Abstract
During muscle contraction, myosin heads (M) bound to actin (A) perform power stroke associated with reaction, AMADPPi → AM + ADP + Pi. In this scheme, A • M is believed to be a high-affinity complex after removal of ATP. Biochemical studies on extracted protein samples show that, in the AM complex, actin-binding sites are located at both sides of junctional peptide between 50K and 20K segments of myosin heavy chain. Recently, we found that a monoclonal antibody (IgG) to the junctional peptide had no effect on both in vitro actin-myosin sliding and skinned muscle fiber contraction, though it covers the actin-binding sites on myosin. It follows from this that, during muscle contraction, myosin heads do not pass through the static rigor AM configuration, determined biochemically and electron microscopically using extracted protein samples. To study the nature of AM and AMADP myosin heads, actually existing in muscle, we examined mechanical responses to ramp-shaped releases (0.5% of Lo, complete in 5ms) in single skinned rabbit psoas muscle fibers in high-Ca (pCa, 4) and low-Ca (pCa, >9) rigor states. The fibers exhibited initial elastic tension drop and subsequent small but definite tension recovery to a steady level. The tension recovery was present over many minutes in high-Ca rigor fibers, while it tended to decrease quickly in low-Ca rigor fibers. EDTA (10mM, with MgCl2 removed) had no appreciable effect on the tension recovery in high-Ca rigor fibers, while it completely eliminated the tension recovery in low-Ca rigor fibers. These results suggest that the AMADP myosin heads in rigor muscle have long lifetimes and dynamic properties, which show up as the tension recovery following applied release. Possible AM linkage structure in muscle is discussed in connection with the X-ray diffraction pattern from contracting muscle, which is intermediate between resting and rigor muscles.
Highlights
To study the nature of AM and AMADP myosin heads, existing in muscle, we examined mechanical responses to ramp-shaped releases (0.5% of Lo, complete in 5ms) in single skinned rabbit psoas muscle fibers in highCa and low-Ca rigor states
EDTA (10mM, with MgCl2 removed) had no appreciable effect on the tension recovery in high-Ca rigor fibers, while it completely eliminated the tension recovery in low-Ca rigor fibers. These results suggest that the AMADP myosin heads in rigor muscle have long lifetimes and dynamic properties, which show up as the tension recovery following applied release
Muscle contraction results from relative sliding between actin and myosin filaments, coupled with ATP hydrolysis [1,2], which in turn is produced by attachment-detachment cycle between the myosin head extending from myosin filaments and the sites on actin filaments
Summary
Muscle contraction results from relative sliding between actin and myosin filaments, coupled with ATP hydrolysis [1,2], which in turn is produced by attachment-detachment cycle between the myosin head extending from myosin filaments and the sites on actin filaments. On the basis of actomyosin ATPase reaction steps in solution [3], myosin head (M), in the form of M ADP Pi first attaches to actin (A), and performs a power stroke, associated with reaction, AMADPPi withtin (A), and so that at the end of power stroke, M takes the form AM, i.e. rigor (or rigor-like) configuration. Upon binding with a new ATP, M detaches from A to perform a recovery stroke, associated with reaction, MATP performn head extendintaches to A. In this scheme, it is generally believed that the AM corresponds to a high-affinity complex between actin and myosin head in the absence of ATP, i.e. the AM complex present in rigor muscle. Based on the static rigor AM linkages, it is implicitly believed that, in rigor fibers, tension is only passively maintained
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