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 interactions between the catalytic regions of the subunit of the bifunctional enzyme aspartokinase I-homoserine dehydrogenase I, although not necessary for the catalytic functions, are shown to be of utmost importance for the regulation of the activities of the enzyme. The study of the effect of a number of –SH reagents on the protein shows that the inhibition of the homoserine dehydrogenase activity by the allosteric effector l-threonine is closely correlated with the existence of an active aspartokinase region. The kinetic study of the proteolytic reaction leading to the previously described homoserine dehydrogenase fragment indicates that a dissociation is a necessary requirement for the digestion of the aspartokinase region. Cross-linking with dimethylsuberimidate and proteolytic degradation of the fully cross-linked tetramer have been used to show that the oligomeric interactions, necessary to build the tetrameric structure, involve the kinase region only. A tentative model is proposed showing aspartokinase I-homoserine dehydrogenase I as an isologous tetramer in which the aspartokinase regions, organized as a “core”, are surrounded by the homoserine dehydrogenase regions.