Kinetic experiments with a substrate series of phenylacetyl-arylamides reveal that at least one polar group in the amine moiety is required for the proper orientation of the substrate in the large nucleophile-binding subsite of penicillin acylase ofEscherichia coli. Quantum mechanical molecular model-ling of enzyme–substrate interactions in the enzyme active site shows that in the case of substrates lacking local sym-metry, the productive binding implies two nonsymmetrical arrangements with respect to the two positively charged guanidinium residues of ArgA145 and ArgB263
