Cadmium carbonic anhydrase

In these microbes, cadmium sticks in the place of zinc and does zinc s chemistry. Y. Xu et al. Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms. 2008. Nature 452(7183), p. 56. DOI 10.1038/nature06636. 

Cadmium is a toxic element (see Chapters 1,14,15) that accumulates especially in kidney and liver [4] being bound preferably to metallothionein (Chapters 6,11). On the other hand, the chemical similarity of Cd " " to Zn " is confirmed by the fact that carbonic anhydrase of marine phytoplankton contains Cd (Chapter 16), whereas the corresponding zinc enzymes are found in organisms from aU kingdoms [5] catalyzing the reversible hydration of carbon dioxide. In marine diatoms cadmium, cobalt, and zinc can functionally substitute for one another to maintain optimal growth [6]. Cadmium-carbonic anhydrase is involved in the acquisition of inorganic carbon for photosynthesis [6]. 

Keywords cadmium carbonic anhydrase phytochelatin phytoplankton uptake... 

Cadmium carbonic anhydrase (CDC A) is the first member of a new class of carbonic anhydrases, the class. CDCAl, which uses Cd as its metal cofactor when Zn is limiting, was isolated from the marine diatom T. weissflogii. The amino acid sequence of CDCAl contains a triple repeat with 85% identity between repeats [10]. CDCAl is a key enzyme in the carbon concentrating mechanism (CCM) through which T. weissflogii increases the concentration of CO2 at the site of fixation byRuBisCO [92]. 

The volume terminates with Chapter 16 in which also the essentiality of Cd " for certain diatoms is pointed out. The distribution of Cd " in the ocean is very similar to that of major nutrients suggesting that it may be taken up by marine phytoplankton at the surface and remineralized at depth. At high concentration, Cd is toxic to phytoplankton as it is for many organisms. However, at relatively low concentrations, Cd " can enhance the growth of a number of phytoplankton species under Zn limitation possibly Cd is taken up either by the Mn or the Zn transport system. The otdy known biological function of Cd is to serve as a metal ion cofactor in cadmium-carbonic anhydrase (CDCA) in diatoms. The expression of CDCA is regulated by the availabilities of Cd " and Zn " both Zn " and Cd can be used as the metal ion cofactor and be exchanged for each other in certain marine phytoplankton species. 

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. 

It should be noted that activity of cadmium-substituted carbonic anhydrase may be enhanced at higher pH values (ca. pH 9). See Ref. (182b) and Bauer, R. Limkilde, P. Johansen, J. T. Biochemistry 1976,15, 334. 

The X-ray structure of the unsubstituted tris(pyrazolyl)borato zinc nitrate has been solved showing a unidentate coordination mode for nitrate, in contrast with the t-butyl substituted ligand, which shows anisobidentate nitrate coordination due to the steric effects.232 A partial explanation of the reduced activity of cadmium-substituted carbonic anhydrase is offered by Parkin on the basis of the comparison of nitrate coordination to cadmium and zinc trispyrazo-lylborate moieties. A contributing factor may be the bidentate coordination supported by the cadmium that does not allow the facile access to a unidentate bicarbonate intermediate, which could be highly important to carbonic anhydrase activity.233... 

Although zinc, cadmium, and mercury are not members of the so-called main-group elements, their behavior is very similar because of their having complete d orbitals that are not normally used in bonding. By having the filled s orbital outside the closed d shell, they resemble the group IIA elements. Zinc is an essential trace element that plays a role in the function of carboxypeptidase A and carbonic anhydrase enzymes. The first of these enzymes is a catalyst for the hydrolysis of proteins, whereas the second is a catalyst for the equilibrium involving carbon dioxide and carbonate,... 

From a nutritional viewpoint, Cu2+ competes with zinc ion, as does the very toxic Cd2+. The latter accumulates in the cortex of the kidney. Dietary cadmium in concentrations less than those found in human kidneys shortens the lives of rats and mice. However, some marine diatoms contain a cadmium-dependent carbonic anhydrase.11 Although zinc deficiency was once regarded as unlikely in humans, it is now recognized as occurring mider a variety of circumstances0 p and is well-known in domestic animals.01 Consumption of excessive amounts of protein as well as alcoholism, malabsorption, sickle cell anemia, and chronic kidney disease can all be accompanied by zinc deficiency. 

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). 

Armitage, I.M., Pajer, R.T., Uiterkamp, A.J.M.S., Chlebowski,J.F. and Coleman, J.E. (1976) Cadmium-133 Fourier Transform nuclear magnetic resonance of cadmium(II) carbonic anhydrase and cadmium(II) alkaline phosphatase./. Am. Chem. Soc., 98, 5710-5712. 

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. 

Phytoplankton particulate matter (organic and biomineralized) contains many trace elements. The most abundant are magnesium, cadmium, iron, calcium, barium, copper, nickel, zinc, and aluminum (Table 1), which are important constituents of enzymes, pigments, and structural materials. Carbonic anhydrase requires zinc or cadmium (Price and Morel, 1990 Lane and Morel, 2000), nitrate reductase requires iron (Geider and LaRoche, 1994), and chlorophyll contains magnesium. Additionally, elements such as sodium, magnesium, phosphorus, chlorine, potassium, and calcium may be present as ions... 

Cadmium is generally considered to be toxic to organisms and how the marine phytoplankton utilize their cadmium is unknown. Cadmium may substitute for zinc in carbonic anhydrase at times when zinc is limiting (Price and Morel, 1990 Lane and Morel, 2000). It is possible that cadmium may play a role in polyphosphate bodies, a form of cellular storage of phosphorus that has been shown to contain significant quantities of elements such as calcium, zinc, and magnesium (Ruiz et al, 2001). 

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). 

Figure 20 Relative levels of carbonic anhydrase (CA) activity (A), amounts of TWCAl protein (a zinc containing CA) and amounts of CdCA protein (a cadmium containing CA) in the marine diatom Thalassiosira weissflogii as a function of metal treatment and CO2 levels. -hZn =15 pM Zn, —Zn = 3 pM Zn, -hCo = 21 pM Co, -hCd = 45 pM Cd (after Morel et al, 2002).

---