Cross-bridges with myosin

Thus, in the cross-bridge cycle, myosin is bound with high affinity alternately to actin and to ATP. Since the energy changes associated with myosin binding to actin and MgATP are internal to the system, the only free energy... 

As the name implies, smooth muscle lacks the highly ordered sarcomere structure of striated muscle, having thick and thin filaments in less orderly arrays with relatively less myosin (one fifth as much) than in striated muscle. Smooth muscle thin filaments have tropomyosin but generally lack troponin. Myosin in smooth muscle is found in monomeric form as well as small thick filaments, and phosphorylation is almost essential for condensation of monomeric myosin into filaments. Thus, the amount of myosin available to cross-bridge with actin may be physiologically adjustable. Like other myosin II types, smooth muscle myosin is a hexamer, and several isoforms of the heavy chains and both light chains are known. The SM-1 isoform (M.W. 204,000) has an unusually long COOH-... 

Figure 8.56. Stereo view in space-filling representation of a scallop cross-bridge (SI) view of the actin binding site on myosin cross-bridge, with continuity of the cleft from the ADP-BeF, binding site to the actin binding site that dissociates on ATP binding (A) Near-rigor state without nucleotide in the binding...

Within each sarcomere the relative sliding of thick and thin filaments is brought about by "cross-bridges," parts of the myosin molecules that stick out from the myosin filaments and interact cyclically with the thin actin filaments, transporting them hy a kind of rowing action. During this process, the hydrolysis of ATP to ADP and phosphate couples the conformational... 

Figure 14.17 A sequence of events combining the swinging cross-bridge model of actin and myosin filament sliding with structural data of myosin with and without bound nucleotides.

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

Figure 13.5 Electron micrograph of part of a longitudinal section of a myofibril. Identification of components and the mechanism of contraction. When a muscle fibre is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no change in length of either myofilament. The thick myosin strands in the A band are relatively stationary, whereas the thin actin filaments, which are attached to the Z discs, extend further into the A band and may eventually obliterate the H band. Because the thin filaments are attached to Z discs, the discs are drawn toward each other, so that the sarcomeres, the distance between the adjacent Z-discs, are compressed, the myofibril is shortened, and contraction of the muscle occurs. Contraction, therefore, is not due to a shortening of either the actin or the myosin filaments but is due to an increase in the overlap between the filaments. The force is generated by millions of cross-bridges interacting with actin filaments (Fig. 13.6). The electron micrograph was kindly provided by Professor D.S. Smith.

It is troponin that responds to the increased Ca centration. The Ca ions bind to Tn-C, which then binds to Tn-I and causes a conformational change in Tn-T. This results in a conformational change in tropomyosin that exposes sites on the actin filament for binding with the myosin head of the cross-bridge. The process can be summarised as follows (see also Figure 13.16) ... 

At the ultrastructural level, flatworm muscle resembles smooth muscle with individual, non-striated myofibrils being delimited by the sarcolemma and interconnected by gap junctions. Also, flatworm muscles lack a T-tubule system that is characteristic of striated muscle in other animal groups. The contractile portion of flatworm myofibrils contains thick myosin and thin actin filaments that connect with the sarcolemma via attachment plaques or desmo-somes. Actomyosin cross-bridges have been reported and where overlap has been observed, ratios that vary from 9 1 to 12 1 have been observed. Although flatworm muscle is mostly non-striated, pseudo-striated (e.g. in the tail of schistosome cercariae Dorsey et al., 2002 Mair et al., 2003) and obliquely striated (e.g. tentacular bulb of the trypanorhynch, Crillotia eri-naceus Ward et al., 1986) muscles have been reported. It is presumed that the role played by these structures has demanded the development... 

FIGURE 13-3 Possible mechanism of action of dantrolene sodium (DantriurrQ. Dantrolene blocks channels in the sarcoplasmic reticulum, thus interfering with calcium release onto the contractile [actin, myosin] filaments. Muscle contraction is reduced because less calcium is available to initiate cross-bridge formation between actin and myosin filaments. 

Dynein is a very large protein of 1000-2000 kDa consisting of one, two or three heads depending on the source. Like the heads of myosin (see Topic Nl), the heads of dynein form cross-bridges, in this case with the B subfibers, and... 

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