Method of screening compounds

A method of screening for compounds that bind to the activation loop of kidney-type glutaminase (“GLS”) protein, said method comprising:

• contacting GLS protein, wherein the GLS protein comprises an activation loop and comprises a phenylalanine to tryptophan substitution at the residue corresponding to position 322 of SEQ ID NO:
• 2, with a test compound, wherein the test compound is a compound that potentially binds to the activation loop, to permit binding between the activation loop and the test compound;
• detecting fluorescence of the substituted tryptophan in the presence of the test compound;
• comparing the detected fluorescence to fluorescence of a control;
• and
• identifying, based on said comparing, that the test compound is a compound that binds to the activation loop of GLS, wherein a change in fluorescence relative to the control indicates that the test compound binds to the activation loop.