Size and Shape of the α Subunit of the (Ca,Mg)‐dependent Adenosinetriphosphatase from Escherichia coli in Solution in the Presence and Absence of ATP

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Abstract
The .alpha. subunit of the (Ca, Mg)-dependent ATPase from E. coli was studied in solution by X-ray scattering experiments at variable constrast and in the presence of ATP. On an absolute scale in the range of (45.0 nm-1) < h < (1.5 nm-1) at pH 8.0. This system and the complex of the .alpha. subunit with ATP can be considered to be ideal and monodisperse so that the following parameters for the .alpha. subunit and its complex with ATP were determined: the radius of gyration, the volume, the degree of hydration, the maximum particle diameter and the MW. The MW of the .alpha. subunit was determined as 58,500 .+-. 3000 by means of light scattering measurements, and 57,700 .+-. 2500 by the small-angle X-ray scattering experiments. The radius of gyration of the .alpha. subunit at pH 8.0 was 2.64 .+-. 0.02 nm, the maximum chord in solution was 10.0 .+-. 0.5 nm, the volume was 102.4 .+-. 2.5 nm3, and the degree of hydration was 0.34 .+-. 0.02 ml/g. Binding of ATP to the .alpha. subunit causes structural changes. These changes are reflected in the radius of gyration, Rg = 2.45 .+-. 0.015 nm, in the volume, 117.6 .+-. 1.5 mm3, and in the maximum chord length in solution, 7-8 .+-. 0.5 nm. These changes imply a decrease of the axial ratio from 2.4 to 1.4, i.e, ATP-binding induced an increase in isometry of the .alpha. subunit.

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