Kinetics of steady state ATPase activity and rigor complex formation of acto-heavy meromyosin

Abstract

1. Actin and heavy meromyosin, initially mixed in a Mg-ATP solution, began to form the rigor complex slowly after ATP in the solution had been completely hydrolyzed. 2. This was because the heavy meromyosin-product complex formed via ATP hydrolysis was almost completely dissociated from actin even in the absence of ATP and as soon as this heavy meromyosin-product complex was decomposed, the heavy meromyosin combined with actin forming the rigor complex. 3. Linear plots were obtained when the reciprocal of the excess rate of the actin-accelerated rigor complex formation was plotted against the reciprocal of the added actin concentration as similar with those made on the steady acto-heavy meromyosin ATPase. 4. The V of the rigor complex formation process was about 1/5 of that of the steady acto-heavy meromyosin ATPase activity, showing that the actomyosin ATPase activity could not be explained merely by the actin-accelerated decomposition of the heavy meromyosin-product complex. 5. The same analyses were carried out on myosin subfragment 1. 6. Our results could be explained by considering the two non-identical active sites of myosin, and we propose the following scheme for the actomyosin ATPase. 7. Actin accelerates the rate-limiting bond hydrolysis in the ATPase occurring at one active site of myosin, as well as the rate-limiting decomposition of the heavy meromyosin-product complex formed at another site.