The Threonine-Sensitive Homoserine Dehydrogenase and Aspartokinase Activities of Escherichia coli K 12. 4. Isolation, Molecular Weight, Amino Acid Analysis and Behaviour of the Sulfhydryl Groups of the Protein Catalyzing the Two Activities

Abstract

The complex protein carrying the two theronine sensitive activities aspartokinase I and homoserine dehydrogenase I has been obtained in a homogenous state from extracts of Escherichia coli K 12. The molecular weight of the protein has been determined by sedimentation equilibrium to be 360,000 dalton. The amino acid composition is given. Titration of sulfhydryl groups by 5–5′-dithiobis-(2-nitrobenzoic)acid leads to an estimation of 16 of 18 reactive cysteines per mole of the protein, all of which can be protected by addition of the allosteric inhibitor, L-theronine.