Muscle contraction occurs when thick filaments translocate along adjacent thin filaments by a process analogous to climbing a rope hand over hand. The hands in this instance are the S-1 domains of the myosin head, which ascend via a repeated cycle of attachment, an ATP-powered conformational change or power stroke, and detachment. Although individually minute in terms of both distance covered and the energy unleashed, when multiplied by the 1−2 × 10¹⁸ myosin molecules in a human bicep, both great force and rapid motion can be generated.

A schematic diagram of the cycle of events involved in each power stroke is presented in Figure 1.

Fig1. The hydrolysis of ATP drives the cyclic association and dissociation of actin and myosin in five reactions described in the text.

At rest, the S-1 head of myosin contains bound ADP and P i , remnants of the last prior power stroke. On stimulation by the second messenger Ca2+ (see later), actin becomes accessible to the S-1 head of myosin which finds it and binds it, thereby forming across-bridgelinking the thick and thin filaments. As depicted in Figure 2, the clusters of cross-bridges (abbreviated as arrowheads with the remainder of the thin filaments not shown) at ends of a thick filament have opposite polarities. The two polar regions of the thick filament are separated by a 150-nm segment (the M band, not labeled in the figure) which does not participate in cross-bridge formation.

Fig2. Arrangement of filaments in striated muscle. (A) Extended. The positions of the I, A, and H bands in the extended state are shown. The thin filaments partly overlap the ends of the thick filaments, and the thin filaments are shown anchored in the Z lines (often called Z disks). In the lower part of Figure 2A, “arrowheads,” pointing in opposite directions, are shown emanating from the myosin (thick) filaments. Four actin (thin) filaments are shown attached to two Z lines via α-actinin. The central region of the three myosin filaments, free of arrowheads, is called the M band (not labeled). Cross sections through the M bands, through an area where myosin and actin filaments overlap and through an area in which solely actin filaments are present, are shown. (B) Contracted. The actin filaments are seen to have slipped along the sides of the myosin fibers toward each other. The lengths of the thick filaments (indicated by the A bands) and the thin filaments (distance between Z lines and the adjacent edges of the H bands) have not changed. However, the lengths of the sarcomeres have been reduced (from 2300 to 1500 nm), and the lengths of the H and I bands are also reduced because of the overlap between the thick and thin filaments. These morphologic observations provided part of the basis for the sliding filament model of muscle contraction.

Once the actin:myosin:ADP:Pi cross-bridge complex is formed, the bound Pi is released. This is followed by release of the bound ADP, triggering a large conformational change in the head of myosin in relation to its tail (Figure 3), the power stroke, that pulls the actin about 10 nm toward the center of the sarcomere. This nucleotide-free actin-myosin complex represents its low-energy state.

Fig3. Representation of the active cross-bridges between thick and thin filaments. HE Huxley proposed that the force involved in muscular contraction originates in a tendency for the myosin head (S-1) to rotate relative to the thin filament and is transmitted to the thick filament by the S-2 portion of the myosin molecule acting as an inextensible link. Flexible points at each end of S-2 permit S-1 to rotate and allow for variations in the separation between filaments. This figure is based on HE Huxley’s proposal, and also incorporates elastic (the coils in the S-2 portion) and stepwise shortening elements (depicted here as four sites of interaction between the S-1 portion and the thin filament). The strengths of binding of the attached sites are higher in position 2 than in position 1 and higher in position 3 than position 2. The myosin head can be detached from position 3 with the utilization of a molecule of ATP; this is the predominant process during shortening. The myosin head is seen to vary in its position from about 90° to about 45°, as indicated in the text. (S-1, myosin head; S-2, portion of the myosin molecule; LMM, light meromyosin).

Next, a molecule of ATP binds to the S-1myosin head, which drastically lowers the affinity of the myosin head for actin. Actin is thus released, or detached. Finally, in the relaxation phase of muscle contraction, the S-1 head of myosin hydrolyzes ATP to ADP and Pi , but these products remain bound. The hydrolysis of ATP to ADP-Pi-myosin raises myosin into a so-called high-energy conformation. The myosin:ADP:Pi complex now stands ready to engage in another cycle and move another 10 nm along the thin filaments, provided Ca2+ levels remain elevated.

Calculations have indicated that the thermodynamic efficiency of muscle contraction is about 50%, which compares quite favorably to the 20% or less of the internal combustion engine. It should also be noted that, while hydrolysis of ATP ultimately powers the cycle, the release of ADP provides the immediate driver for the conformational change in S-1 that translates chemical energy into the mechanical power stroke. Rigor mortis, the stiffening of the body that occurs postmortem, occurs because the resulting fall in intracellular ATP levels prevents the dissociation of the myosin S-1 head from actin.

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مواضيع ذات صلة

Cardiac Muscle Resembles Skeletal Muscle in Many Respects

Contraction is Orchestrated by the Second Messenger Ca2+

Cholesterol Side Chain Cleavage Enzyme (p450scc)

Steroidogenic Acute Regulatory Protein (StAR)

Cytochrome P450 Steroid Hydroxylases

Major Protein Components of Muscle Fibers

The Major Components of Cartilage are Type II Collagen & Certain Proteoglycans

Bone is Affected by many Metabolic & Genetic Disorders

The Assembly of Membranes is Complex

Transport Vesicle Formation and Function Involves SNAREs & Other Factors

Misfolded Proteins Undergo Endoplasmic Reticulum Associated Degradation

Proteoglycans & Glycosaminoglycans