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The structural stability of these enzymes at different pH values was examined by optical rotatory dispersion, free-boundary electrophoresis, and esterase activity. The increase in esterase and decrease in peptidase activities of carboxypeptidase acetylated either with acetic anhydride or acetylimidazole are not accompanied by changes in conformation of the enzyme detectable by techniques presently available for this purpose. Such changes as could be detected in the enzyme modified with acetic anhydride could be shown to be owing to denaturation of a frac-* This work was supported by the Howard Hughes

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In metalloenzymes, metals which affect function directly by participation in catalysis appear to have ligand sites which differ markedly from those of metals which influence function indirectly through modulation of protein structure. Thus, in both E. coli alkaline phosphatase and equine liver alcohol dehydrogenase, metals positioned at the active site interact selectively with chelating agents, undergo isotope exchange, and display distinctive physical chemical characteristics. Such active site metals may have an irregular geometry which facilitates their catalytic role. In contrast, nonactive site metals exhibit physical properties more like those of simple, bidentate model complexes; they frequently appear to stabilize structure or influence subunit interactions as shown by their effects on sedimentation or hydrogen exchange rates of proteins. Such "structural" metals may function importantly in control mechanisms for biochemical reactions.Touring the past two decades, considerable progress in understanding **** the role of metals in biological processes has resulted from the discovery and purification of a large number of metalloproteins and from efforts to ascertain their function-e.g., in catalysis, electron transport, or gas exchange (I). Studies of the physical chemical properties of this group of proteins have been of particular interest because of the presence of the inorganic moiety. In principle, metals should be excellent labels of the protein sites to which they bind owing to the distinctive physical properties of metal complexes with common ligands-e.g., color, in the case of copper, cobalt, or iron salts. Hence, one might have hoped that the basic characteristics of the metal, known from studies of inorganic complexes, would be preserved in biological systems and aid identification of the ligand groups and their properties at protein loci of special