N-Acetylglutamate 5-phosphotransferase (acetylglutamate kinase, EC has been isolated from pea (Pisum sativum) cotyledons and purified 312-fold by using heat treatment, (NH4)2SO4 fractionation, affinity chromatography on ATP--Sepharose and ion-exchange chromatography on DEAE-cellulose. This preparation was shown on polyacrylamide-gel electrophoresis to yield one band staining with Coomassie Blue. The enzyme was shown by a variety of techniques to be composed of two different kinds of subunits, of mol.wts. 43000 and 53000 respectively. These subunits are arranged to give either a dimeric or tetrameric enzyme composed of equal numbers of each type of subunit. The dimeric and tetrameric enzyme forms are thought to be interconvertible, the equilibrium between these forms being influenced by the type of ligand bound to the subunits. Kinetic studies performed on the purified enzyme, indicated a random Bi Bi type of mechanism. The enzyme displayed apparent negative co-operativity with respect to one of its substrates, N-acetylglutamate; as a result, two Km values were found for this substrate, one at 1.9 X 10(-3) M and the other at 6.2 X 10(-3) M. A single Km value for ATP was found to be 1.7 X 10(-3) M. Allosteric regulation by arginine was also shown. A model, based on the Koshland, Némethy & Filmer [(1966) Biochemistry 5, 365-385] Sequential model, which adequately describes the kinetic and structural properties of N-acetylglutamate 5-phosphotransferase, is presented.

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