Abstract
X-ray diffraction data were measured by x-ray diffractometry to 5-A resolution for both the monoclinic form of rabbit skeletal muscle aldolase (EC 4.1.2.13) and a platinum derivative. The heavy atom difference patterson was solved at 6-A resolution yielding eight distinct heavy atom sites. Choice was made of the enantiomorph and protein phases were calculated on the basis of single isomorphous replacement differences. The electron density map calculated from these phases was averaged according to the non-crystallographic molecular symmetry. Rotational symmetry analysis of native patterson and site symmetry analysis of refined heavy atom positions are consistent with the aldolase tetramer possessing a very high degree of 222 internal symmetry. The subunits in the tetramer are positioned in a tetrahedral configuration displaying a slight square planar deformation. Each subunit is roughly ellipsoidal in shape with the major axis nearly parallel to a local 2-fold axis. Prominent at the surface of each subunit were structural features resembling alpha helices. Each subunit contributes to its boundary surface at least six helices which are arranged in a barrel-like manner and possessing a right handed twist with respect to each other. Density associated with binding of substrate on the enzyme was located on the surface of each subunit. Cooperative aspects of the conformational changes produced upon substrate binding are discussed.
Highlights
Tetramer possessing vaery high degreeof 222 internal enzyme
The electron density map was of sufficient quantity such eachsubunit.Cooperativeaspects of theconforma- that molecular and subunit boundaries were readily traced in tional changes produced upon substrate binding are most parts on the map
The interpretation of the electron density map at 5-A resolution suggests a packing of the aldolase subunits corresponding to a compact tetrahedral or pseudotetrahedral spatial arrangement of subunits that has been proposed from observations of electron micrographs of aldolase [1].Each subunit in the electron density map is in close contact with only two neighboring subunits related through the local 2fold axes that are oriented nearly parallel to the crystallographic a and c* axis, respectively
Summary
Tetramer possessing vaery high degreeof 222 internal enzyme. In a preliminary step we wish to report on aspects of symmetry. Density associawteidth binding of through the rabbit muscle aldolase tetramer is shown in Fig. substrate on the enzymewas located on the surface of 1.
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