I will advice you to test Prodan (Nature 319:70 - 73; 1986) or Acrylodan (JBC 258:7541-7544;1983 ). These probes show a large blue shift when they interact to globular proteins in respect to their free state. I am not sure if they will specifically bind to the active cleft of the protein, but if this protein site is hydrophobic you may have a good chance. Good luck