Threonine inhibition of the aspartokinase-homoserine dehydrogenase I of Escherichia coli. Threonine binding studies

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Abstract
Both activities of the aspartokinase-homoserine I (AK-HSD) of E. coli are inhibited by threonine. Careful threonine binding studies have been done which have distinguished the various effects of threonine on the enzyme. The ultrafiltration technique for measuring ligand binding was comparable with equilibrium dialysis techniques. Reduction in error by utilization of this procedure enabled us to obtain evidence for 2 different sets of theronine sites by direct binding studies. The binding data were mathematically consistent with 2 independent classes of threonine sites, each of which contained 4 sites/tetramer and had a Hill coefficient of about 2.3-2.5. Kd for the 2nd set of sites was 5- to 10-fold greater than the high affinity sites, depending upon conditions. The sequential model for site-site interactions adequately describes the cooperativity of threonine binding to the high affinity set of sites.

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