Intracellular beta-carbonic anhydrase of the unicellular green alga Coccomyxa. Cloning of the cdna and characterization of the functional enzyme overexpressed in Escherichia coli

Structural insights into novel mechanisms of inhibition of the major β-carbonic anhydrase CafB from the pathogenic fungus Aspergillus fumigatus

In fungi the β-class of carbonic anhydrases (β-CAs) are zinc metalloenzymes that are essential for growth, survival, differentiation, and virulence. Aspergillus fumigatus is the most important pathogen responsible for invasive aspergillosis and possesses two major β-CAs, CafA and CafB. Recently we reported the biochemical characterization and 1.8 Å crystal structure of CafA. Here, we report a crystallographic analysis of CafB revealing the mechanism of enzyme catalysis and establish the relationship of this enzyme to other β-CAs. While CafA has a typical open conformation, CafB, when exposed to acidic pH and/or an oxidative environment, has a novel type of active site in which a disulfide bond is formed between two zinc-ligating cysteines, expelling the zinc ion and stabilizing the inactive form of the enzyme. Based on the structural data, we generated an oxidation-resistant mutant (Y159A) of CafB. The crystal structure of the mutant under reducing conditions retains a catalytic zinc at the expected position, tetrahedrally coordinated by three residues (C57, H113 and C116) and an aspartic acid (D59), and replacing the zinc-bound water molecule in the closed form. Furthermore, the active site of CafB crystals grown under zinc-limiting conditions has a novel conformation in which the solvent-exposed catalytic cysteine (C116) is flipped out of the metal coordination sphere, facilitating release of the zinc ion. Taken together, our results suggest that A. fumigatus use sophisticated activity-inhibiting strategies to enhance its survival during infection.

Expression of seven carbonic anhydrases in red alga Gracilariopsis chorda and their subcellular localization in a heterologous system, Arabidopsis thaliana

Red alga, Gracilariopsis chorda, contains seven carbonic anhydrases that can be grouped into α-, β- and γ-classes. Carbonic anhydrases (CAHs) are metalloenzymes that catalyze the reversible hydration of CO₂. These enzymes are present in all living organisms and play roles in various cellular processes, including photosynthesis. In this study, we identified seven CAH genes (GcCAHs) from the genome sequence of the red alga Gracilariopsis chorda and characterized them at the molecular, cellular and biochemical levels. Based on sequence analysis, these seven isoforms were categorized into four α-class, one β-class, and two γ-class isoforms. RNA sequencing revealed that of the seven CAHs isoforms, six genes were expressed in G. chorda in light at room temperature. In silico analysis revealed that these seven isoforms localized to multiple subcellular locations such as the ER, mitochondria and cytosol. When expressed as green fluorescent protein fusions in protoplasts of Arabidopsis thaliana leaf cells, these seven isoforms showed multiple localization patterns. The four α-class GcCAHs with an N-terminal hydrophobic leader sequence localized to the ER and two of them were further targeted to the vacuole. GcCAHβ1 with no noticeable signal sequence localized to the cytosol. The two γ-class GcCAHs also localized to the cytosol, despite the presence of a predicted presequence. Based on these results, we propose that the red alga G. chorda also employs multiple CAH isoforms for various cellular processes such as photosynthesis.

Redox regulation of carbonic anhydrases via thioredoxin in chloroplast of the marine diatom Phaeodactylum tricornutum

Thioredoxins (Trxs) are important regulators of photosynthetic fixation of CO(2) and nitrogen in plant chloroplasts. To date, they have been considered to play a minor role in controlling the Calvin cycle in marine diatoms, aquatic primary producers, although diatoms possess a set of plastidic Trxs. In this study we examined the influences of the redox state and the involvement of Trxs in the enzymatic activities of pyrenoidal carbonic anhydrases, PtCA1 and PtCA2, in the marine diatom Phaeodactylum tricornutum. The recombinant mature PtCA1 and -2 (mPtCA1 and -2) were completely inactivated following oxidation by 50 μm CuCl(2), whereas DTT activated CAs in a concentration-dependent manner. The maximum activity of mPtCAs in the presence of 6 mm reduced DTT increased significantly by addition of 10 μm Trxs from Arabidopsis thaliana (AtTrx-f2 and -m2) and 5 μm Trxs from P. tricornutum (PtTrxF and -M). Analyses of mPtCA activation by Trxs in the presence of DTT revealed that the maximum mPtCA1 activity was enhanced ∼3-fold in the presence of Trx, whereas mPtCA2 was only weakly activated by Trxs, and that PtTrxs activate PtCAs more efficiently compared with AtTrxs. Site-directed mutagenesis of potential disulfide-forming cysteines in mPtCA1 and mPtCA2 resulted in a lack of oxidative inactivation of both mPtCAs. These results reveal the first direct evidence of a target of plastidic Trxs in diatoms, indicating that Trxs may participate in the redox control of inorganic carbon flow in the pyrenoid, a focal point of the CO(2)-concentrating mechanism.

Structural studies of β-carbonic anhydrase from the green alga Coccomyxa: inhibitor complexes with anions and acetazolamide

The β-class carbonic anhydrases (β-CAs) are widely distributed among lower eukaryotes, prokaryotes, archaea, and plants. Like all CAs, the β-enzymes catalyze an important physiological reaction, namely the interconversion between carbon dioxide and bicarbonate. In plants the enzyme plays an important role in carbon fixation and metabolism. To further explore the structure-function relationship of β-CA, we have determined the crystal structures of the photoautotroph unicellular green alga Coccomyxa β-CA in complex with five different inhibitors: acetazolamide, thiocyanate, azide, iodide, and phosphate ions. The tetrameric Coccomyxa β-CA structure is similar to other β-CAs but it has a 15 amino acid extension in the C-terminal end, which stabilizes the tetramer by strengthening the interface. Four of the five inhibitors bind in a manner similar to what is found in complexes with α-type CAs. Iodide ions, however, make contact to the zinc ion via a zinc-bound water molecule or hydroxide ion — a type of binding mode not previously observed in any CA. Binding of inhibitors to Coccomyxa β-CA is mediated by side-chain movements of the conserved residue Tyr-88, extending the width of the active site cavity with 1.5-1.8 Å. Structural analysis and comparisons with other α- and β-class members suggest a catalytic mechanism in which the movements of Tyr-88 are important for the CO₂-HCO₃^- interconversion, whereas a structurally conserved water molecule that bridges residues Tyr-88 and Gln-38, seems important for proton transfer, linking water molecules from the zinc-bound water to His-92 and buffer molecules.

The unique structure of carbonic anhydrase αCA1 from Chlamydomonas reinhardtii

Chlamydomonas reinhardtii α-type carbonic anhydrase (Cr-αCA1) is a dimeric enzyme that catalyses the interconversion of carbon dioxide and carbonic acid. The precursor form of Cr-αCA1 undergoes post-translational cleavage and N-glycosylation. Comparison of the genomic sequences of precursor Cr-αCA1 and other αCAs shows that Cr-αCA1 contains a different N-terminal sequence and two insertion sequences. A 35-residue peptide in one of the insertion sequences is deleted from the precursor during maturation. The crystal structure of the mature form of Cr-αCA1 has been determined at 1.88 Å resolution. Each subunit is cleaved into the long and short peptides, but they are linked together by a disulfide bond. The two subunits are linked by a disulfide bond. N-Glycosylations occur at three asparagine residues and the attached N-glycans protrude into solvent regions. The subunits consist of a core β-sheet structure composed of nine β-strands. At the centre of the β-sheet is the catalytic site, which contains a Zn atom bound to three histidine residues. The amino-acid residues around the Zn atom are highly conserved in other monomeric and dimeric αCAs. The short peptide runs near the active site and forms a hydrogen bond to the zinc-coordinated residue in the long chain, suggesting an important role for the short peptide in Cr-αCA1 activity.

Characterization and expression of the maize β-carbonic anhydrase gene repeat regions

In maize, carbonic anhydrase (CA; EC 4.2.1.1) catalyzes the first reaction of the C(4) photosynthetic pathway; it catalyzes the hydration of CO(2) to bicarbonate and provides an inorganic carbon source for the primary carboxylation reaction catalyzed by phosphoenolpyruvate (PEP) carboxylase. The β-CA isozymes from maize, as well as other agronomically important NADP-malic enzyme (NADP-ME) type C(4) crops, have remained relatively uncharacterized but differ significantly from the β-CAs of other C(4) monocot species primarily due to transcript length and the presence of repeat sequences. This research confirmed earlier findings of repeat sequences in maize CA transcripts, and demonstrated that the gene encoding these transcripts is also composed of repeat sequences. One of the maize CA genes was sequenced and found to encode two domains, with distinct groups of exons corresponding to the repeat regions of the transcript. We have also shown that expression of a single repeat region of the CA transcript produced active enzyme that associated as a dimer and was composed primarily of α-helices, consistent with that observed for other plant CAs. As the presence of repeat regions in the CA gene is unique to NADP-ME type C(4) monocot species, the implications of these findings in the context of the evolution of the location and function of this C(4) pathway enzyme are strongly suggestive of CA gene duplication resulting in an evolutionary advantage and a higher photosynthetic efficiency.

Cloning, polymorphism, and inhibition of beta-carbonic anhydrase of Helicobacter pylori

BACKGROUND

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2) to bicarbonate and a proton, and alpha-class CA has been reported to facilitate the acid acclimation of Helicobacter pylori (hpalphaCA). The purpose of this study was to characterize the beta-class CA of H. pylori (hpbetaCA) and elucidate the role of this enzyme as a possible drug target for eradication therapy.

METHODS

We isolated DNA clones of independent H. pylori strains obtained from patients with gastritis (n = 15), gastric ulcer (n = 6), or gastric cancer (n = 16), and then studied genetic polymorphisms. In addition, the susceptibility of H. pylori to sulpiride, an antiulcer drug and efficient inhibitor of both hpalphaCA and hpbetaCA, was studied with an in vitro killing assay.

RESULTS

DNA sequences of all 37 hpbetaCA clones encoded a 221 amino acid polypeptide with a variety of polymorphisms (57 types of amino acid substitution at 48 residue positions). There was no polymorphism functionally relevant to the gastric lesion type. One strain included unique residues that were not seen in the other 36 clones from Japanese patients but which were found in a strain obtained from the United Kingdom. Sulpiride had killing effects at concentrations greater than 200 microg/ml for H. pylori, including strains resistant to clarithromycin, metronidazole, or ampicillin.

CONCLUSIONS

Helicobacter pylori might have evolved independently in the Caucasian and Japanese populations. Dual inhibition of alpha-and beta-class CAs could be applied as alternative therapy for eradication of H. pylori.

Extracellular carbonic anhydrases of the stromatolite-forming cyanobacterium Microcoleus chthonoplastes

Active extracellular carbonic anhydrases (CAs) were found in the alkaliphilic stromatolite-forming cyanobacterium Microcoleus chthonoplastes. Enzyme activity was detected in intact cells and in the cell envelope fraction. Western blot analysis of polypeptides from the cell envelope suggested the presence of at least two polypeptides cross-reacting with antibodies against both alpha and beta classes of CA. Immunocytochemical analysis revealed putative alpha-CA localized in the glycocalyx. This alpha-CA has a molecular mass of about 34 kDa and a pI of 3.5. External CAs showed two peaks of activity at around pH 10 and 7.5. The possible involvement of extracellular CAs of M. chthonoplastes in photosynthetic assimilation of inorganic carbon and its relationship to CaCO(3) deposition during mineralization of cyanobacterial cells are discussed.

Carbonic anhydrase inhibitors: the beta-carbonic anhydrase from Helicobacter pylori is a new target for sulfonamide and sulfamate inhibitors

DNA clones for the beta-class carbonic anhydrase (CA, EC 4.2.1.1) of Helicobactor pylori (hpbetaCA) were obtained. A recombinant hpbetaCA protein lacking the N-terminal 15-amino acid residues was produced and purified, representing a catalytically efficient CA. hpbetaCA was strongly inhibited (K(I)s in the range of 24-45 nM) by many sulfonamides/sulfamates, among which acetazolamide, ethoxzolamide, topiramate, and sulpiride, all clinically used drugs. The dual inhibition of alpha- and/or beta-class CAs of H. pylori might represent a useful alternative for the management of gastritis/gastric ulcers, as well as gastric cancer. This is also the first study showing that a bacterial beta-CA can be a drug target.

The thylakoid carbonic anhydrase associated with photosystem II is the component of inorganic carbon accumulating system in cells of halo- and alkaliphilic cyanobacterium Rhabdoderma lineare

The organization of carbonic anhydrase (CA) system in halo- and alkaliphilic cyanobacterium Rhabdoderma lineare was studied by Western blot analysis and immunocytochemical electron microscopy. The presence of putative extracellular alpha-CA of 60 kDa in the glycocalyx, forming a tight sheath around the cell, and of two intracellular beta-CA is reported. We show for the first time that the beta-CA of 60 kDa is expressed constitutively and associated with polypeptides of photosystem II (beta-CA-PS II). Another soluble beta-CA of 25 kDa was induced in low-bicarbonate medium. Induction of synthesis of the latter beta-CA was accompanied by an increase in the intracellular pool of inorganic carbon, which suggests an important role of this enzyme in the functioning of a CO(2)-concentrating mechanism.

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.

Structural mechanics of the pH-dependent activity of beta-carbonic anhydrase from Mycobacterium tuberculosis

Carbonic anhydrases catalyze the reversible hydration of carbon dioxide to form bicarbonate, a reaction required for many functions, including carbon assimilation and pH homeostasis. Carbonic anhydrases are divided into at least three classes and are believed to share a zinc-hydroxide mechanism for carbon dioxide hydration. beta-carbonic anhydrases are broadly spread among the domains of life, and existing structures from different organisms show two distinct active site setups, one with three protein coordinations to the zinc (accessible) and the other with four (blocked). The latter is believed to be inconsistent with the zinc-hydroxide mechanism. The Mycobacterium tuberculosis Rv3588c gene, shown to be required for in vivo growth of the pathogen, encodes a beta-carbonic anhydrase with a steep pH dependence of its activity, being active at pH 8.4 but not at pH 7.5. We have recently solved the structure of this protein, which was a dimeric protein with a blocked active site. Here we present the structure of the thiocyanate complexed protein in a different crystal form. The protein now forms distinct tetramers and shows large structural changes, including a carboxylate shift yielding the accessible active site. This structure demonstrated for the first time that a beta-carbonic anhydrase can switch between the two states. A pH-dependent dimer to tetramer equilibrium was also demonstrated by dynamic light scattering measurements. The data presented here, therefore, suggest a carboxylate shift on/off switch for the enzyme, which may, in turn, be controlled by a dimer-to-tetramer equilibrium.

The carbonic anhydrase gene families ofChlamydomonas reinhardtii

Carbonic anhydrases (CAs) are zinc-containing metalloenzymes that catalyze the reversible interconversion of CO2and HCO3–. Aquatic photosynthetic organisms have evolved different forms of CO2-concentrating mechanisms to aid Rubisco in capturing CO2from the surrounding environment. One aspect of all CO2-concentrating mechanisms is the critical roles played by various specially localized extracellular and intracellular CAs. There are three evolutionarily unrelated CA families designated α-, β-, and γ-CA. In the green alga, Chlamydomonas reinhardtii Dangeard, eight CAs have now been identified, including three α-CAs and five β-CAs. In addition, C. reinhardtii has another CA-like gene, Glp1 that is similar to known γ-CAs. To characterize these different CA isoforms, some of the CA genes have been overexpressed to determine whether the proteins have CA activity and to generate antibodies for in vivo immunolocalization. The CA proteins Cah3, Cah6, and Cah8, and the γ-CA-like protein, Glp1, have been overexpressed. Cah3, Cah6, and Cah8 have CA activity, but Glp1 does not. At least two of these proteins, Cah3 and Cah6, are localized to the chloroplast. Using immunolocalization and sequence analyses, we have determined that Cah6 is located to the chloroplast stroma and confirmed that Cah3 is localized to the chloroplast thylakoid lumen. Activity assays show that Cah3 is 100 times more sensitive to sulfonamides than Cah6. We present a model on how these two chloroplast CAs might participate in the CO2-concentrating mechanism of C. reinhardtii. Key words: carbonic anhydrase, CO2-concentrating mechanism, Chlamydomonas, immunolocalization.

CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO(2) conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO(2) conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO(2) and/or HCO(3) (-) which increased the internal DIC concentration. The feature is referred to as the 'CO(2)-concentrating mechanism (CCM)'. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO(2) and/or HCO(3) (-), depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO(3) (-) transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric beta-type of CA composed of two internally repeating structures. An increase in the CO(2) concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO(2) concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO(2) concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H(+)-transport in extremely high-CO(2) conditions. This same state transition has also been observed when high-CO(2) cells were transferred to low CO(2) conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO(2) conditions.

Physiological and molecular biological characterization of intracellular carbonic anhydrase from the marine diatom Phaeodactylum tricornutum

A single intracellular carbonic anhydrase (CA) was detected in air-grown and, at reduced levels, in high CO(2)-grown cells of the marine diatom Phaeodactylum tricornutum (UTEX 642). No external CA activity was detected irrespective of growth CO(2) conditions. Ethoxyzolamide (0.4 mM), a CA-specific inhibitor, severely inhibited high-affinity photosynthesis at low concentrations of dissolved inorganic carbon, whereas 2 mM acetazolamide had little effect on the affinity for dissolved inorganic carbon, suggesting that internal CA is crucial for the operation of a carbon concentrating mechanism in P. tricornutum. Internal CA was purified 36.7-fold of that of cell homogenates by ammonium sulfate precipitation, and two-step column chromatography on diethylaminoethyl-sephacel and p-aminomethylbenzene sulfone amide agarose. The purified CA was shown, by SDS-PAGE, to comprise an electrophoretically single polypeptide of 28 kD under both reduced and nonreduced conditions. The entire sequence of the cDNA of this CA was obtained by the rapid amplification of cDNA ends method and indicated that the cDNA encodes 282 amino acids. Comparison of this putative precursor sequence with the N-terminal amino acid sequence of the purified CA indicated that it included a possible signal sequence of up to 46 amino acids at the N terminus. The mature CA was found to consist of 236 amino acids and the sequence was homologous to beta-type CAs. Even though the zinc-ligand amino acid residues were shown to be completely conserved, the amino acid residues that may constitute a CO(2)-binding site appeared to be unique among the beta-CAs so far reported.

Inhibition of photosynthesis by intracellular carbonic anhydrase in microalgae under excess concentrations of CO(2)

When cells of Chlorococcum littorale that had been grown in air (air-grown cells) were transferred to extremely high CO(2) concentrations (>20%), active photosynthesis resumed after a lag period which lasted for 1-4 days. In contrast, C. littorale cells which had been grown in 5% CO(2) (5% CO(2)-grown cells) could grow in 40% CO(2) without any lag period. When air-grown cells were transferred to 40% CO(2), the quantum efficiency of PS II (Phi(II)) decreased greatly, while no decrease in Phi(II) was apparent when the 5% CO(2)-grown cells were transferred to 40% CO(2). In contrast to air-grown cells, 5% CO(2)-grown cells showed neither extracellular nor intracellular carbonic anhydrase (CA) activity. Upon the acclimation of 5% CO(2)-grown cells to air, photosynthetic susceptibility to 40% CO(2) was induced. This change was associated with the induction of CA. In addition, neither suppression of photosynthesis nor arrest of growth was apparent when ethoxyzolamide (EZA), a membrane-permeable inhibitor of CA, had been added before transferring air-grown cells of C. littorale to 40% CO(2). The intracellular pH value (pH(i)) decreased from 7.0 to 6.4 when air-grown C. littorale cells were exposed to 40% CO(2) for 1-2 h, but no such decrease in pH(i) was apparent in the presence of EZA. Both air- and 5% CO(2)-grown cells of Chlorella sp. UK001, which was also resistant to extremely high CO(2) concentrations, grew in 40% CO(2) without any lag period. The activity of CA was much lower in air-grown cells of this alga than those in air-grown C. littorale cells. These results prompt us to conclude that intracellular CA caused intracellular acidification and hence inhibition of photosynthetic carbon fixation when air-grown C. littorale cells were exposed to excess concentrations of CO(2). No such harmful effect of intracellular CA was observed in Chlorella sp. UK001 cells.

Prokaryotic carbonic anhydrases

Carbonic anhydrases catalyze the reversible hydration of CO(2) [CO(2)+H(2)Oright harpoon over left harpoon HCO(3)(-)+H(+)]. Since the discovery of this zinc (Zn) metalloenzyme in erythrocytes over 65 years ago, carbonic anhydrase has not only been found in virtually all mammalian tissues but is also abundant in plants and green unicellular algae. The enzyme is important to many eukaryotic physiological processes such as respiration, CO(2) transport and photosynthesis. Although ubiquitous in highly evolved organisms from the Eukarya domain, the enzyme has received scant attention in prokaryotes from the Bacteria and Archaea domains and has been purified from only five species since it was first identified in Neisseria sicca in 1963. Recent work has shown that carbonic anhydrase is widespread in metabolically diverse species from both the Archaea and Bacteria domains indicating that the enzyme has a more extensive and fundamental role in prokaryotic biology than previously recognized. A remarkable feature of carbonic anhydrase is the existence of three distinct classes (designated alpha, beta and gamma) that have no significant sequence identity and were invented independently. Thus, the carbonic anhydrase classes are excellent examples of convergent evolution of catalytic function. Genes encoding enzymes from all three classes have been identified in the prokaryotes with the beta and gamma classes predominating. All of the mammalian isozymes (including the 10 human isozymes) belong to the alpha class; however, only nine alpha class carbonic anhydrase genes have thus far been found in the Bacteria domain and none in the Archaea domain. The beta class is comprised of enzymes from the chloroplasts of both monocotyledonous and dicotyledonous plants as well as enzymes from phylogenetically diverse species from the Archaea and Bacteria domains. The only gamma class carbonic anhydrase that has thus far been isolated and characterized is from the methanoarchaeon Methanosarcina thermophila. Interestingly, many prokaryotes contain carbonic anhydrase genes from more than one class; some even contain genes from all three known classes. In addition, some prokaryotes contain multiple genes encoding carbonic anhydrases from the same class. The presence of multiple carbonic anhydrase genes within a species underscores the importance of this enzyme in prokaryotic physiology; however, the role(s) of this enzyme is still largely unknown. Even though most of the information known about the function(s) of carbonic anhydrase primarily relates to its role in cyanobacterial CO(2) fixation, the prokaryotic enzyme has also been shown to function in cyanate degradation and the survival of intracellular pathogens within their host. Investigations into prokaryotic carbonic anhydrase have already led to the identification of a new class (gamma) and future research will undoubtedly reveal novel functions for carbonic anhydrase in prokaryotes.

The active site architecture of Pisum sativum beta-carbonic anhydrase is a mirror image of that of alpha-carbonic anhydrases

We have determined the structure of the beta-carbonic anhydrase from the dicotyledonous plant Pisum sativum at 1.93 A resolution, using a combination of multiple anomalous scattering off the active site zinc ion and non-crystallographic symmetry averaging. The mol- ecule assembles as an octamer with a novel dimer of dimers of dimers arrangement. Two distinct patterns of conservation of active site residues are observed, implying two potentially mechanistically distinct classes of beta-carbonic anhydrases. The active site is located at the interface between two monomers, with Cys160, His220 and Cys223 binding the catalytic zinc ion and residues Asp162 (oriented by Arg164), Gly224, Gln151, Val184, Phe179 and Tyr205 interacting with the substrate analogue, acetic acid. The substrate binding groups have a one to one correspondence with the functional groups in the alpha-carbonic anhydrase active site, with the corresponding residues being closely superimposable by a mirror plane. Therefore, despite differing folds, alpha- and beta-carbonic anhydrase have converged upon a very similar active site design and are likely to share a common mechanism.

A plant-type (beta-class) carbonic anhydrase in the thermophilic methanoarchaeon Methanobacterium thermoautotrophicum

Carbonic anhydrase, a zinc enzyme catalyzing the interconversion of carbon dioxide and bicarbonate, is nearly ubiquitous in the tissues of highly evolved eukaryotes. Here we report on the first known plant-type (beta-class) carbonic anhydrase in the archaea. The Methanobacterium thermoautotrophicum DeltaH cab gene was hyperexpressed in Escherichia coli, and the heterologously produced protein was purified 13-fold to apparent homogeneity. The enzyme, designated Cab, is thermostable at temperatures up to 75 degrees C. No esterase activity was detected with p-phenylacetate as the substrate. The enzyme is an apparent tetramer containing approximately one zinc per subunit, as determined by plasma emission spectroscopy. Cab has a CO(2) hydration activity with a k(cat) of 1.7 x 10(4) s(-1) and K(m) for CO(2) of 2.9 mM at pH 8.5 and 25 degrees C. Western blot analysis indicates that Cab (beta class) is expressed in M. thermoautotrophicum; moreover, a protein cross-reacting to antiserum raised against the gamma carbonic anhydrase from Methanosarcina thermophila was detected. These results show that beta-class carbonic anhydrases extend not only into the Archaea domain but also into the thermophilic prokaryotes.