.lastname: Pougeois
authors.forename: M
authors.initials: M
authors.lastname: Ronjat
authors.forename: S
authors.initials: S
authors.lastname: Verjovsky-Almeida
mesh_terms: D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D005456:Fluorescent Dyes / Q000494:pharmacology*; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D009132:Muscles / Q000201:enzymology; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D005456:Fluorescent Dyes; C045674:2',3'-O-(2,4,6-trinitro-cyclohexadienylidine)adenosine 5'-triphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases