S.  R. Kumar, Prabhune, A., Pundle, A. V., Karthikeyan, M., and Suresh, C. G., ?Tryptophan residue is identified in the substrate binding of penicillin G acylase from Kluyvera citrophila?, Enzyme and Microbial Technology, vol. 40, no. 5, pp. 1389-1397, 2007.