Reaction mechanism of calcium-ATPase of sarcoplasmic reticulum. Substrates for phosphorylation reaction and back reaction, and further resolution of phosphorylated intermediates

Reaction of the purified Ca2+-ATPase of sarcoplasmic reticulum at 0 degrees C at low gamma-32P]ATP (0.1 to 0.67 microM) and enzyme (0.025 to 0.24 microM) concentration in the presence of 0.11 to 30 mM Ca2+ without added Mg2+ has resulted in the formation of phosphorylated intermediate (EP:maximal level of EP = 0.45 mol/mol of enzyme) at a very slow rate. Under these conditions, the reaction steps in which EP decomposition takes place are completely prevented. This has permitted us to study the EP formation reaction and its reversal specifically, with a considerably improved time resolution. An apparent rate constant of EP formation (Vf) increases in parallel with the concentration of Ca . ATP, but not with those of Mg . ATP, or of protonated or fully ionized free ATP. This suggests that Ca . ATP is the substrate under these conditions. If Co2+ or Mn2+ are in excess over the other ions during the reaction, Vf varies in parallel with Co . ATP] or Mn . ATP]. Thus, it appears that either Ca2+, Co2+, or Mn2+ can be complexed with ATP to form the effective substrate. An apparent rate constant of the back reaction of EP initiated by addition of ADP to EP (Vr) increases in proportion to ADP] or H . ADP], but is inhibited by increasing concentrations of the ADP complex with Ca2+ or Mg2+, indicating that free ADP or protonated ADP, or both, are actual substrates for the back reaction of EP. These results suggest a new type of site to which the metal moiety of metal . ATP complex remains bound after the release of ADP from the enzyme. An acid-stable phosphorylated intermediate (EP) produced in the presence of high Ca2+ concentrations (e.g. 0.11 mM) without added Mg2+ does not decompose spontaneously, and the major portion (approximately 90%) of this EP (EPD+) reacts with ADP to form ATP (ADP-sensitive). Upon chelating Ca2+ with ethylene glycol bis(beta-amino-ethyl ether)N,N,N',N'-tetraacetic acid (EGTA), EPD+ is converted to another form of EP (EPD-), which is unreactive with ADP (or ADP-insensitive). Addition of Mg2+, after initiation of the reaction leading to EPD- by EGTA, results in rapid production of Pi from a portion of EPD- with KMg approximately equal to 3.3 x 10(3) M-1. The fraction of EPD- that is Mg2+-sensitive (EPD-,M+) increases with reaction time at a much slower rate than the Mg2+-insensitive portion of EPD- (EPD-,M-). These results suggest that the enzyme reaction involves the sequential formation of at least three forms of acid-stable EP, viz. in the order of formation, EPD+, EPD-,M-, and EPD-,M+. The equilibrium between EPD+ and EPD-,M- is shifted by higher K+] and Ca2+] towards EPD+.