Enzymes metal-activated

Many enzymes require metal ions for maximal activity. If the enzyme binds the metal very tightly or requires the metal ion to maintain its stable, native state, it is referred to as a metalloenzyme. Enzymes that bind metal ions more weakly, perhaps only during the catalytic cycle, are referred to as metal activated. One role for metals in metal-activated enzymes and metalloenzymes is to act as electrophilic catalysts, stabilizing the increased electron density or negative charge that can develop during reactions. Among the enzymes that function in this... 

Cofactors serve functions similar to those of prosthetic groups but bind in a transient, dissociable manner either to the enzyme or to a substrate such as ATP. Unlike the stably associated prosthetic groups, cofactors therefore must be present in the medium surrounding the enzyme for catalysis to occur. The most common cofactors also are metal ions. Enzymes that require a metal ion cofactor are termed metal-activated enzymes to distinguish them from the metalloenzymes for which metal ions serve as prosthetic groups. 

The substitution process permeates the whole realm of coordination chemistry. It is frequently the first step in a redox reaction and in the dimerization or polymerization of a metal ion, the details of which in many cases are still rather scanty (e.g. for Cr(III) ). An understanding of the kinetics of substitution can be important for defining the best conditions for a preparative or analytical procedure. Substitution pervades the behavior of metal or metal-activated enzymes. The production of apoprotein (demetalloprotein and the regeneration of the protein, as well as the interaction of substrates and inhibitors with metalloproteins are important examples. ... 

Most of the literature on cobalt(II) in biochemistry concerns its effects in various metal-activated enzyme systems (3). Many of the typical Mg2+-activated enzymes can work with Co2+, though usually at a low rate. In some other systems, where Mnz+ commonly is the best coenzyme, Co2+ gives high activities while other cations are less effective or inhibitory. A few enzymes, notably some metal-activated peptidases, are most efficient with Co2+, but other metal ions are also functional. It is not believed, however, that Co2+ is an important enzyme activator in vivo (4). 

Because of the ease with which dimercaptopropanol can be broken down in the body there is a danger that chelation, followed by breakdown, will simply result in the translocation of the metal ions to other tissues such as brain or liver. High doses of dimercaptopropanol can adversely affect a number of essential metal-activated enzymes, such as catalase, carbonic anhydrase and peroxidase, and also produce dangerous systemic effects. Dimercaptopropanol cannot be used to remove cadmium because its cadmium complex is toxic to kidney tissue54). 

The general issue of metal-activated enzymes in nucleic acid biochemistry has been addressed in significant detail by Cowan [13], beautifully illustrating the... 

Many enzymes require metal ion cofactors for their activity. Such enzymes are either metalloenzymes, in which case the metal ion is tightly bound, or metal-activated enzymes, in which case the bound metal ion is retained in an equilibrium with free metal ions. 

Enzymes require a metal in their composition (such as Fe+2,Mg2+ Mn2+,Zn2+ ) are known as metalloenzymes. If they bind zmd retain their metal atom(s) under all conditions, that is with very high affinity. Those, which have a lower affinity for metal ion, but still require the metal ion for activity, are known as metal-activated enzymes. 

Patients treated unsuccessfully with large doses of EDTA for severe lead poisoning have, prior to death developed anuria and uraemia and autopsies have revealed that the patients had suffered kidney damage. This nephrotoxicity may have been due to in situ mobilization of specific metals that exchange for calcium, thus impairing the function of the metal-controlled or metal activated enzyme systems in the kidney. 

Metal cofactors in enzymes may be bound reversibly or firmly. Reversible binding occurs in metal-activated enzymes (e.g., many phosphotransferases) firm (or tight) binding occurs in metalloenzymes (e.g., carboxypeptidase A). Metals participate in enzyme catalysis in a number of different ways. An inherent catalytic property of a metal ion may be augmented by the enzyme protein, or metal ions may form complexes with the substrate and the active center of the enzyme and promote catalysis, or metal ions may function in electron transport reactions between substrates and enzymes. 

Early experimentalists in this field, lacking the sophisticated and highly sensitive instrumentation of today, relied mainly on simple kinetic observation and classification of types of metal-activated enzymes. As reflected in early reviews ", this led to highly speculative hypotheses regarding the mechanistic role of metals in enzymic catalysis. 

Modem methods for study of metal-activated enzymes include NMR and ESR spectroscopy, water relaxation rates by pulsed NMR (PRR), atomic absorption, Mbssbauer, X-ray and neutron diffraction, high-resolution electron microscopy, UV/visible/IR spectroscopy, laser lanthanide pertubation methods, fluorescence, and equilibrium and kinetic binding techniques. Studies with Mg(II)-activated enzymes have been hampered by the lack of paramagnetic or optical properties that can be used to probe its environment, and the relative lack of sensitivity of other available methods initial velocity kinetics, changes in ORD/CD, fluorescence, or UV properties of the protein, atomic absorption assays for equilibrium binding, or competition with bound Mn(II) °. Recent developments in Mg and 0-NMR methodology have shown some promise to provide new insights . ... 

As there are many different types of active sites, it would be impossible to examine all of them here, but we can consider one important class of active sites, those that include metal ions. The inclusion of the metal ion in the enzyme may either be from strong coordinate covalent bonds, or via a looser association in the active site. When the metal is tightly bound, the protein is called a metalloenzyme. When the binding is not covalent, a metal-activated enzyme results. In either case, the metal ion is likely to be an important part of the active site. 

Loosely bound (forming metal-activated enzymes)... 

Metal-activated enzymes may have an absolute requirement for the metal ion, or they may simply have enhanced activity in the presence of the metal ion. Phosphofructokinase is an example of a metal-activated enzyme, which catalyzes the reaction... 

A divalent metal ion (Mg ) is needed to coordinate the phosphate groups on the ATP molecule in order for phosphofructokinase to successfully catalyze this reaction. Mg, Mn, Ca, and often function as cofactors for metal-activated enzymes. 

Divalent cations, univalent cations, or both are essential cofactors for a large number of enzymes. Kinases as a class, for example, share the requirement for such cations, while in other instances, other metal ions are inhibitors of metal-activated enzymes, e.g., activation by Mg " and inhibition by Ca for many kinases and synthetases. Mildvan (1970) has reviewed the models that have been proposed to account for activation (or inhibition) of enzymes by metal ions. The "substrate bridge" and "metal bridge" models conceive of the metal ions either combining with the substrate to form a chelate or interacting with the enzyme to complete the required binding site. These complexes usually involve the active site, but Schramm (1974) has demonstrated activation of AMP nucleosidase by MgATP at a modifier site instead. 

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