Actinic activation

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

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. 

Enzymatic activities. The hydrolysis of ATP by actin-activated myosin is the characteristic enzymatic activity of muscle, smooth muscle included. All forms of smooth muscle myosin are slower than those of other muscles. The binding site for ATP and a reduced enzymatic activity are still present in monomeric myosin. The enzymatic activity of monomeric myosin is altered by a conformational change, (the 10S-6S transition) and the species of cations present in the reaction mixture. These differences relate to the possible mechanisms of regulation. 

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 ... 

Eisenberg, E. Moos, C. (1968). Adenosine triphosphate activity of acto-heavy meromyosin A kinetic analysis of actin activation. Biochemistry 7, 1486-1489. 

Effect of protein-bound Ca TpC 4Ca antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) Calmodulin 4Ca activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin-myosin interaction (allows F-actin activation of ATPase). 

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)... 

Matsuura, Y., Stewart, M., Kawamoto, M., Kamiya, N., Saeki, K., Yasunaga, Y., and Wakabayashi, T. (2000). Structural basis for the higher Ca2+ activation of the regulated actin-activated myosin ATPase observed in Dictyostelium/Tetrahymena chimeras./ Mol. Biol. 296, 579-595. 

Photochemical electron-transfer can be effected by irradiation of the charge-transfer absorption band of the electron donor-acceptor complex.15 Alternatively, photochemical electron-transfer may proceed by actinic activation of RH followed by quenching with A, or by the reverse sequence involving activation of A and quenching with RH. 

Photochemical electron transfer proceeds either by foe prior actinic activation of foe organic donor RH followed by quenching by foe electron acceptor (equation 4), or by foe reverse sequence involving foe prior accei r activation and quenching with donor.Photochemical electron transfer can also be ef-... 

It is important to emphasize the anodic, chemical and actinic activations of electron-transfer oxidation to be complementary methods that all commonly involve the reactive intermediates like those presented in equations (la) and (2). As such, cognizance must always be taken of the subtle differences of concentration, temperature, solvent polarity, etc. that affect the behavior of the transient radicals and ion radi-... 

Eisenberg E, Moos C. Actin activation of heavy meromyosin adenosine triphosphatase. Dependence on adenosine triphosphate and actin concentrations. J. Biol. Chem. 1970 245 2451-2456. 

Actin filaments are the tracks for myosin movement. Biochemically, actin activates the ATP hydrolysis of myosin. Recently, it has been shown that myosin motility is activated through actin conformational changes. Singlemolecule FRET from doubly labeled actin monomers in the filament has revealed that actin has multiple conformations and spontaneously changes between them with time (Fig. 12.4a, b) [34]. The multiple conformations are... 

Calponin is another polypeptide monomer (M.W. 32,000) that can inhibit actin-activated myosin ATPase activity. In contrast to CaD, CaP exerts its effect in the absence of tropomyosin and completely inhibits motility in a 2/3 ratio with actin. CaP inhibits myosin binding to actin, but does so by reducing the affinity of actin for myosin rather than competing for the same site. CaP can be phosphorylated by PKC and CaMKII, both of which reverse CaP s inhibitory activity. As with caldesmon, many questions remain. The ratio of CaP to actin actually observed in smooth muscle is in the range 1 10 to 1 16, far from the 2/3 ratio found to produce near-complete inhibition of motility. Therefore, the importance of CaP and its regulation by phosphorylation is still debatable. 

Myosin II consists of two heavy chains and several light chains. Each heavy chain has a head (motor) domain, which Is an actin-activated ATPase a neck domain, which Is associated with light chains and a long rodlike tail domain that organizes the dimeric molecule and binds to thick filaments In muscle cells (see Figure 3-24). 

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