EFFECTS OF FEEDBACK MODIFIERS ON THE STATE OF AGGREGATION OF HOMOSERINE DEHYDROGENASE OF RHODOSPIRILLUM RUBRUM

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Abstract
The mechanism of control of Rhodospirillum rubrum homoserine dehydrogenase activity by amino acids from homoserine was studied using density gradient centrifugation and gel filtration. The centrifugation experiments indicate that threonlne causes aggregation of the enzyme to a catalytically inactive form, presumably a dimer, characterized by markedly increased sedimentation rate. Aggregation of the dehydrogenase in the presence of threonine also results in extensive displacement of the filtration profile on Sephadex G-200, permitting distinction of the 2 forms of the enzyme. The L-isomers of homoserine, isoleucine and methionine reverse the feedback inhibition of catalytic activity by L-threonine and the threonine-induced aggregation. The monomer-polymer interconversions may play a decisive role in regulating the activity of the homoserine dehydrogenase.