The slow skeletal muscle isoform of myosin shows kinetic features common to smooth and non-muscle myosins.
Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the beta-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actin-myosin dissociates at up to approximately 1000 s(-1), a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (approximately 20 s(-1)) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actin-myosin.ADP and provide evidence of three closely related actin-myosin.ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (approximately 20 s(-1)), and a third complex that releases ADP too slowly (approximately 6 s(-1)) to be on the main ATPase pathway. The role of these actin-myosin.ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.
Iorga
Bogdan
B
Department of Vegetative Physiology, Faculty of Medicine, University of Cologne, Cologne 50931, Germany.
Adamek
Nancy
N
Geeves
Michael A
MA
D000818:Animals; D007700:Kinetics; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000201:enzymology / Q000378:metabolism*; D015879:Myosin Subfragments / Q000378:metabolism*; D024507:Nonmuscle Myosin Type IIA / Q000378:metabolism*; D024501:Nonmuscle Myosin Type IIB / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D011817:Rabbits; D024745:Smooth Muscle Myosins / Q000378:metabolism*
D015879:Myosin Subfragments; D020033:Protein Isoforms; D024507:Nonmuscle Myosin Type IIA; D024501:Nonmuscle Myosin Type IIB; D024745:Smooth Muscle Myosins