Actin ATPase activity

ADP-ribosylation completely blocks the actin ATPase activity and increases the rate of ATP exchange by about twofold (Geipel ef al., 1990 Geipel ef al., 1989). This effect is not due to inhibition of polymerization, because the basal ATPase activity of G-actin is also inhibited. Moreover, the ATPase activity of actin is blocked even in the quasi-monomeric actin-DNAse I complex after stimulation with the mycotoxin cytochalasin (Geipel ef al., 1990). Thus, by analogy with the ADP-ribosylation of G-proteins by cholera toxin, which inhibits G-protein-associated GTP hydrolysis, the ADP-ribosylation of actin inhibits its intrinsic ATPase activity. 

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. 

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... 

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. 

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. 

Inhibitor of F-actin-myosin interaction (inhibitor of F-actin-dependent activation of ATPase) Troponin system (Tpl) Unphosphorylated myosin light chain... 

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly. 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

One possesses the ATPase activity whereas the other possesses the actin-binding site. 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

This ATPase activity [EC 3.6.1.32] is directly responsible for muscle contraction. In the absence of actin filaments, myosin is a feeble ATPase with a kcat of only 0.05 s because product release is much slower than the rapid release of a proton in the P—O—P bond-cleavage step forming ADP and Pi from bound ATP. Interaction with... 

Ngai, P.K. Walsh, M.P. Inhibition of smooth muscle actin-activated myosin Mg -ATPase activity by caldesmon. J. Biol. Chem., 259, 13656-13659 (1984)... 

Maruta, H. Korn, E.D. Acanthamoeba cofactor protein is a heavy chain kinase required for actin activation of the Mg -ATPase activity of Acanthamoeba myosin I. J. Biol. Chem., 252, 8329-8332 (1977)... 

That actin and myosin are jointly responsible for contraction was demonstrated long before the fine structure of the myofibril became known. In about 1929, ATP was recognized as the energy source for muscle contraction, but it was not until 10 years later that Engelhardt and Ljubimowa showed that isolated myosin preparations catalyzed the hydrolysis of ATP.138 Szent-Gyorgi139 140 showed that a combination of the two proteins actin (discovered by F. Straub141) and myosin was required for Mg2+-stimulated ATP hydrolysis (ATPase activity). He called this combination actomyosin. 

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