Studies on aspartase

Aspartase (l-aspartate ammonia-lyase, EC 4.3.1.1) of Escherichia coli is composed of four subunits of seemingly identical molecular weight (Suzuki, S., Yamaguchi, J. and Tokushige, M. (1973) Biochim. Biophys. Acta 321, 369–381). The subunit arrangement of the enzyme was studied by two distinct methods, cross-linking of subunits with a bifunctional reagent, dimethyl suberimidate, and statistical classification of negatively stained electron microscopic images. In the former method, the densitometric patterns of the cross-linked aspartase were analyzed quantitatively after separating each component by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and the results were compared with the theoretical distribution. In the latter method, a number of electron microscopic images were classified into several groups according to their characteristic appearance. The results obtained by these two methods are compatible with the possibility that the enzyme has a tetrameric structure consisting of two pairs of dimers, in which the two pairs of rod-shape subunits meet perpendicularly, being typical of D₂ symmetry.