Myosin titration curves

A spectrophotometric titration of the phenolic groups of myosin and its subunits has been reported by Stracher (1960). The data resemble those shown for ribonuclease in Fig. 11. About two-thirds of the tyrosine residues are titrated normally, and about one-third appear inaccessible in native myosin. An interesting feature is that 6 M urea has no effect at all on the titration curve. 

Myosin is another protein to which the theory of Linderstr0m-Lang in its present form is not applicable, since in myosin the ratio of molecular length to width is 100/1—far from the sphericity on which the theory is based. Thus experimental values of the parameter w cannot be easily interpreted quantitatively. Myosin is soluble in the presence of salt on the alkaline side of its isoionic point only, and thus should behave as a soluble protein above pH 5.7 to 5.8 and as an insoluble one below this. Mihdlyi (1950) has studied the effect of salt on the titration of myosin and reports that its insolubility in acid in the presence of greater than 0.05 M KCl does not affect the reversibility of the titration nor are there any obvious discontinuities in his titration curves, shown in Fig. 4. The data for basic solutions appear to be affected by salt very much as those of other soluble proteins, and reach an apparent limiting curve at a fairly low ionic strength (0.15). In acid solution where the protein is insoluble, however, the effect of salt closely resembles that for wool, except that the displacements of the parallel central portions of the curves are somew hat less than for wool, consistent with a lower affinity of myosin for chloride ion. The slopes of these portions of the curves are within 10 % of those observed for... 

Fio. 4. Titration curves of myosin at different ionic strengths. Temperature 25°C. Empty circles 0. M KCI vertical circles (right side black) 0.05 M KCl full black circles 0.15 M KCl horizontal half circles (black top) 0.30 M KCl vertical half circles (left black) 0.60 M KCl horizontal half circles (white top) 1.20 M KCl. Ordinate equivalents of hydrogen ions bound or dissociated by lO g. myosin. Ab scissa pH. The curve in the inset shows the difference in the number of equivalents of base bound by 10 g. myosin in the presence of potassium chloride (T/2 > 0.15) and in the absence of salts. From Mihilyi (1950). 

There is not much information on the charge of the fibrous muscle proteins. Older work on the titration curve, alkali binding and I.P. (Hollwede and H. H. Weber, 1938 Dubuisson, 1941 Dubuisson and Hamoir, 1943) is not as valuable as it might be, since unfractionated myosin was used. It is not certain even whether the I.P. of L-myosin and actomyosin are identical (c/. Table VI). 

Fig. 16. Electrophoretic mobility and H+ binding of L-myosin. pH titration curve from Dubuisson, 1941 - - mobility from Dubpisson (1946b, 1948b, and 1950b) (c/. Table VIII) mobility from Erdos and Snellman (1948).

The titration of myosin has been studied by Mihalyi (1950). The coimt of groups in the various regions of the curve agrees with analysis to better than 10%, except for the carboxyl region, where titration indicates 165 groups per 100,000 gm, as compared with the analytical figure of 132. It is likely that the analytical assay for amide groups is the source of the error. 

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