Tension Generation In Muscle Research Articles

Mechanism of Tension Generation in Muscle: An Analysis of the Forward and Reverse Rate Constants

Tension generation in muscle occurs during the attached phase of the ATP-powered cyclic interaction of myosin heads with thin filaments. The transient nature of tension-generating intermediates and the complexity of the mechanochemical cross-bridge cycle have impeded a quantitative description of tension generation. Recent experiments performed under special conditions yielded a sigmoidal dependence of fiber tension on temperature—a unique case that simplifies the system to a two-state transition. We have applied this two-state analysis to kinetic data obtained from biexponential laser temperature-jump tension transients. Here we present the forward and reverse rate constants for de novo tension generation derived from analysis of the kinetics of the fast laser temperature-jump phase τ 2 (equivalent of the length-jump phase 2 slow). The slow phase τ 3 is temperature-independent indicating coupling to rather than a direct role in, de novo tension generation. Increasing temperature accelerates the forward, and slows the reverse, rate constant for the creation of the tension-generating state. Arrhenius behavior of the forward and anti-Arrhenius behavior of the reverse rate constant is a kinetic signature of multistate multipathway protein-folding reactions. We conclude that locally unfolded tertiary and/or secondary structure of the actomyosin cross-bridge mediates the power stroke.

Kinetic and physical characterization of force generation in muscle: a laser temperature-jump and length-jump study on activated and contracting rigor fibers.

Experiments are presented that probe the mechanism of contraction in normal activated muscle fibers and in heated rigor fibers. In activated fibers we subdivide the partial recovery of isometric tension during the Huxley-Simmons phase 2 into temperature-independent and temperature-dependent steps termed, respectively, phase 2fast and phase 2slow. Evidence is presented to show that phase 2fast arises from the perturbation of a damped elastic element in the cross-bridge and that phase 2slow is the manifestation of an endothermic, order-disorder transition responsible for de novo tension generation. These responses are common to both frog and rabbit fibers. The only difference between animals is that the kinetics of phase 2slow appears to scale with the working temperature of the muscle and not absolute temperature. Rigor fibers heated above the working temperature of the muscle contract. Tension generation is, as with activated fibers, endothermic. Tension transients following a laser temperature-jump of activated and heated rigor fibers are virtually indistinguishable on the basis of either the form or magnitude of the response. In length-jump experiments, tension recovery by heated rigor fibers consists of three exponentials with a tension-dependent rate for the medium speed step. Preliminary data indicate that the rigor cross-bridge operates over a distance of between 13.5 and 18 nm. Collectively, these data imply that tension generation in muscle arises from accessible conformational states in the proteins of the cross-bridge alone. ATP hydrolysis in active fibers and the heating of rigor fibers simply serve to shift these intrinsic conformational equilibria towards tension generation.

Possible role of helix-coil transitions in the microscopic mechanism of muscle contraction

Local helix-coil transitions in the coiled coil portion of myosin have long been implicated as a possible origin of tension generation in muscle. From a statistical mechanical theory of conformational transitions in coiled coils, the free energy required to form a randomly coiled bubble in the hinge region of myosin of the type conjectured by Harrington (Harrington, W. F., 1979, Proc. Natl. Acad. Sci. USA, 76:5066-5070) is estimated to be approximately 25 kcal/mol. Unfortunately this is far more than the free energy available from ATP hydrolysis if the crossbridges operate independently. Thus, in solution such bubbles are predicted to be absent, and the theory requires that the rod portion of myosin be a hingeless, continuously deforming rod. While such bubble formation in vivo cannot be entirely ruled out, it appears to be unlikely. We further conjecture that in solution the swivel located between myosin subfragments 1 and 2 (S-2 and S-1) is due to a locally random conformation of the chains caused by the presence of a proline residue at the point that physically separates the coiled coil from the globular portion of myosin. On attachment of S-1 to actin in the strong binding state, the configurational entropy of the random coil in the swivel region is greatly reduced relative to the case where the ends are free. This produces a spontaneous coil-to-helix transition in the swivel region that causes rotation of S-1 and the translation of actin. Thus, the model predicts that the actin filaments are pushed rather than pulled past the thick filaments by the crossbridges. The specific mechanism of force generation is examined in detail, and a simple statistical mechanical realization of the model is proposed. We find that the model gives a substantial number of qualitative and at times quantitative predictions in accord with experiment, and is particularly appealing in that it provides a simple means of free energy transduction--the well known fact that topological constraints shift the equilibrium between helical and random coil states.