Crystal-structure of the Ternary Complex Between Carboxypeptidase-a, L-phenylalanine, and the Azide Ion

The structure of the ternary complex of carboxypeptidase A (CPA) with the amino acid L-phenylalanine and the azide anion is reported as determined by X-ray crystallography to a resolution of 2.0 angstrom. The structure has been refined to an R factor of 0.151 over 7234 reflections having I > 2-sigma(I). The binding of the amino acid molecule to the enzyme hydrophobic pocket closely resembles that of D-phenylalanine with the carboxylate linked to Arg-145 and Asn-144 and its alpha-amino group linked to Glu-270. The azide anion binds to Arg-145, which is proposed as the high-affinity binding site for anions. Upon L-phe binding the Tyr-248 residue moves in the "down" position at contact distances with both the amino and the carboxylate groups of the bound amino acid. A region of electron density near the metal has been interpreted as a second azide ion directly coordinated to zinc as suggested by spectroscopic studies on cobalt-substituted CPA. The present structure confirms the proposed mechanism of CPA sensitivity toward anions.