Carbonic anhydrase metal-substituted

Carbonic anhydrase an insight into the zinc binding site and into the active cavity through metal substitution. I. Bertini, C. Luchinat and A. Scozzafava, Struct. Bonding (Berlin), 1982, 48, 46-92 (296). 

Nickel is required by plants when urea is the source of nitrogen (Price and Morel, 1991). Bicarbonate uptake by cells may be limited by Zn as HCOT transport involves the zinc metal-loenzyme carbonic anhydrase (Morel et al., 1994). Cadmium is not known to be required by organisms but because it can substitute for Zn in some metalloenzymes it can promote the growth of Zn-limited phytoplankton (Price and Morel, 1990). Cobalt can also substitute for Zn but less efficiently than Cd. 

Bertini I, Luchinat C, Scozzafava A (1982) Carbonic Anhydrase An Insight into the Zinc Binding Site and into the Active Cavity Through Metal Substitution. 48 45-91 Bertrand P (1991) Application of Electron Transfer Theories to Biological Systems. 75 1-48 Bill E, see Trautwein AX (1991) 78 1-96 Bino A, see Ardon M (1987) 65 1-28 Blanchard M, see Linares C (1977) 33 179-207 Blasse G, see Powell RC (1980) 42 43-96... 

Bertini, /., Luchinat, C., Scozzafava, A. Carbonic Anhydrase An Insight into the Zinc Binding Site and into the Active Cavity Through Metal Substitution. Vol. 48, pp. 45-91. 

This suggestion is not necessarily intended to imply that the bicarbonate intermediate of the carbonic anhydrase cycle must exhibit unidentate coordination in the ground state. Rather, it is intended to imply that a unidentate species should be readily accessible. Moreover, it is not intended to suggest that this factor alone is responsible for influencing the activity of metal-substituted carbonic anhydrases,... 

As an example of tetra-coordinate cobalt(II) systems, the NMRD profile of cobalt(II)-substituted carbonic anhydrase (MW 30,000) at high pH is reported (Fig. 14). The metal ion is coordinated to three histidines and to a hydroxide ion (48). The NMRD profile shows a cos Cg dispersion centered around 10 MHz, which qualitatively sets the correlation time around 10 s. As the reorientational correlation time of the molecule is much longer, this value is a measure of the effective electronic relaxation time. A quantitative... 

Substitution of foreign metals for the metals in metalloenzymes (those that contain metals as part of their structures) is an important mode of toxic action by metals. A common mechanism for cadmium toxicity is the substitution of this metal for zinc, a metal that is present in many metalloenzymes. This substitution occurs readily because of the chemical similarities between the two metals (for example, Cd2+ and Zn2+ behave alike in solution). Despite their chemical similarities, however, cadmium does not fulfill the biochemical function of zinc and a toxic effect results. Some enzymes that are affected adversely by the substitution of cadmium for zinc are adenosine triphosphate, alcohol dehydrogenase, and carbonic anhydrase. 

It is known that a vast variety of enzymes use metal ions in acid/base catalysts. In some cases the role of the metal is to activate water directly, e.g. Zn(OH)2 becomes Zn(OH ) in carbonic anhydrase, but in others it may be that the metal just forms a particularly constructive (useful) H-bond network, e.g. calcium in phospholipase A2 and in staph, nuclease. Substitution of one metal by other metals is now a critical test of the precision of the catalytic site and we know that nickel does not substitute for zinc in carbonic anhydrase, although it binds, and that Sr(II) has a different activity in lipases and nucleases from Ca(II). It is the water in the coordination sphere which is partly responsible for these changes. 

It appears that cobalt plays a particularly important role in the growth of cyanobacteria (Saito et al, 2002 Sunda and Huntsman, 1995b). Both Prochlorococcus and Synechococcus show an absolute cobalt requirement that zinc cannot substitute for (Figure 18(a)). The growth rate of Synechococcus is little affected by low zinc concentrations, except in the presence of cadmium which then becomes extremely toxic (Saito et al, personal communication). The biochemical processes responsible for the major cellular utilization of zinc and cobalt in marine cyanobacteria are unknown, however. These metals may be involved in carbonic anhydrase and/or other hydrolytic enzymes. Cobalamin (vitamin B12) synthesis is a function of cobalt in these organisms, yet B12 quotas tend to be very small (on the order of only 0.01 p.mol (mol C) ) and hence are not likely represent a significant portion of the cellular cobalt (Wilhelm and Trick, 1995). 

In the presence of human serum albumin, the H spectrum of acetyl-salicyclic acid is specifically shifted and broadened [119]. The interpretation of changes in T, and T2 require several theoretical assumptions. These have been discussed in detail [120] for JV-acetylsulphanilamide and acetate binding to the active site of carbonic anhydrase. It was concluded that the acetyl groups of these inhibitors have a motion additional to that of the enzyme. It can be shown by NMR that acetate binds to two sites on the enzyme, only one of which is inhibitory to esterase activity (methyls are 4.3 and 4.8 A from the metal in the Mn substituted enzyme [121]). Strict care must be taken to avoid paramagnetic impurities when NMR relaxation enhancement by diamagnetic macromolecules is being studied. A preparation of carbonic anhydrase, for example, can contain 0.24 paramagnetic Cu atoms per Zn atom [122]. 

Cobalt has recently been used as an ESR active substitute in zinc metalloenzymes. Whilst liquid helium temperatures may be needed and theoretical aspects of the spectra are not yet as well understood, cobalt has two important advantages over copper as a metal substitute, namely that many cobalt derivatives show some enzymic activity (e.g. cobalt in carbonic anhydrase, alkaline phosphatase and superoxide dismutase) and that g values and hyperfine splitting are more sensitive to ligand environment, particularly when low spin. ESR data have been reported for cobalt substituted thermolysin, carboxypeptidase A, procarboxypeptidase A and alkaline phosphatase [51]. These are all high spin complexes. Cobalt carbonic anhydrase has been prepared and reacted with cyanide [52]. In... 

Numerous metalloenzymes have the ability to remain functional even after the metal, which presumably is present at their active center, has been replaced by another metal (13). Thus in zinc deficiency, if the apoenzyme is synthesized, as has been observed in the case of . coli alkaline phosphatase (13), then other metals which might have accumulated or are normally within the cell could substitute for zinc and generate an active enzyme. Although this is a possibility in the case of microorganisms, it certainly does not appear to be true in the case of experimental animals and man, in that the apoenzymes of alkaline phosphatase, carbonic anhydrase, carboxypeptidase, alcohol dehydrogenase, and de-oxythymidine kinase do not accumulate in zinc-deficient tissues. Thus, one may conclude that a deficiency of zinc does specifically aflFect the activities of zinc-dependent enzymes in sensitive tissues. 

From X-ray crystal structures the AT of the imidazole rings of the two histidine residues are coordinated to the Cu in plastocyanin (3, 4, 7), azurin (8), pseudoazurin (12), and CBP il3). However, in studies on Co(II)-substituted stellacyanin (71), it has been demonstrated that both histidines bind the metal via the N atom. Similar differences have been observed in the case of binuclear Fe proteins for example. Thus in ribonucleotide reductase the of histidine is coordinated, whereas in hemerythrin it is the N atom which is involved (85). In carbonic anhydrase the two coordinated imidazoles have and N atoms respectively bonded to the same Zn (85). The differences are most likely attributable to steric factors involving the polypeptide. 

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