Transient-state kinetics of phosphoenzyme transformation in the rabbit skeletal sarcoplasmic reticulum calcium-dependent adenosine triphosphatase reaction. Two distinct modes of ADP and K+ regulation

Abstract

We have observed two modes each of ADP and K+ regulation of phosphoenzyme (EP) intermediates formed in the early phase of skeletal sarcoplasmic reticulum hydrolysis of ATP at 20 degrees C, using, for the first time, a five-syringe quench flow apparatus for transient-state kinetic measurements. The total acid-stable EP formed for 20.5 and 116 ms in the K+ medium appears to be composed of either two monomers in rapid equilibrium, E1P in equilibrium E'1P, or a dimer of the two subunits, PE1E'1P. The ADP-sensitive E1P may form an acid-labile ADP X E1P (or ATP X E1) complex rapidly, giving ATP as a consequence of acid quenching. The ADP may also induce decomposition of the ADP-reactive E'1P. Monomeric and dimeric mechanisms are introduced to account for the hyperbolic relation between the rate constant of the ADP-induced E'1P decomposition and [ADP], consistent with the fact that the E'1P may also give ATP in the presence of ADP. As to the K+ effects, the K+, which is bound to the unphosphorylated enzyme and possibly becomes occluded during EP formation, may either facilitate the one-to-one E1P in equilibrium E'1P equilibrium or maintain the dimeric functional unit. The subsequent forward transformation of the E'1P to the ADP-insensitive K+-sensitive E'2P, possibly the rate-determining step for the catalytic cycle, is found to be K+ independent. The major effect of the K+ in the medium is its catalytic cleavage of the E'2P, which is detected as the missing EP under these conditions. When K+ is not involved in the EP formation, the forward sequential transformation E1P----E'1P----E'2P----E2P or PE1E'1P----PE'2E2P is apparent in the time range from 20.5 to 116 ms after EP formation, and the E'2P may accumulate in the K+ devoid medium and be detected as the major component of the total acid-stable EP. The Mg2+-sensitive E2P represents the EP missing in the medium containing no ADP and K+.