Phosphoenzyme decomposition in dog cardiac sarcoplasmic reticulum Ca2+-ATPase.
A five-syringe quench-flow apparatus was used in the transient-state kinetic study of intermediary phosphoenzyme (EP) decomposition in a Triton X-100 purified dog cardiac sarcoplasmic reticulum (SR) Ca2+-ATPase at 20 degrees C. Phosphorylation of the enzyme by ATP in the presence of 100 mM K+ for 116 ms gave 32% ADP-sensitive E1P, 52% ADP- and K+-reactive E2P, and 16% unreactive residual EPr. The EP underwent a monomeric, sequential E1P 17 s-1----E2P 10.5 s-1----E2 + Pi transformation and decomposition in the ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid quenched Ca2+-devoid medium. The calculated rate constant for the total EP (i.e., E1P + E2P) dephosphorylation was 7.8 s-1. The E1P had an affinity for ADP with an apparent Kd congruent to 100 microM. When the EP was formed in the absence of K+ for 116 ms, no appreciable amount of the ADP-sensitive E1P was detected. The EP comprised about 80% ADP- and K+-reactive E2P and 20% residual EPr, suggesting a rapid E1P----E2P transformation. Both the E2P's formed in the presence and absence of K+ decomposed with a rate constant of about 19.5 s-1 in the presence of 80 mM K+ and 2 mM ADP, showing an ADP enhancement of the E2P decomposition. The results demonstrate mechanistic differences in monomeric EP transformation and decomposition between the Triton X-100 purified cardiac SR Ca2+-ATPase and deoxycholate-purified skeletal enzyme [Wang, T. (1986) J. Biol. Chem. 261, 6307-6319].
Wang
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Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Ohio 45267-0575.
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004285:Dogs; D007700:Kinetics; D009206:Myocardium / Q000201:enzymology*; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases