Conformational Changes in the Activation Loop of Mitochondrial Glutaminase C: A Direct Fluorescence Read-Out that Distinguishes the Binding of Allosteric Inhibitors from Activators [Signal Transduction]

The first step in glutamine catabolism is catalyzed by the mitochondrial enzyme glutaminase, with a specific isoform, glutaminase C (GAC), being highly expressed in cancer cells. GAC activation requires the formation of homo-tetramers, promoted by anionic allosteric activators such as inorganic phosphate. This leads to the proper orientation of a flexible loop proximal to the dimer-dimer interface that is essential for catalysis (i.e. the activation loop). A major class of allosteric inhibitors of GAC, with the prototype being BPTES (bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfide), and the related molecule CB-839, binds to the activation loop and induces the formation of an inactive tetramer (2 inhibitors bound per active tetramer). Here, we describe a direct readout for monitoring the dynamics of the activation loop of GAC in response to these allosteric inhibitors, as well as allosteric activators, through the substitution of phenylalanine at position 327 with tryptophan (F327W). The tryptophan fluorescence of the GAC(F327W) mutant undergoes a marked quenching upon the binding of BPTES or CB-839, yielding titration profiles that make it possible to measure the binding affinities of these inhibitors for the enzyme. Allosteric activators like phosphate induce the opposite effect (i.e. a fluorescence enhancement). These results describe direct read-outs for the binding of the BPTES-class of allosteric inhibitors, as well as for inorganic phosphate and related activators of GAC, which should facilitate screening for additional modulators of this important metabolic enzyme.

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