Bicarbonate binding activity of the CmpA protein of the cyanobacterium Synechococcus sp. strain PCC 7942 involved in active transport of bicarbonate

The cmpABCD operon of the cyanobacterium Synechococcus sp. strain PCC 7942 encodes an ATP-binding cassette transporter involved in HCO(3)(-) uptake. The three genes, cmpBCD, encode membrane components of an ATP-binding cassette transporter, whereas cmpA encodes a 42-kDa cytoplasmic membrane protein, which is 46.5% identical to the membrane-anchored substrate-binding protein of the nitrate/nitrite transporter. Equilibrium dialysis analysis using H(14)CO(3)(-) showed that a truncated CmpA protein lacking the N-terminal 31 amino acids, expressed in Escherichia coli cells as a histidine-tagged soluble protein, specifically binds inorganic carbon (CO(2) or HCO(3)(-)). The addition of the recombinant CmpA protein to a buffer caused a decrease in the concentration of dissolved CO(2) because of the binding of inorganic carbon to the protein. The decrease in CO(2) concentration was accelerated by the addition of carbonic anhydrase, indicating that HCO(3)(-), but not CO(2), binds to the protein. Mass spectrometric measurements of the amounts of unbound and bound HCO(3)(-) in CmpA solutions containing low concentrations of inorganic carbon revealed that CmpA binds HCO(3)(-) with high affinity (K(d) = 5 microm). A similar dissociation constant was obtained by analysis of the competitive inhibition of the CmpA protein on the carboxylation of phosphoenolpyruvate by phosphoenolpyruvate carboxylase at limiting concentrations of HCO(3)(-). These findings showed that the cmpA gene encodes the substrate-binding protein of the HCO(3)(-) transporter.     

NasFED Proteins Mediate Assimilatory Nitrate and Nitrite Transport in Klebsiella oxytoca (pneumoniae) M5al

Klebsiella oxytoca can use nitrate and nitrite as sole nitrogen sources. The enzymes required for nitrate and nitrite assimilation are encoded by the nasFEDCBA operon. We report here the complete nasFED sequence. Sequence comparisons indicate that the nasFED genes encode components of a conventional periplasmic binding protein-dependent transport system consisting of a periplasmic binding protein (NasF), a homodimeric intrinsic membrane protein (NasE), and a homodimeric ATP-binding cassette (ABC) protein (NasD). The NasF protein and the related NrtA and CmpA proteins of cyanobacteria contain leader (signal) sequences with the double-arginine motif that is hypothesized to direct prefolded proteins to an alternate protein export pathway. The NasE protein and the related NrtB and CmpB proteins of cyanobacteria contain unusual variants of the EAA loop sequence that defines membrane-intrinsic proteins of ABC transporters. To characterize nitrate and nitrite transport, we constructed in-frame nonpolar deletions of the chromosomal nasFED genes. Growth tests coupled with nitrate and nitrite uptake assays revealed that the nasFED genes are essential for nitrate transport and participate in nitrite transport as well. Interestingly, the ΔnasF strain exhibited leaky phenotypes, particularly at elevated nitrate concentrations, suggesting that the NasED proteins are not fully dependent on the NasF protein.     

The nitrate/nitrite ABC transporter of Phormidium laminosum: phosphorylation state of NrtA is not involved in its substrate binding activity

Most cyanobacteria take up nitrate or nitrite through a multisubunit ABC transporter (ATP-binding cassette) located in the cytoplasmic membrane. Nitrate and nitrite transport activity is instantaneously blocked by the presence of ammonium in the medium. Previous biochemical studies reported the existence of phosphorylation/dephosphorylation events of the nitrate transporter (NRT) related to the presence of ammonium-sensitive kinase/phosphatase activities in plasma membranes of the cyanobacterium Synechococcus elongatus PCC 6301. In this work, we have analyzed the biochemical properties of the periplasmic nitrate/nitrite-binding subunit (NrtA) of NRT from the thermophilic nondiazotrophic cyanobacterium Phormidium laminosum. Our results show that cyanobacterial NrtA is phosphorylated in vivo. However, substrate binding activity in vitro is not affected by the phosphorylation state of the protein, ruling out the possibility that phosphorylation/dephosphorylation of NrtA is involved in the regulation of the nitrate/nitrite uptake by NRT transporter. Moreover, NrtA is present as multiple isoforms showing the same molecular mass but different isoelectric points ranging from pI 5 to 6. Mass spectrometric characterization of NrtA isoforms shows that the protein is phosphorylated at residue Tyr203, and contains several methionine sulphoxide residues which account for the observed isoforms. Both phosphorylated and non-phosphorylated forms of NrtA are active in vitro, showing comparable binding affinity for nitrate and nitrite. Both substrates behave as pure competitive inhibitors with a binding stoichiometry of one molecule of anion per NrtA monomer.     

The nitrate/nitrite ABC transporter of Phormidium laminosum: Phosphorylation state of NrtA is not involved in its substrate binding activity

Most cyanobacteria take up nitrate or nitrite through a multisubunit ABC transporter (ATP-binding cassette) located in the cytoplasmic membrane. Nitrate and nitrite transport activity is instantaneously blocked by the presence of ammonium in the medium. Previous biochemical studies reported the existence of phosphorylation/dephosphorylation events of the nitrate transporter (NRT) related to the presence of ammonium-sensitive kinase/phosphatase activities in plasma membranes of the cyanobacterium Synechococcus elongatus PCC 6301. In this work, we have analyzed the biochemical properties of the periplasmic nitrate/nitrite-binding subunit (NrtA) of NRT from the thermophilic nondiazotrophic cyanobacterium Phormidium laminosum. Our results show that cyanobacterial NrtA is phosphorylated in vivo. However, substrate binding activity in vitro is not affected by the phosphorylation state of the protein, ruling out the possibility that phosphorylation/dephosphorylation of NrtA is involved in the regulation of the nitrate/nitrite uptake by NRT transporter. Moreover, NrtA is present as multiple isoforms showing the same molecular mass but different isoelectric points ranging from pI 5 to 6. Mass spectrometric characterization of NrtA isoforms shows that the protein is phosphorylated at residue Tyr²⁰³, and contains several methionine sulphoxide residues which account for the observed isoforms. Both phosphorylated and non-phosphorylated forms of NrtA are active in vitro, showing comparable binding affinity for nitrate and nitrite. Both substrates behave as pure competitive inhibitors with a binding stoichiometry of one molecule of anion per NrtA monomer.     

Proton transport by phosphate diffusion--a mechanism of facilitated CO2 transfer

We have measured CO2 fluxes across phosphate solutions at different carbonic anhydrase concentrations, bicarbonate concentration gradients, phosphate concentrations, and mobilities. Temperature was 22-25 degrees C, the pH of the phosphate solutions was 7.0-7.3. We found that under physiological conditions of pH and pCO2 a facilitated diffusion of CO2 occurs in addition to free diffusion when (a) sufficient carbonic anhydrase is present, and (b) a concentration gradient of HCO3- is established along with a pCO2 gradient, and (c) the phosphate buffer has a mobility comparable to that of bicarbonate. When the phosphate was immobilized by attaching 0.25-mm-long cellulose particles, no facilitation of CO2 diffusion was detectable. A mechanism of facilitated CO2 diffusion in phosphate solutions analogous to that in albumin solutions was proposed on the basis of these findings: bicarbonate diffusion together with a facilitated proton transport by phosphate diffusion. A mathematical model of this mechanism was formulated. The CO2 fluxed predicted by the model agree quantitatively with the experimentally determined fluxes. It is concluded that a highly effective proton transport mechanism acts in solutions of mobile phosphate buffers. By this mechanism; CO2 transfer may be increased up to fivefold and proton transfer may be increased to 10,000-fold.     

Chloride--bicarbonate exchange in red blood cells: physiology of transport and chemical modification of binding sites

About 80% of the CO2 formed by metabolism is transported from tissues to lungs as bicarbonate ions in the water phases of red cells and plasma. The catalysed hydration of CO2 to bicarbonate takes place in the erythrocytes but most of the bicarbonate thus formed must be exchanged with extracellular chloride to make full use of the carbon dioxide transporting capacity of the blood. The anion transport capacity of the red cell membrane is among the largest ionic transport capacities of any biological membrane. Exchange diffusion of chloride and bicarbonate is nevertheless a rate-limiting step for the transfer of CO2 from tissues to lungs. Measurements of chloride and bicarbonate self-exchange form the basis for calculations that demonstrate that the ionic exchange processes cannot run to complete equilibration at capillary transit times less than 0.5 s. The anion exchange diffusion is mediated by a large transmembrane protein constituting almost 30% of the total membrane protein. The kinetics of exchange diffusion must depend on conformational changes of the protein molecule, associated with the binding and subsequent translocation of the transported anion. We have characterized the nature of anion-binding sites facing the extracellular medium by acid-base titration of the transport function and modification of the transport protein in situ with group-specific amino acid reagents. Anion binding and translocation depend on the integrity and the degree of protonation of two sets of exofacial groups with apparent pK values of 12 and 5, respectively. From the chemical reactivities towards amino acid reagents it appears that the groups whose pK = 12 are guanidino groups of arginyl residues, while the groups whose pK = 5 are likely to be carboxylates of glutamic or aspartic acid. Our studies suggest that the characteristics of anion recognition sites in water-soluble proteins and in the integral transport proteins are closely related.     

Independent induction of two blue light-dependent monovalent anion transport systems in the plasma membrane of Monoraphidium braunii

In the plasma membrane of the green alga Monoraphidium braunii there are at least two monovalent anion transport systems. One of them is specific for bicarbonate. This transport system is activated by blue light and its induction is triggered by a decrease in the external CO2 concentration. The second transport system is responsible for nitrate uptake at least. This transport system is also activated by blue light and its induction occurs when there is no ammonium in the external medium. Both transport systems are synthesized independently. Hence, when M. braunii cells grow with nitrate as the only nitrogen source under high CO2, they have a nitrate transport system but lack a bicarbonate transporter. Conversely, cells grown with ammonium under low CO2, have a bicarbonate transport system but lack a nitrate transporter. Both transport systems are induced in cells irradiated with white light in the absence of a carbon source, suggesting that there may be precursors in the plasma membrane that only need the synthesis and assembly of some component(s) to become fully active. The induction of nitrate and nitrite reductases, however, only takes place when a carbon source is supplied to the cells.     

Bicarbonate, not CO2, is the species required for the stimulation of Photosystem II electron transport

Evidence is presented that the bicarbonate ion (HCO3-), not CO2, H2CO3 or CO32-, is the species that stimulates electron transport in Photosystem II from spinach (Spinacia oleracea). Advantage was taken of the pH dependence of the ratio of HCO3- to CO2 at equilibrium in order to vary effectively the concentration of one species while holding the other constant. The Hill reaction was stimulated in direct proportion with the equilibrium HCO3- concentration, but it was independent of the equilibrium CO2 concentration. The other two carbonic species, H2CO3 and CO32-, are also shown to have no direct involvement. It is suggested that HCO3- is the species which binds to the effector site.     

Investigation of the system copper(II) carbonic anhydrase and HCO3-/CO2

Copper(II) substituted human and bovine carbonic anhydrases B in the presence of bicarbonate have been investigated in solution through water-solvent proton nuclear magnetic resonance (nmr) at variable magnetic fields. HCO3-, contrary to all the other monoanionic inhibitors, partially reduces the water proton relaxation rates. This has been accounted for on the basis of the availability within the active cavity of two coordination positions partially overlapping. 13C-nmr measurements on both CO2 and HCO3- confirm that HCO3- binds the metal, whereas CO2 interacts with the paramagnetic center at nonbonding distance. The upper limit for the CO2 in equilibrium HCO3- interconversion has been estimated to be 10 sec-1.     

Structure, Function and Regulation of the Nitrate Transport System of the Cyanobacterium Synechococcus sp. PCC7942

The active nitrate transport system of the cyanobacterium Synechococcussp. PCC7942 is encoded by the four genes nrtA, nrtB, nrtC andnrtD. It is essential for the growth of the cyanobacterium atphysiological concentrations of nitrate and has been shown tobe involved in the active transport of nitrite as well. Thededuced amino acid sequences of the NrtB, NrtC and NrtD proteinsindicate that the transporter is a member of the ABC (ATP-bindingcassette) superfamily of active transporters. Among the prokaryoticABC transporters, the cyanobacterial nitrate/nitrite transporteris unique in having a membrane-bound protein NrtA and an NrtA-likeextra domain linked to one of the ATP-binding subunits (C-terminaldomain of NrtC). Molecular biological, biochemical and physiologicalstudies suggest that NrtA is the substrate-binding protein requiredfor the transport of nitrate/nitrite and that the C-terminaldomain of NrtC has a regulatory role. Comparison of the structuresof nitrate transporters from eukaryotic and prokaryotic, photosyntheticand non-photosynthetic organisms indicate that the nrt nitrate/nitritetransporter represents a prokaryotic nitrate transporter distinctfrom the nitrate transporters of eukaryotes. ¹Recipient of the JSPP Young Investigator Award, 1994.     

Nitrite transport is mediated by the nitrite-specific high-affinity NitA transporter and by nitrate transporters NrtA, NrtB in Aspergillus nidulans

Disruption of the Aspergillus nidulans high-affinity nitrate transporter genes (nrtA and nrtB) prevents growth on nitrate but not nitrite. We identified a distinct nitrite transporter (K(m)=4.2+/-1 microM, V(max)=168+/-21 nmolmg(-1)DW(-1)h(-1)), designated NitA. Disruption of nrtA, nrtB and nitA blocked growth on nitrite, despite low rates of nitrite depletion we ascribe to passive nitrous acid permeation. Growth of the single mutant nitA16 on nitrite was wild-type, suggesting that NrtA and/or NrtB transports nitrite as well as nitrate. Indeed, NrtA and NrtB transport nitrite at higher rates than NitA; K(m) and V(max) values were 16+/-4 microM and 808+/-67 nmolmg(-1)DW(-1)h(-1) (NrtA) and 11+/-1 microM and 979+/-17 nmolmg(-1)DW(-1)h(-1) (NrtB). We suggest that NrtA is a nitrate/nitrite transporter, NrtB absorbs nitrite in preference to nitrate and NitA is exclusively a nitrite transporter.     

Mini Review

To allow cells to control their pH and bicarbonate levels, cells express bicarbonate transport proteins that rapidly and selectively move bicarbonate across the plasma membrane. Physical interactions have been identified between the carbonic anhydrase isoform, CAII, and the erythrocyte membrane [Formula: See Text] anion exchanger, AE1, mediated by an acidic motif in the AE1 C-terminus. We have found that the presence of CAII attached to AE1 accelerates AE1 [Formula: See Text] transport activity, as AE1 moves bicarbonate either into or out of the cell. In efflux mode the presence of CAII attached to AE1 will increase the local concentration of bicarbonate at the AE1 transport site. As bicarbonate is transported into the cell by AE1, the presence of CAII on the cytosolic surface accelerates transport by consumption of bicarbonate, thereby maximizing the transmembrane bicarbonate concentration gradient experienced by the AE1 molecule. Functional and physical interactions also occur between CAII and [Formula: See Text] co-transporter isoforms NBC1 and NBC3. All examined bicarbonate transport proteins, except the DRA (SLC26A3) [Formula: See Text] exchange protein, have a consensus CAII binding site in their cytoplasmic C-terminus. Interestingly, CAII does not bind DRA. CAIV is anchored to the extracellular surface of cells via a glycosylphosphatidyl inositol linkage. We have identified extracellular regions of AE1 and NBC1 that directly interact with CAIV, to form a physical complex between the proteins. In summary, bicarbonate transporters directly interact with the CAII and CAIV carbonic anhydrases to increase the transmembrane bicarbonate flux. The complex of a bicarbonate transporter with carbonic anhydrase forms a "Bicarbonate Transport Metabolon."     

The bicarbonate transport metabolon

To allow cells to control their pH and bicarbonate levels, cells express bicarbonate transport proteins that rapidly and selectively move bicarbonate across the plasma membrane. Physical interactions have been identified between the carbonic anhydrase isoform, CAII, and the erythrocyte membrane Cl- /HCO3(-) anion exchanger, AE1, mediated by an acidic motif in the AE1 C-terminus. We have found that the presence of CAII attached to AE1 accelerates AE1 HCO3(-) transport activity, as AE1 moves bicarbonate either into or out of the cell. In efflux mode the presence of CAII attached to AE1 will increase the local concentration of bicarbonate at the AE1 transport site. As bicarbonate is transported into the cell by AE1, the presence of CAII on the cytosolic surface accelerates transport by consumption of bicarbonate, thereby maximizing the transmembrane bicarbonate concentration gradient experienced by the AE1 molecule. Functional and physical interactions also occur between CAII and Na+/HCO3(-) co-transporter isoforms NBC1 and NBC3. All examined bicarbonate transport proteins, except the DRA (SLC26A3) Cl-/HCO3(-) exchange protein, have a consensus CAII binding site in their cytoplasmic C-terminus. Interestingly, CAII does not bind DRA. CAIV is anchored to the extracellular surface of cells via a glycosylphosphatidyl inositol linkage. We have identified extracellular regions of AE1 and NBC1 that directly interact with CAIV, to form a physical complex between the proteins. In summary, bicarbonate transporters directly interact with the CAII and CAIV carbonic anhydrases to increase the transmembrane bicarbonate flux. The complex of a bicarbonate transporter with carbonic anhydrase forms a "Bicarbonate Transport Metabolon."     

Chemical kinetic and diffusional limitations on bicarbonate reabsorption by the proximal tubule.

It is accepted that bicarbonate reabsorption in the proximal tubule is mediated by H+ secretion, but several aspects of this process have remained controversial. To examine some of these issues, we have developed a model that allows for spatial variations in the concentrations of CO2, HCO3-, and H2CO3 within the tubule lumen and cell cytoplasm, passive transport of these substances across cell membranes, carbonic anhydrase-catalyzed interconversion of HCO3- and CO2 within the cell and at the luminal membrane surface, and the corresponding uncatalyzed reactions in lumen and cell. Most of the required kinetic and transport parameters were estimated from physicochemical data in the literature, whereas intracellular pH and HCO3- permeability at the basal cell membrane, found to be the most significant parameters under normal conditions, were adjusted to yield reabsorption rates of "total CO2" (tCO2, the sum of CO2, HCO3- and H2CO3) comparable to measured values in the rat. Our results suggest that for normal carbonic anhydrase activity, almost all tCO2 leaves the lumen as CO2, yet the transepithelial differences in CO2 partial pressure does not exceed approximately 2 mm Hg. Electrochemical potential gradients favor substantial passive backleak of HCO3- from cell to lumen. Gradients in CO2 partial pressure remain small during simulated inhibition of carbonic anhydrase, with approximately 70% of tCO2 leaving the lumen as H2CO3 in this case, and the remainder as CO2. Predicted tCO2 reabsorption rates for carbonic anhydrase inhibition are approximately of normal, in good agreement with recent measurements in the rat, indicating that the concept of "carbonic acid recycling" is viable.     

Nitrite transport is mediated by the nitrite-specific high-affinity NitA transporter and by nitrate transporters NrtA, NrtB in Aspergillus nidulans.

Disruption of the Aspergillus nidulans high-affinity nitrate transporter genes (nrtA and nrtB) prevents growth on nitrate but not nitrite. We identified a distinct nitrite transporter (K(m)=4.2+/-1 microM, V(max)=168+/-21 nmolmg(-1)DW(-1)h(-1)), designated NitA. Disruption of nrtA, nrtB and nitA blocked growth on nitrite, despite low rates of nitrite depletion we ascribe to passive nitrous acid permeation. Growth of the single mutant nitA16 on nitrite was wild-type, suggesting that NrtA and/or NrtB transports nitrite as well as nitrate. Indeed, NrtA and NrtB transport nitrite at higher rates than NitA; K(m) and V(max) values were 16+/-4 microM and 808+/-67 nmolmg(-1)DW(-1)h(-1) (NrtA) and 11+/-1 microM and 979+/-17 nmolmg(-1)DW(-1)h(-1) (NrtB). We suggest that NrtA is a nitrate/nitrite transporter, NrtB absorbs nitrite in preference to nitrate and NitA is exclusively a nitrite transporter.     

A transport metabolon. Functional interaction of carbonic anhydrase II and chloride/bicarbonate exchangers

The cytoplasmic carboxyl-terminal domain of AE1, the plasma membrane chloride/bicarbonate exchanger of erythrocytes, contains a binding site for carbonic anhydrase II (CAII). To examine the physiological role of the AE1/CAII interaction, anion exchange activity of transfected HEK293 cells was monitored by following the changes in intracellular pH associated with AE1-mediated bicarbonate transport. AE1-mediated chloride/bicarbonate exchange was reduced 50-60% by inhibition of endogenous carbonic anhydrase with acetazolamide, which indicates that CAII activity is required for full anion transport activity. AE1 mutants, unable to bind CAII, had significantly lower transport activity than wild-type AE1 (10% of wild-type activity), suggesting that a direct interaction was required. To determine the effect of displacement of endogenous wild-type CAII from its binding site on AE1, AE1-transfected HEK293 cells were co-transfected with cDNA for a functionally inactive CAII mutant, V143Y. AE1 activity was maximally inhibited 61 +/- 4% in the presence of V143Y CAII. A similar effect of V143Y CAII was found for AE2 and AE3cardiac anion exchanger isoforms. We conclude that the binding of CAII to the AE1 carboxyl-terminus potentiates anion transport activity and allows for maximal transport. The interaction of CAII with AE1 forms a transport metabolon, a membrane protein complex involved in regulation of bicarbonate metabolism and transport.     

Plasma membrane Cl-/HCO3-exchangers: Structure,mechanism and physiology

Anion exchanger proteins facilitate the exchange of bicarbonate for chloride across the plasma membrane. When bicarbonate combines with a proton it undergoes conversion into CO2, either spontaneously, or catalyzed by carbonic anhydrase enzymes. The CO2/HCO3- equilibrium is the body’s central pH buffering system. Rapid bicarbonate transport across the plasma membrane is essential to maintain cellular and whole body pH, to dispose of metabolic waste CO2, and to control fluid movement in our bodies. Cl-/HCO3- exchangers are found in two distinct gene families: SLC4A and SLC26A. Differences in the tissue distribution, electrogenicity, and regulation of the specific anion exchanger proteins allow for precise regulation of bicarbonate transport throughout the human body. This review provides a look into the structural and functional features that make this family of proteins unique, as well as the physiological significance of the different anion exchangers.     

Equilibrium of CO2 reactions in the pulmonary capillary

Steady-state CO2 excretion was measured in isolated blood-free rabbit lungs perfused with bicarbonate solutions. CO2 in the expired ventilation was either present initially in the perfusate as dissolved CO2 or produced from bicarbonate during pulmonary capillary transit. The two components were separated by measurement of simultaneous acetylene excretion. Bovine carbonic anhydrase and acetazolamide were sequentially added to the perfusate to determine the effects of maximal enzyme catalysis and inhibition of native lung carbonic anhydrase on CO2 production. Control CO2 production was significantly greater than that observed during inhibition of native lung carbonic anhydrase, confirming previous observations that bicarbonate has access to the tissue enzyme. Addition of excess carbonic anhydrase increased CO2 production by a statistically, but not physiologically, significant amount. These data demonstrate that CO2 reactions outside the erythrocyte attain 97% completion during pulmonary capillary transit. Under control and catalyzed conditions, alveolar and venous CO2 tens ions and pH were essentially identical to equilibrium values determined by in vitro tonometry.     

Characterization of the carboxysomal carbonic anhydrase CsoSCA from Halothiobacillus neapolitanus

In cyanobacteria and many chemolithotrophic bacteria, the CO(2)-fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) is sequestered into polyhedral protein bodies called carboxysomes. The carboxysome is believed to function as a microcompartment that enhances the catalytic efficacy of RubisCO by providing the enzyme with its substrate, CO(2), through the action of the shell protein CsoSCA, which is a novel carbonic anhydrase. In the work reported here, the biochemical properties of purified, recombinant CsoSCA were studied, and the catalytic characteristics of the carbonic anhydrase for the CO(2) hydration and bicarbonate dehydration reactions were compared with those of intact and ruptured carboxysomes. The low apparent catalytic rates measured for CsoSCA in intact carboxysomes suggest that the protein shell acts as a barrier for the CO(2) that has been produced by CsoSCA through directional dehydration of cytoplasmic bicarbonate. This CO(2) trap provides the sequestered RubisCO with ample substrate for efficient fixation and constitutes a means by which microcompartmentalization enhances the catalytic efficiency of this enzyme.     

碳酸酐酶的生理功能、多样性及其在CO2捕集中的应用

碳酸酐酶（carbonic anhydrase）作为一种活性中心含有锌离子的金属酶,能够可逆催化CO2生成碳酸氢盐的水合反应,该反应在生物体内承担着多样的生理学功能,具有高度的生物学意义。除广泛存在于真核生物以外,该酶在淡水、海水、嗜常温、嗜热、厌氧、好氧、致病、产酸、自养、异养等多种原核微生物中也有广泛的分布,并参与光合作用、呼吸作用和以CO2作为底物的反应,维持生理pH以及离子转运等生理过程。近年来,随着温室效应的日益加剧．生物固定CO2作为该酶的一种全新应用引起了研究者的广泛关注。回顾了碳酸酐酶作为催化剂参与CO2固定过程的历史、现状和最新发现,同时展望了未来应用的趋势。