Enzymes metal ions

Metal-dependent enzymes have been divided into two groups,80-83 the metalloenzymes and the enzyme-metal-ion complexes. Metalloenzymes are those that contain one or more functional metal atoms per enzyme molecule. The metal is firmly bound to the protein, and the enzyme can be purified without any loss in activity. The content of functional metal in the preparation approaches a limiting value during purification. Enzyme-metal-ion complexes are more readily dissociable than metalloenzymes. It is necessary to add the functional, metal ion during or after purification, in order to maintain or restore full catalytic activity. 

There is now considerable evidence that indicates that metal ion-activated enzymes and metalloenzymes catalyze reactions via ternary complexes consisting of a 1 1 1 ratio of enzyme metal ion substrate. Three coordination schemes are possible ... 

Metal labels have been proposed to resolve problems connected with enzymes. Metal ions [13-16], metal-containing organic compounds [17,18], metal complexes [19-21], metalloproteins or colloidal metal particles [22-28] have served as labels. Spectrophotometric [22,25], acoustic [25], surface plasmon resonance, infrared [24] and Raman spectroscopic [28] methods, etc. were used. A few papers have been dealing with electrochemical detection. However, electrochemical methods of metal label detection may be viewed as very promising taking into account their high sensitivity, low detection limit, selectivity, simplicity, low cost and the availability of portable instruments. 

Elucidating all the fascinating details of this reaction will require further mechanistic, structural, and model studies. Finally, the discovery of self-processing redox enzymes (see Section 6 see Metal-mediated Protein Modification) may be relevant to understanding aspects of the evolution of enzymes. Metal-ion mediated redox chemistry with oxygen can modify several amino acids, especially tyrosine, tryptophan, cysteine, and histidine. This may have provided a path to generate new redox cofactors prior to the advent of the complex biosynthetic pathways. 

Hershberg, R. D., Reed, G. H., Slotboom, A. J., and de Haas, G. H. (1976). Phospholipase A2 complexes with gadolinium (III) and interaction of the enzyme-metal ion complex with monomeric and micellar alkylphosphorylcholines. Water proton nuclear magnetic relaxation studies. Biochemistry 15,2268-2274. 

Although one hesitates to speculate on a mechanism for what is obviously a complex overall reaction, it seems likely that most of the enzymic metal ions are involved in feeding electrons to the ultimate acceptor and that the selenium participates in the actual formate oxidation step. If this is so, a reasonable mechanism for that process, based on the characteristics of selenium already discussed, appears in equation (b). If this is the mechanism, in the catalytic cycle the reduced enzyme is presumably reoxidized by the ultimate electron acceptor in a mechanism mediated by the other components of the enzyme. 

There has been considerable interest in the chemistry of ternary complexes of copper(II) containing a bidentate aromatic nitrogen base such as 1,10-phen-anthroline (phen) or 2,2,-bipyridine (bpy) and a bidentate oxygen donor ligand or an amino acid,1 6 as some of these could possibly serve as models for enzyme-metal ion-substrate complexes. Two procedures are described below for the convenient, high-yield preparation of two such complexes. 

ASV integrase acts on the phosphodiester DNA. By analogy to crystallographic studies of the 3 -5 -exonuclease domain of E. coli DNA polymerase [65], E. coli ribonuclease H [66,67], and the ribonuclease H domain of HTV-l reverse transcriptase [65], it is suggested [62] that an enzyme-metal ion complex may possibly stabilize a pentacoordinate phosphate intermediate during two successive nucleophilic attacks. These two reactions are ... 

Coenzymes and metal ions. Besides their amino acid side chains, enzymes can provide other reactive groups. Coenyzmes are biomolecules that provide chemical groups that help catalysis. Like enzymes themselves, coenzymes are not changed during catalysis. This distinguishes them from other substrates, such as ATP, which are changed by enzyme action. Coenzymes, however, are not made of protein, as are most enzymes. Metal ions can also be found in the active sites of a number of enzymes, bound to the enzyme and sometimes to the substrate. 

Besides the large group of hydrolytic enzymes, metal ions are often present in enzymes, which catalyze redox processes. Nature provides a large number of oxidoreductases, which catalyze diverse reactions. Many of them are copper enzymes that use O2 as the ultimate oxidant. A prominent example for such a type-2 copper enzyme is galactose oxidase. The structure of galactose oxidase and its mechanism... 

Enzyme Metal ion composition Biological function Research incentive... 

This enzyme catalyses the decarboxylation of the ) -ketoacid oxaloacetate, with the same stoichiometry as acetoacetate decarboxylase. The former however, requires a Mn ion for activity and is insensitive to the action of sodium borohydride. This duality of mechanism is not unlike the one observed for enzymatic aldol condensation, where enzymes of Class 1 react by forming Schiff-base intermediates, whereas enzymes of Class II show metal ion requirements [47]. Oxaloacetate decarboxylase from cod also catalyses the reduction by borohydride of the enzymatic reaction product pyruvate. This is evidenced by the accumulation of D-lactate in presence of enzyme, reducing agent, and manganous ions. It has been proposed that both reduction and decarboxylation occur by way of an enzyme-metal ion-substrate complex in which the metal ion acts as an electron sink, thereby stabilizing the enolate ion formed in the decarboxylation reaction [48] ... 

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