Metals, coenzyme

Gantzer CJ, LP Wackett (1991) Reductive dechlorination catalyzed by bacterial transition-metal coenzymes. Environ Sci Technol 25 715-722. 

Once formed, the metal coenzymes are more easily selected by proteins than the parent ions due to the idiosyncratic nature of each ring. 

Schanke and Wackett [379] reported TeCA degradation by transition-metal coenzymes. cDCE (53%), tDCE (29%), VC (14%), ethylene (1%), and traces of 1,1,2-TCA were the products from this abiotic transformation with vitamin B12 and titanium(III) citrate. Both dechlorination and dichloroelimination had occurred the major route of degradation was reductive dihaloelimina-tion. 

Table I lists isomorphous replacements for various metalloproteins. Consider zinc enzymes, most of which contain the metal ion firmly bound. The diamagnetic, colorless zinc atom contributes very little to those physical properties that can be used to study the enzymes. Thus it has become conventional to replace this metal by a different metal that has the required physical properties (see below) and as far as is possible maintains the same activity. Although this aim may be achieved to a high degree of approximation [e.g., replacement of zinc by cobalt(II)], no such replacement is ever exact. This stresses the extreme degree of biological specificity. The action of an enzyme depends precisely on the exact metal it uses, stressing again the peculiarity of biological action associated with the idiosyncratic nature of active sites. (The entatic state of the metal ion is an essential part of this peculiarity.) Despite this specificity, the replacement method has provided a wealth of information about proteins that could not have been obtained by other methods. Clearly, there will often be a compromise in the choice of replacement. Even isomorphous replacement that should retain structure will not necessarily retain activity at all. However, it has become clear that substitutions can be made for structural studies where the substituted protein is inactive (e.g., in the copper proteins and the iron-sulfur proteins). It is also possible to substitute into metal coenzymes. Many studies have been reported of the...

Schanke, G A. Wackett, L. P. (1992). Environmental reductive elimination reactions of polychlorinated ethanes mimicked by transition-metal coenzymes. Environmental Science Technology, 26, 830—3. 

NAD. The several possibilities for Zn -NAD interaction have been reviewed. However, the wide range of techniques that have been applied have failed to give conclusive evidence for direct metal-coenzyme interaction. Many studies have been inconclusive or contradictory. On the other hand there is some positive evidence for the maintenance of a four-coordinate geometry for the metal in the binary complex. Thus the shape and intensity of the electronic spectrum of Co(c)2Zn(n)2-LADH (discussed below) are consistent with the expected tetrahedral structure for the catalytic metal ion. However, there are no major changes in the spectrum of the binary complex, suggesting that the four-coordinate structure is maintained and that the coenzyme does not bind to the metal. 

Enzyme Abbre- viation Mole- cular weight Per cent metal Metal/ Coen-Mole- zyme/ cular Molecular Weight Weight Metal/ Coenzyme Jhnpirical formula References... 

Reaction inhibitors slow reaction rates. Nitrogen mineralization and nitrification (conversion of organic nitrogen and ammonium to nitrate) rates in soils, for example, can be slowed temporarily by chemicals that specifically slow or stop the microorganisms involved. Toxic metals can also operate as enzyme inhibitors, by replacing the metal coenzyme portion of an enzyme and thereby inactivating it. 

Several chemical substances have been proposed as electron mediators including natural organic matter (NOM) (Dunnivant et al., 1992), iron porphyrins (Baxter, 1990 Holmstead, 1976 Khalifa et al., 1976 Quirke et al., 1979 Klecka and Gon-sior, 1984 Wade and Castro, 1973), corrinoids (Krone et al., 1989) and bacterial transition-metal coenzymes such as vitamin B,2 and hematin (Gantzer and Wackett,... 

Table 4. Experimental evidence for ribonucleotide reduction and its metal (coenzyme) requirement in various groups of organisms...

Many similarities, and strong dissimilarities of the known enzymes have been mentioned above. They are all identical in that a nucleotide-binding protein, with redox-active cysteine/cystine residues, combines with another metal-containing polypeptide or a metal coenzyme in which radical intermediates can be generated and stabilized. The binding protein carries several nucleotide sites so that reaction rates are influenced by allosteric effects, with the same specificity pattern everywhere. On closer inspection it becomes apparent that the considerable individual differences in subunit composition and in the nature of the second, catalytic component can indeed be integrated into a general concept of ribonucleotide reduction. 

The biochemistry tmderlying reductive dechlorination was recently examined in vitro using a series of transition-metal coenzymes and other biochemical reductants [49]. Vitamin and coenzyme F... 

Unlike flavins and metal coenzymes, the nicotinamide nucleotide coenzymes do not remain bound to the enzyme, but act as substrates, binding to the enzyme, undergoing reduction and then leaving. The reduced coenzyme is then reoxidized either by reaction with another enzyme, for which it acts as a hydrogen donor, or by way of the mitochondrial electron transport chain (section 3.3.1.2). Cells contain only a small amount of NAD(P) (of the order of400 nmol/g in liver), which is rapidly cycled between the oxidized and reduced forms by different enzymes. 

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