Cytosolic carbonic anhydrase

Bayram, E. Senturk, M. Kufrevioglu, O.I. Supuran, CT. (2008). In vitro inhibition of salicylic acid derivatives on human cytosolic carbonic anhydrase isozymes I and II. Bioorganic and Medicinal Chemistry, 16,9101-9105. 

Carbonic anhydrase (CA, also called carbonate dehydratase) is an enzyme found in most human tissues. As well as its renal role in regulating pH homeostasis (described below) CA is required in other tissues to generate bicarbonate needed as a co-substrate for carboxylase enzymes, for example pyruvate carboxylase and acetyl-CoA carboxylase, and some synthase enzymes such as carbamoyl phosphate synthases I and II. At least 12 isoenzymes of CA (CA I—XII) have been identified with molecular masses varying between 29 000 and 58 000 some isoenzymes are found free in the cytosol, others are membrane-bound and two are mitochondrial. 

Carbonic anhydrase is a metalloprotein with a co-ordinate bonded zinc atom immobilized at three histidine residues (His 94, His 96 and Hisl 19) close to the active site of the enzyme. The catalytic activity of the different isoenzymes varies but cytosolic CA II is notable for its very high turnover number (Kcat) of approximately 1.5 million reactions per second. 

Figure 3. A schematic diagram of the renal proximal tubule showing how the Na+/H+ exchanger, by acidifying tubular fluids, promotes the regeneration of bicarbonate. Interconversions of C02, water, H+, OH and HCOj are catalyzed by carbonic anhydrase (membrane bound and cytosolic).

CO2 can diffuse across the plasma membrane and become hydrated to HC03 via the enzyme carbonic anhydrase, which occurs in the cytosol of mesophyll cells of C4 plants. Hie Michaelis constant of PEP carboxylase for HC03 is about 200 mmol m-3 (0.2 mM), which suggests that the cytosolic pH must be above 7 to get sufficient HC03 formation (see Table 8-3) to match the COz uptake rates of C4 plants. 

The overall process by which parietal cells acidify the stomach lumen is Illustrated in Figure 7-28. In a reaction catalyzed by carbonic anhydrase the excess cytosolic OH combines with CO2 that diffuses in from the blood, forming HCO3 . Catalyzed by the basolateral anion antiporter, this bicarbonate ion is exported across the basolateral membrane (and ultimately into the blood) in exchange for a Cl ion. The Cl ions then exit through Cl channels in the apical membrane, entering the stomach lumen. To preserve electroneutrality, each Cl ion that moves into the stomach lumen across the apical membrane is accompanied by a ion that moves outward through a separate channel. In this way, the excess ions pumped Inward by the H /K ... 

H /K ATPase and exit via a channel. The basolateral membrane contains an anion antiporter that exchanges HCOT and cr ions. The combined operation of these four different transport proteins and carbonic anhydrase acidifies the stomach lumen while maintaining the neutral pH and electroneutrality of the cytosol. See the text for more details. 

The combined action of carbonic anhydrase and four different transport proteins permits parietal cells in the stomach lining to secrete HCl into the lumen while maintaining their cytosolic pH near neutrality (see Figure 7-28). 

F. Abbate, A. Casini, T. Owa, A. Scozzafava and C. T. Supuran, Carbonic anhydrase inhibitors E7070, a sulfonamide anticancer agent, potently inhibits cytosolic isozymes 1 and 11, and transmembrane, tumor-associated isozyme IX, Bioorg. Med. Chem., 2004,14, 217-223. 

Carbonic anhydrase P increases in cytosol Atkins (1974) Christeller et al. (1977)... 

The distribution of carbonic anhydrase is also in accord with the alkaline stress theory. This enzyme is highly active in C3 chloroplasts whereas it is absent from C4 bundle-sheath chloroplasts conducting carboxlation - nor is it needed there. Bicarbonate generated on nitrite reduction in the mesophyll chloroplasts could be transferred to the cytosol at a leisurely rate for PEP carboxylase activity there. ... 

A set of bisphosphonate matrix metalloproteinase (MMP) inhibitors (666) has been investigated for inhibitoiy activity against several carbonic anhydrase (CA) isozymes. Some of the bisphosphonates (666) have been found to selectively inhibit the isoforms involved in bone remodeling, over the ubiquitous cytosolic CA I. ... 

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