Muscle Fibres

Myofibril Contraction

Upon contraction of a myofibril, the "walking" of the myosin heads along the thin filaments means that the overlap of the two types of filament, i.e. the width of the AI band, increases; this is at the expense of the AH zone, so that the total width of AI and AH (i.e. the length of a myosin thick filament) is invariant. As the length of the thin filaments is also constant, the increase in the width of AI is matched by a decrease in that of I.

The Contraction Cycle

In relaxed muscle, the S1 heads of the myosin molecules of the thick filaments are detached from the thin filaments, and orientated perpendicular to them. One molecule of ADP and one Pi (phosphate) group are bound to the myosin head.

The contraction cycle can be described in 4 stages:

Stimulation of the muscle results in the S1-ADP-Pi complexes binding to the adjacent thin filaments, still perpendicular to them.
The interaction between myosin and actin results in the release of Pi, followed by ADP, which induces a conformational change in the myosin molecules: hinge bending tilts the head through approximately 45°. This motion pulls the thin filaments approximately 100Å towards the M line, in a "rowing" action: thus the "power stroke".
Binding of ATP to the S1 heads causes them to detach from the thin filaments, still in the tilted conformation.
The bound ATP is hydrolyzed (see below), returning the S1 head to the former relaxed conformation.

Diagram.

During the contraction of a muscle, this cycle occurs many times as the myosin heads walk along the thin filaments; the length of a contracted muscle may be as little as two-thirds of its fully extended state.

The ATP hydrolysis in Step 4 above is carried out by myosin itself (the globular S1 heads are ATPases; see Page 1.) In solution studies, the turnover number of this reaction is found to be increased (by a factor of 200) by actin, by means of accelerating the release of ADP and Pi from the actomyosin complex (step 2); the hydrolysis step itself is carried out rapidly by myosin alone. Magnesium ions are required for these reactions. Addition of ATP to a solution of the complex is found to decrease the affinity of actin for myosin; this corresponds to step 3.

The role of Troponin and Tropomyosin

Thin filaments consist of actin filaments with one troponin-tropomyosin complex for each 7 actin monomers; refer to the section in the previous chapter .

Actomyosin complexes obtained from purified actin and myosin exhibit contraction upon the addition of ATP.
Actomyosin complexes prepared from muscle tissue (which therefore include tropomyosin and troponin in the thin filaments) do not contract upon the addition of ATP, unless calcium ions are present.

This indicates that the troponin-tropomyosin complex regulates muscle contraction in response to the levels of Ca²+ ions. Only the troponin subunit TnC binds Ca²+.

An allosteric mechanism is believed to regulate the binding of myosin to actin, and thus muscle contraction. In the relaxed state, the tropomyosin molecule binds along the groove in the actin double helix, and blocks the S1-binding sites of the seven actin monomers. Binding of Ca²+ to troponin C causes a conformational change; interaction between troponin and tropomyosin moves the latter approximately 10Å deeper into the groove, exposing the myosin-binding sites. Refer to Zot and Potter (1987).

The Ca²+ ions are delivered from the lumen of the sarcoplasmic reticulum, a network of flattened membrane-bound sacs which surrounds all the myofibrils in a muscle cell; the membrane is made temporarily permeable to Ca²+ ions upon the arrival of a nerve impulse.

References

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J. Walshaw