E. Mintz et al., THE MODULATION OF CA2-RETICULUM ATPASE BY ATP ANALOGS IS PH-DEPENDENT( BINDING TO SARCOPLASMIC), The Journal of biological chemistry, 270(45), 1995, pp. 27160-27164
Excess ATP is known to enhance Ca2+-ATPase activity and, among other e
ffects, to accelerate the Ca2+ binding reaction, In previous work, we
studied the pH dependence of this reaction and proposed a 3H(+)/2Ca(2) exchange at the transport sites, in agreement with the H+/Ca2+ count
er transport, Here we studied the effect of ADP and nonhydrolyzable AT
P analogues on the Ca2+ binding reaction at various pH values. At pH 6
, where Ca2+ binding is monophasic and slow, ADP, adenosine 5'-(beta,g
amma-methylene)triphosphate (AMP-PCP), or adenyl-5'-yl imidodiphosphat
e (AMPPNP) increased the Ca2+ binding rate constant 20-fold, At pH 7 a
nd 8, where Ca2+ binding is biphasic, the nucleotides induce fast and
monophasic Ca2+ binding. At pH 7, AMP-PCP accelerated Ca2+ binding wit
h an apparent dissociation constant of 10 mu M. At acidic pH, ADP, AMP
PCP, or AMPPNP increased the equilibrium affinity of Ca2+ for ATPase,
whereas at alkaline pH, these nucleotides had no effect, At pH 5.5, AM
PPCP increased equilibrium Ca2+ binding with an apparent dissociation
constant of 1 mu M.