ATPase, Actin

The expression of the different actin genes is tissue specific. For example, the expression of y-actin is highest in visceral smooth muscles and the expression of a-actin is highest in vascular smooth muscles (Fatigati and Murphy 1984, Hartshorne 1987). Some smooth muscle tissues contain a mixture of both a and y isoforms others express one or the other exclusively (Hartshorne 1987). The p or "cytoskeletal isoform of actin appears to be a significant component of all smooth muscle tissues. No functional differences among these isoforms or the skeletal muscle isoform have been discerned with respect to the activation of myosin ATPase, actin filament... 

Ishijima A, Kojima H, Funatsu T, Tokunaga M, Higuchi H, Tanaka H and Yanagida T 1998 Simultaneous observation of individual ATPase and mechanical events by a single myosin molcule during interaction with actin Ce//92 161-71... 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

The molecular events of contraction are powered by the ATPase activity of myosin. Much of our present understanding of this reaction and its dependence on actin can be traced to several key discoveries by Albert Szent-Gyorgyi at the University of Szeged in Hungary in the early 1940s. Szent-Gyorgyi showed that solution viscosity is dramatically increased when solutions of myosin and actin are mixed. Increased viscosity is a manifestation of the formation of an actomyosin complex. 

Szent-Gyorgyi further showed that the viscosity of an actomyosin solution was lowered by the addition of ATP, indicating that ATP decreases myosin s affinity for actin. Kinetic studies demonstrated that myosin ATPase activity was increased substantially by actin. (For this reason, Szent-Gyorgyi gave the name actin to the thin filament protein.) The ATPase turnover number of pure myosin is 0.05/sec. In the presence of actin, however, the turnover number increases to about 10/sec, a number more like that of intact muscle fibers. 

The specific effect of actin on myosin ATPase becomes apparent if the product release steps of the reaction are carefully compared. In the absence of actin, the addition of ATP to myosin produces a rapid release of H, one of the products of the ATPase reaction ... 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

The myosins are a superfamily of proteins that have the ability to convert energy released by ATP is hydrolysis into mechanical work. There are many forms of myosin, all of which have ATPase activity and an actin-binding site that is located... 

Myosin as an ATPase Activation of Myosin ATPase by Actin Lymn and Taylor Model 1971 Eisenberg and Hill Model 1985... 

Huxley suggested that crossbridges can move out in this way and bind to actin because S-2 of HMM acted as a flexible link between LMM in the thick filament backbone and S-1. This was based on the observation that heavy meromyosin could be digested by chymotrypsin into two further subffagments (Lowey et al., 1966), S-1 and S-2, as described above, and that S-1 contained the ATPase and actin binding sites, whereas S-2 did not moreover, S-2 did not self-aggregate, as did the rod or LMM portion of myosin. 

The simplest mechanism to explain the much faster rate of dissociation of actomyosin-S-1 by ATP than that of ATP cleavage is that actin activates the myosin ATPase by accelerating the rate at which ADP and Pj are released. That is when ATP is added to actomyosin-S-1, ATP rapidly binds and dissociates actomyosin, myosin ATPase then hydrolyzes ATP to form myosin-ADP.Pj, this state then reattaches to actin and phosphate is released much faster from actomyosin. ADP.Pj than it is from myosin.ADP.Pj, as shown in the scheme below ... 

Heavy meromyosin (HMM molecular mass about 340 kDa) is a soluble protein that has both a fibrous portion and a globular portion (Figure 49-4). It exhibits ATPase activity and binds to F-actin. Digestion of HMM with papain generates two subfragments, S-1 and S-2. The S-2 fragment is fibrous in character, has no ATPase activity, and does not bind to F-actin. 

S-1 (molecular mass approximately 115 kDa) does exhibit ATPase activity, binds L chains, and in the absence of ATP will bind to and decorate actin with arrowheads (Figure 49-5). Both S-1 and HMM exhibit ATPase activity, which is accelerated 100- to 200-fold by complexing with F-actin. As discussed below, F-actin greatly enhances the rate at which myosin ATPase releases its products, ADP and Pj. Thus, although F-actin does not affect the hydrolysis step per se, its ability to promote release of the products produced by the ATPase activity greatly accelerates the overall rate of catalysis. 

Relaxation occurs when sarcoplasmic Ca falls below 10 mol/L owing to its resequestration into the sarcoplasmic reticulum by Ca ATPase. TpC.dCa thus loses its Ca. Consequently, troponin, via interaction with tropomyosin, inhibits further myosin head and F-actin interaction, and in the presence of ATP the myosin head detaches from the F-actin. 

A decrease in the concentration of ATP in the sarcoplasm (eg, by excessive usage during the cycle of con-traction-relaxation or by diminished formation, such as might occur in ischemia) has two major effects (1) The Ca ATPase (Ca + pump) in the sarcoplasmic reticulum ceases to maintain the low concentration of Ca + in the sarcoplasm. Thus, the Interaction of the myosin heads with F-actin is promoted. (2) The ATP-depen-dent detachment of myosin heads from F-actin cannot occur, and rigidity (contracmre) sets in. The condition of rigor mortis, following death, is an extension of these events. 

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. 

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