Structure and Function of Metalloenzymes

Abstract

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

pH-dependent migration of copper(II) to the vacant zinc-binding site of zinc-free bovine erythrocyte superoxide dismutase

“…6 it can be estimated that at pH 7.8 at least 10% of the subunits are in the apo form. The apoprotein is much less stable to thermal inactivation than are derivatives that contain metals (28,29). It is possible, therefore, that the source of the apparent thermal instability of the Zn-free SODase at pH 7.8 relative to the native protein is the apo subunits that are present at that pH.…”

Cadmium-Binding Components in Soybean Plants

“…This accumulation at higher concentrations in various plant species was reported to be toxic (10,11,17). As low as 18 ,UM Cd in the nutrient solution inhibited pod fresh weight accumulation, nitrogenase activities, and photosynthetic rates in soybeans (1 1).There is abundant literature pertaining to the binding of Cd in animal tissues (3,5,6,15,16,18,21,22,24) …”

“…There is abundant literature pertaining to the binding of Cd in animal tissues (3,5,6,15,16,18,21,22,24) …”

“…There is abundant literature pertaining to the binding of Cd in animal tissues (3,5,6,15,16,18,21,22,24) and in shellfish tissues (19). The most important finding was that 75 to 90% of the Cd that accumulated in the mammalian liver or kidney and shellfish tissues was bound by a cytoplasmic soluble protein of low mol wt.…”

Bioinorganic interactions

“…They possess two special chemical features which can readily be utilized in cyclic biological processes, namely (i) their capacity to bond reversibly and with a particular stereochemistry to electron donor atoms such as oxygen, nitrogen and sulphur, and (ii) their ability to participate in facile electron transfer reactions [4]. These properties have led to their incorporation in one-third of all known enzymes [5].…”