CONTROL OF ENZYME ACTIVITY BY CONCERTED FEEDBACK INHIBITION

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Abstract
The phenomenon of concerted feedback inhibition of enzyme activity is described. This type of feedback control depends on the simultaneous action of two end products of a branched biosynthe-tic pathway. The enzyme studied, viz., [beta]-aspartokinase from the photo-synthetic bacterium Rhodopseudomonas capsulatus, is virtually insensitive to individual end products. The specific combination of L-lysine + L-threonine, however, causes effective inhibition. Concerted inhibition due to these amino acids is nontotal, reversible, and noncompetitive with respect to one of the substrates of the enzymatic reaction, namely, L-aspartate. Evidence is presented for the conclusion that the [beta]-aspartokinase possesses separate "regulatory sites" for L-lysine and L-threonine, which are distinct from the substrate (L-aspartate) sites. The probable role of concerted feedback inhibition in regulation of synthesis of aspartic family amino acids (lysine, threonine, isoleucine, methio-nine) in Rps capsulatus, and in branched pathways in general, is discussed.