Metals as cofactors

P-Gal has a molecular weight of 540,000 and is composed of four identical subunits of MW 135,000, each with an independent active site (Melchers and Messer, 1973). The enzyme has divalent metals as cofactors, with chelated Mg+2 ions required to maintain active site conformation. The presence of NaCl or dilute solutions (5 percent) of low-molecular-weight alcohols (methanol, ethanol, etc.) causes enhanced substrate turnover. P-Gal contains numerous sulfhy-dryl groups and is glycosylated. 

It is necessary now to classify the metals with regard to their function in biological systems metals as cofactors of proteins, metalloen2ymes, communicative functions of metals, interaction of metal ions with polynucleotides, biometal-organic chemistry (e.g. metals in medicine). 

Metals as cofactors in proteins and metalloproteins oxygen binding and transport, electron transfer, energy transfer followed by electron transfer, and several regulating functions. 

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. 

Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin (Chapter 45). FMN and FAD are usually tighdy—but not covalendy—bound to their respecdve apoenzyme proteins. Metalloflavopro-teins contain one or more metals as essential cofactors. 

Enzymes may not function well or at all unless some other species known as a cofactor is present. An enzyme alone is referred to as the apoenzyme and the combination of enzyme and cofactor is known as the holoenzyme. Among the species that function as cofactors are organic compounds that interact with the enzyme. If the organic moiety is strongly attached to the enzyme, it is called a prosthetic group, but if it is loosely bound to the enzyme, it is referred to as a coenzyme. For the purposes of this discussion, the most interesting cofactors are metal ions. Depending on the type of enzyme, the appropriate metal ion cofactor may be Mg2+, Ca2+, K+, Fe2+, or Cu2+. A sizeable number of enzymes are sometimes called metalloenzymes because they have active sites that contain a metal. 

A rather new approach for detecting metal ions with very high sensitivity and selectivity utilizes DNAzymes. DNAzymes are a special class of enzymes formed from DNA nucleotides. Compared to proteins and ribozymes, they are more stable, structurally simpler, and therefore cheaper. As DNAzymes often require metal ion cofactors, they are interesting sensing platforms for these metal ions [149]. 

Since the second complex contains two molecules of drug, therefore, we refer to it as dimer complex. Keeping in view the milli molar concentration of the metal ion present in the cell, possibility of the formation of dimer complex is more under in vivo conditions. However, in certain cases of cancer the metal ion concentration goes down to micro molar range. Under these unusual conditions, complex I is formed. Recently we have shown that mithramycin forms only dimer complex with Zn +, another metal ion playing an important role as cofactor in many enzymes and DNA binding proteins like transcription factors. 

For all these reasons, some chemical or genetical modifications have been applied into the binding sites of antibodies in order to improve their reactivity [22]. Antibodies can be modified by the incorporation of natural or synthetic catalysts into the antibody recognition site, as for instance transition metal complexes, cofactors, and bases or nucleophiles, to carry other catalytic functions, which open the way to... 

Metal ions can function in several different ways as cofactors in ri-bozyme-catalyzed reactions, as described above, and proposed mechanisms for the reactions catalyzed by several ribozymes have taken advantage of such functions. Significant aspects of these functions of metal ions might be subsumed by nucleobases if their pKa values could be adjusted appropriately. The full details of the mechanisms of action of metalloenzymes remain to be elucidated. 

Aldolases are part of a large group of enzymes called lyases and are present in all organisms. They usually catalyze the reversible stereo-specific aldol addition of a donor ketone to an acceptor aldehyde. Mechanistically, two classes of aldolases can be recognized [4] (i) type I aldolases form a Schiff-base intermediate between the donor substrate and a highly conserved lysine residue in the active site of the enzyme, and (ii) type II aldolases are dependent of a metal cation as cofactor, mainly Zn, which acts as a Lewis acid in the activation of the donor substrate (Scheme 4.1). 

The antitumor antibiotic bleomycin (BLM) is believed to cause cytotoxicity through its ability, in the combined presence of dioxygen and a metal ion cofactor (204), to bind to and degrade DNA (205). Iron complexes of BLM have aroused special attention, as such complexes are the first (vide supra concerning the discussion of hemerythrin and hemocyanin) non-heme-iron complexes with a significant capacity for dioxygen activation (206). 

The three in vitro activities of integrase require divalent metal ions as cofactors. The only two metals that support these activities are Mn2+ and Mg2+. Since quite high metal concentrations must be added to assays (1-10 m Mfor optimal activity), it has been presumed that Mg2+ is the ion used in vivo. 

In addition to cobalt and iron (discussed above), other metals frequently function as cofactors in enzyme-catalyzed reactions. Like coenzymes, they are useful because they offer something not available in amino acid side chains. The most important of such features of metals are their high concentration of positive charge, their directed valences for interacting with two or more ligands, and their ability to exist in two or more valence states. 

An enzyme cofactor can be either an inorganic ion (usually a metal cation) or a small organic molecule called a coenzyme. In fact, the requirement of many enzymes for metal-ion cofactors is the main reason behind our dietary need for trace minerals. Iron, zinc, copper, manganese, molybdenum, cobalt, nickel, and selenium are all essential trace elements that function as enzyme cofactors. A large number of different organic molecules also serve as coenzymes. Often, although not always, the coenzyme is a vitamin. Thiamine (vitamin Bj), for example, is a coenzyme required in the metabolism of carbohydrates. 

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