Subunit Structure of l-Ribulokinase from Escherichia coli

Abstract l-Ribulokinase from Escherichia coli B/r has a molecular weight of 98,000 by sedimentation-diffusion, sedimentation equilibrium, and osmotic pressure measurements. Protonation of l-ribulokinase or treatment with 8 m urea results in the dissociation of the native enzyme into two subunits of molecular weight 49,500 (pH 2.0) and 50,200 (urea). The subunits are identical by size and charge, as evidenced by their sedimentation and electrophoretic behavior. Both subunits appear to have masked N-terminal end groups. There are 14 methionine residues per molecule of native l-ribulokinase, and after cleavage with cyanogen bromide we observed 7 bands by electrophoresis in acrylamide gels. All evidence is consistent with l-ribulokinase being a dimer of identical polypeptide chains.