Diatom carbonic anhydrase

Roberts et al. reported a 27 kDa monomeric carbonic anhydrase, TWCAl, from the marine diatom Thalassiosira weissflogii (221). X-ray absorption spectroscopy indicated that the catalytic zinc is coordinated by three histidines and a water molecule, similar to the active sites of the a- and y-CAs (222). Also, the active site geometry is similar to that of a-CAs. Based on these results the catalytic mechanism is expected to be similar to that of the -class carbonic anhydrases. Tripp et al. (223) proposed that this TWCAl is the prototype of a fourth class carbonic anhydrase designated as 8-class CAs. In the... 

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. 

Figure 10 A hypothetical model of carbon acquisition in the marine diatom Thalassiosira weissflogii. Solid circles represent transporters. Catalyzing enzymes CA, carbonic anhydrase PEPC, phosphoenol pyruvate carboxylase PEPCK phospoenol pyruvate carboxykinase RUBISCO, Ribulose 1-5 bisphosphate Carboxylase Oxygenase (after...

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).

Figure 21 Active centers of carbonic anhydrase (CA) from the marine diatom Thalassiosira weissflogii. The zinc and cobalt centers are found in the protein TWCAl (Cox et al., 2000) Cox, unpublished data). The Cd center is found in the protein CdCA and is hypothetically based on unpublished XANES data showing sulfur binding and unpublished protein sequence data.

Lane T. W. and Morel E. M. M. (20(X)b) Regulation of carbonic anhydrase expression by zinc, cobalt, and carbon dioxide in the marine diatom Thalassiosira weissflogii. Plant Physiol. 123, 345-352. 

Roberts S. B., FaneT. W., and Morel F. M. M. (1997) Carbonic anhydrase in the marine diatom Thalassiosira weissflogii (Bacillariophyceae). J. Phycol. 33, 845-850. 

The trace metals Ni, Zn, Co, and Cd are essential elements for enz5mies that carry out various functions of metabolism. Zinc is the most predominant metal in the enzyme carbonic anhydrase (CA), which catalyzes the transformation of HCO3 to CO2. This mechanism is important in the sea because the pool of HCO3 contains 100 times more carbon than the pool of CO2 at the pH of seawater, and it is usually CO2 that is reduced enzymatically to organic carbon. Diatoms and some cyanobacteria also use CA to concentrate CO2, and it has been observed that in some cases both Co and Cd can substitute for Zn in the carbonic anhydrase enzyme. 

The Idnetic rate constants for CO2 hydration determined in the laboratory in sterile seawater (Table 4.6) are known sufficiently well that this value should create little uncertainty in the above calculation. However, in natural waters the reaction rates may be enzymatically catalyzed. Carbon dioxide hydration catalysis by carbonic anhydrase (CA) is the most powerful enzyme reaction known (see the discussion in Section 9.3). The catal5dic turnover number (the number of moles of substrate reacted, divided by the number of moles of enz5mie present) is 8 x 10 min for CA (Table 9.7), and marine diatoms are loiown to produce carbonic anhydrase (Morel et al, 1994). The calculations presented in Fig. 10.14 indicate that increasing the CO2 hydration rate constant by 10-fold should increase the gas exchange rate of CO2 in the ocean by 10%-50%. 

The marine diatom Thalassiosira weissflogli has a Cd -containing carbonic anhydrase. 

Cadmium, again with a much higher affinity to thiol compounds, is a toxic-only element. Only one case of Cd(II) as a trace element has been reported in the absence of Zn(II), Cd(II) was used by some diatoms in the active site of carbonic anhydrase (Lane and Morel 2000). Cd(II) enters the cell by (CorA- and NRAMP-like) uptake systems (Nies 1999), binds to thiol compounds (thereby exerting toxicity), and is then reexported by efflux systems (P-, CBA, or CDF type) (Nies 2003). 

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]. 

After many years of research, the first protein that uses Cd naturally has been discovered in marine diatoms a carbonic anhydrase with Cd as its catalytic center (CDCA) [9,10]. It appears that CDCA plays a pivotal role in the acquisition of inorganic carbon in diatoms, and thus the use of Cd in CDCA provides a link between the biogeochemical cycles of carbon and Cd. The existence of CDCA is an example of the unique mechanisms phytoplankton have evolved over geological times as an adaptation to life in the metal-poor environment of surface seawater. But CDCA may not be the only biological use of Cd in seawater. While we are beginning to understand how and how much Cd is utilized by phytoplankton cells, there are still many challenging questions that need to be answered. 

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. 

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