Calcium carbonate, carbonic anhydrase

Enzyme active sites, 136,148, 225. See also Protein active sites in carbonic anhydrase, 197-199 in chymotrypsin, 173 in lysozyme, 153, 157 nonpolar (hypothetical site), 211-214 SNase, 189-190,190 steric forces in, 155-158, 209-211, 225 in subtilisin, 173 viewed as super solvents, 227 Enzyme cofactors calcium ... 

Figure 48-12. Schematic illustration of some aspects of the role of the osteoclast in bone resorption. Lysosomal enzymes and hydrogen ions are released into the confined microenvironment created by the attachment between bone matrix and the peripheral clear zone of the osteoclast. The acidification of this confined space facilitates the dissolution of calcium phosphate from bone and is the optimal pH for the activity of lysosomal hydrolases. Bone matrix is thus removed, and the products of bone resorption are taken up into the cytoplasm of the osteoclast, probably digested further, and transferred into capillaries. The chemical equation shown in the figure refers to the action of carbonic anhydrase II, described in the text. (Reproduced, with permission, from Jun-queira LC, Carneiro J BasicHistology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

In the other subdivision, water activation occurs in the first step of the enzymatic cycle. This activation is achieved by a carboxylate group in aspartic hydrolases (Fig. 3.10), Zn2+ and a carboxy group in metallopep-tidases (Fig. 3.12 ), a histidine side chain in calcium-dependent hydrolases (Fig. 3.14), or a Zn2+ in carbonic anhydrase (Fig. 3.15). The substrate, on the other hand, is polarized (activated) by a carboxy group in aspartic hydrolases or by a cation in metallopeptidases and calcium-dependent hydrolases. In this manner, the reactivity of both the water molecule and the substrate is enhanced and fine-tuned to drive formation of a tetrahedral intermediate that will break down to form the hydrolysis products. 

Phosphaturia and hypercalciuria occur during the bicarbonaturic response to inhibitors of carbonic anhydrase. Renal excretion of solubilizing factors (eg, citrate) may also decline with chronic use. Calcium salts are relatively insoluble at alkaline pH, which means that the potential for renal stone formation from these salts is enhanced. 

A third problem with the mitochondrial theory of biomineralization is that many mineralized tissues contain carbonate rather than phosphate. Since bicarbonate ions do not pass across mitochondrial membranes with any ease, it has now been shown that in phosphate-free buffers, calcium will enter mitochondria if dissolved carbon dioxide is available. It appears that some mitochondria possess carbonic an-hydrase activity on the inner membrane or in the mitochondrial matrix and are thus able to synthesize bicarbonate within the organelle. In such cases, inhibitors of carbonic anhydrase block the accumulation of calcium and carbonate ions622) since crystals of calcite have been identified in the mitochondria of earthworms calci-ferous glands623. These cells freqently showed spherical granules in the cytoplasm and lumen of the glands during phases of mineral secretion and it was suggested that they were aspects of cellular breakdown which occurred at these times. 

Isoenzymes III and VII have a more specialized distribution. Carbonic anhydrase III is abundant in adipocytes which use bicarbonate in fatty acid synthesis.7 Isoenzyme V is present in the mitochondrial matrix and is also abundant in both adipocytes and liver.7 8 Isoenzyme IV is a larger membrane-associated form, while VI is secreted into the saliva.10 Carbonic anhydrase has also been identified in E. coli.,11 in a methanobacterium,12 and in green plants.13 133 A 60-kDa carbonic anhydrase called nacrein is found in the organic matrix of the nacreous layer of the pearl oyster, the layer that forms aragonite (orthorhombic calcium carbonate) in the shell and in pearls.14... 

Other actions of some anticonvulsants include inhibition of the enzyme carbonic anhydrase, negative modulation of calcium channel activity, and actions on second messenger systems, including inhibition of phosphokinase C. Beyond the second messenger, there is the possibility that second messenger systems may be affected, analogously to what is hypothesized for lithium. 

FIGURE 7-27. Shown here is an icon of topiramate s pharmacologic actions. By interfering with calcium channels and sodium channels, topiramate is thought both to enhance the inhibitory actions of gamma aminobutyric acid (GABA) and to reduce the excitatory actions of glutamate. Topiramate is also a carbonic anhydrase inhibitor (CAI) and as such has independent anticonvulsant actions. 

By analogy with other calcium-binding proteins, it is expected that the Ca2+-bound intestinal protein is then able to interact with some receptor protein. Suggestions have been made for interaction with alkaline phosphatase or carbonic anhydrase, but another possibility is that the calcium-bound protein is able to activate a gating mechanism for the transport of calcium. 

Several lines of evidence indicate that carbonic anhydrase is related to calcification in two possible ways (1) as a simple catalyst of C02 hydration, and (2) as a protector of fixed calcium at the nucleation site. The idea has even been advanced that in calcareous algae photosynthesis is the driving force in carbonate deposition by consuming C02 molecules from an intracellular pool of bicarbonate. 

Carbonic anhydrase seems to be not only a catalyst in calcification but also a vehicle for the translocation of reserve calcium. In the mantle of freshwater clams, for instance, the ratio of bound to free calcium ions is in the order of 10 to 127S 276. A series of experiments revealed that the dynamic equilibrium between the pools of ionized and combined calcium is entirely controlled by carbonic anhydrase. 

The enzyme functions as a kind of chemical homeostaf by releasing calcium and carbonate to the extrapallial fluid from the reserves present in the mantle tissue. Because of the extreme sensitivity of the carbonate system to slight changes in pH (see p. 19, Fig. 14), carbonic anhydrase must operate in both directions in order to maintain a dynamic equilibrium between the bound and free calcium pools. 

Deposition of mineral matter is limited by diffusion of calcium and/or phosphorus to the site of deposition400-. Since the transfer of both compounds is enzymatically controlled (see p. 21) equilibrium relationships may change environmental settings critical for the deposition of minerals. For instance, by limiting carbonic anhydrase activity in one direction, a pool of H2C03 may build up at the site of deposition in the reverse situation HC03 will concentrate. As a consequence,... 

For the leaching and carbonation of calcium, the enzyme carbonate anhydrase may be used to catalyze the chemistry. A careful control of pH and dissolved metallic species is required, however. 

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. 

When distances of micrometers are involved, and the ions are in solution, diffusion becomes very important. There are several indications that one biological strategy in diffusion control is to separate temporally, and maybe even spatially, the introduction of the cation and anion components into the microenvironment of mineralization. Several observations point to the fact that many of the charged matrix macromolecules that are components of the preformed matrix framework, do not adopt a regular conformation unless calcium is present as a counterion [39, 95]. This implies that the framework loads up first with the cation, and then only when the anion is introduced does mineralization begin. This may well be the reason why the enzymes that control the concentrations of the relevant forms of calcium carbonate and calcium phosphate (carbonic anhydrase and alkaline phosphatase respectively) are so intimately associated with the mineralization site [27, 96]. This is clearly an excellent strategy for maintaining control. 

CARBONIC ANHYDRASE INHIBITORS ANTIEPILEPTICS- BARBITURATES, PHENYTOIN Risk of osteomalacia Barbiturates and phenytoin have a small risk of causing osteomalacia this may be t by acetazolamide-induced urinary excretion of calcium Be aware... 

CAIs alter renal function primarily by inhibiting carbonic anhydrase in the proximal tubule, which results in decreased bicarbonate reabsorption. The net effect of the renal actions of acetazolamide therapy is alkaliniza-tion of the urine and metabolic acidosis. Metabolic acidosis results from the initial bicarbonate loss and persists with continued acetazolamide use. Moderate metabolic acidosis develops in most patients. Reabsorption of bicarbonate independent of carbonic anhydrase prevents severe acidosis. Initially, acetazolamide produces diuresis, but urinary output decreases with the development of metabolic acidosis. In addition, decreased urinary citrate excretion follows acetazolamide therapy and has been attributed to the metabolic acidosis it produces. A high urinary pH and low urinary citrate concentration are conducive to precipitation of calcium phosphate in both the renal papillae and the urinary tract. 

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). 

Step 1 A water molecule is attracted to the zinc ion at the active site of carbonic anhydrase. The positively charged zinc ion displaces a proton from the water molecule. The displaced proton finds a new place of residence — the histidine residue. This histidine residue prt>bably aids in the removal of the proton from the water molecule, in concert wdth the action of the zinc ion. Combination of the zinc ion (Zn ) vedth the hydroxyl group does not form a complex with the structure Zn OH. The zinc atom does not change its valence (Its number of charges). Instead, the complex has the structure Zn (OH ). [Calcium ions behave similarly to zinc ions. In contrast, iron and copper ions readily change their valences when they participate in biochemical reactions.)... 

By inhibiting carbonic anhydrase, topiramate reduces the urinary excretion of citrate and increases urinary pH, leading to higher calcium phosphate saturation and a risk of nephrolithiasis. During 1074 patient-years of topiramate exposure in 1183 patients, 18 (1.5%) had 21 episodes of renal calcuh, suggesting an incidence of nephrohthiasis comparable to that reported for acetazol-amide (SEDA-20, 66). 

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