Quaternary Structure and Certain Allosteric Properties of Aspartase

Aspartase from Enterobacter aerogenes was found to have a molecular weight of approximately 180,000, and was cleaved by p-hydroxymercuribenzoate into four subunits. Active tetramer was regenerated by treating the mercury derivative with mercaptoethanol. A careful examination of velocity-concentration plots over a wide range of pH revealed evidence of homotropic substrate interactions which were pH-dependent. Some confirmatory evidence was obtained for nucleotide activation reported by other workers. The pH optimum and apparent Km at 30° were found to be 8.5 and 1.5 x 10-3 m. The aspartase used in these experiments possessed the highest specific activity yet reported.