Reaction of carboxypeptidase A crystals with diazotized arsanilic acid uniquely modifies Tyr-248 to form a monazo derivative, which-in solution-forms an intramolecular inner-sphere coordination complex in the active site zinc atom. tarsanilazocarboxypeptidase exhibits spectral properties that are closely similar to those of the model complex, tetrazolylazo-N-carbobenzoxytyrosine Zn2+, with a distinctive maximum at 510 nm. In addition, its circular dichroic spectrum reveals a negative extremum at this wavelength, also characteristic of this complex. Both spectra are exquisitely responsive to pth changes and serve to monitor formation and dissociation of the metal-azophenol complex. Two pKapp at 7.7 and 9.5 delineate the pH range over which the probe characteristics most effectively gauge conformational features of the active center of arsanilazcarboxypeptidase. Other environmental parameters, e.g., substrates and inhibitors, as well as crystallization of the enzyme also critically influence the formation and dissociation of the complex; the response of the probe suggests that they induce conformational movement of the azoTyr-248 residue away from the zinc atom. tthe now available chemical, functional, structural data bearing on the spatial relationships of Tyr-248 and Zn, both thought critical to catalysis, are evaluated, based on spectra of arsanilazo- and nitrocarboxypeptidase crystals and solutions as well as on detailed kinetic analyses of the native enzyme in both physical states and based on the X-ray structure analysis of the native enzyme and its Gly-L-Tyr complex. Collectively all of the data show that the conformation of carboxypeptidase in crystals differs from that in solution. Moreover, reexamination of the original X-ray maps reported in 1968 and thought to preclude a Tyr-248-Zn interaction now leads to the conclusion that in up to 25 per cent of the molecules in the crystals ttyr-248 interacts with the active site zinc atom (W.D. Lipscomb (1973), Proc. Nat. Acad. Sci U.S. 70, 3797). Thus, even in the crystals the enzyme exists in at least two different conformations. In one of these Tyr-248 is near while in the other it is far from the zinc atom. The spectral effects of Gly-L-Tyr and beta-phenylpropionate on solutions of arsanilazo- and of nitrocarboxypeptidase demonstrate that during the catalytic process Tyr-248 moves away from the zinc atom. This implies a mechanistic role for Tyr-248 different from that postulated on the basis of X-ray crystallographic analysis. Indeed, the proximity of ttyr-248 to the zinc atom, when altered by substrates and inhibitor, may reflect certain of the properties characteristic of the entatic, active site.