Abstract
A relatively large number of compounds is known to cause activation or inhibition of myosin A-ATPase depending upon their respective concentrations in the incubation mixture. Some of these modifiers, such as ethylene diamine tetraacetic acid (EDTA), p-mercuribenzoate (PCMB), 2,4-dinitrophenol (DNP), are believed to interact with certain “specific groups” in or close to the active center of the enzyme. The mechanism of action of some other compounds such as urea, ethylene glycol and dioxane was explained mainly on the basis of their respective effects on the secondary or tertiary structure of the protein molecule. It was, therefore, thought that a study of the effects of some “specific” modifiers in the presence and absence of the specified solvents may throw more light on the importance of hydrogen and hydrophobic bonds in the enzymatic reactions of the myosin A.
Myosin A was prepared by standard methods(1). All ATP and ITP splitting experiments were carried out at 37°C in a 0.05 M tris buffer. The tecnique for ATPase determination has been published(2).
It may be seen from Fig. 1 that urea concentrations between 6 and 12% doubled the EDTA-activated ATPase activity of myosin A while that of Ca++-ATPase was accelerated to a much smaller extent. Six per cent urea completely abolished the PCMB activation but diminished the DNP effect by only 20%. These results showed clearly that even small amounts of urea caused alterations of the active center of myosin A, so as to produce enzymatic effects which were decidedly different in the presence of various modifiers. The fact that urea enhanced only the EDTA-activated-ATPase would suggest that hydrogen bonding, as it existed on the enzyme, did not provide the best fit for ATP to be split in the presence of EDTA.
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