Myosin species differences

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. 

Fig. 27. Radial net of various crossbridge lattices from different species along with their corresponding computed Fourier transform. The myosin filament is three-stranded in (A) vertebrate muscle, four-stranded in invertebrates (B and C), and seven-stranded in scallop muscle (D). This figure shows the similarity of the surface lattices of the myosin head origins on the myosin filaments in different muscles although the myosin heads have different slew, tilt, and rotations. Images were created using the program HELIX (Knupp and Squire, 2004).

Fig. 30. Comparison of the X-ray-modeled myosin head arrays in relaxed fish muscle (A) and relaxed insect flight muscle (B), with the motor (catalytic) domain of outer myosin heads in each model circled to show the close similarity of their configurations in the two different species. The M-band is at the bottom in both models.

G-actin is very highly conserved, both across actin genes within a species and across species. Apparently, the need for so many functional binding sites in a molecule of that size leaves few options for nonlethal mutations. Among the actins sequenced from 30 widely divergent species, there were only 32 amino acid substitutions. One implication of this is that when differences in contractile properties are observed between various types of muscle, those differences must be due to the motor protein (myosin) or to the various regulatory proteins. 

FIGURE 5 Comparison of amino acid sequence for myosin regulatory light chain proteins in different species. Cce, chicken cellular LC20-A, chicken adult smooth muscle LC20-B1, chicken embryonic smooth muscle Ram, rat aortic muscle Hsm, putative human smooth muscle. Amino acid residues are numbered starting at the Ser residue next to the initiator Met. A dot indicates identity of an amino acid residue with that in the top row. Reproduced with permission from Zavodny et al. (1990, Fig. 3, p. 937), Copyright 1994, American Heart Association. 

In an extremely interesting experiment Oosawa and his colleagues proved that actin from different species indeed can act similarly. All they did was mix actin derived from the slime mold a myxomycete) with myosin from striated rabbit muscle. In purified state the solution of both these proteins showed low specific viscosity. The moment these two proteins were mixed, the specific viscosity shot up (Figure 5.11). Moreover, when ATP was added, the viscosity dropped reminiscent of the previous experiment stated above. This means that aetin and myosin from evolutionarily widely removed species still interacted to give rise to aetomyosin. 

Figure 7.5 Compensation of enthalpy and entropy of activation to maintain a constant Gibbs energy of activation during adaptation of myosin ATPase to different temperature environments. The study was carried out on species of living at temperatures ranging from hot springs to arctic conditions. The plot is recalculated from the data of Johnson Goldspink (1975).

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