Is carbonic anhydrase activity of photosystem II required for its maximum electron transport rate?

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO₃^?/CO₂ as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α?carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of α?carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO₃^?/CO₂. Addition of exogenous HCO₃^? or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.     

Carbon Oxysulfide Inhibition of the CO(2)-Concentrating Process of Unicellular Green Algae

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O₂ evolution, dissolved inorganic carbon accumulation, and photosynthetic CO₂ fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO₂-concentrating process. This COS inhibition can be attributed to inhibition of external HCO₃⁻ conversion to CO₂ and OH⁻ by the carbonic anhydrase component of an active CO₂ pump. At a low pH of 5 to 6, COS stimulated O₂ evolution during photosynthesis by algae with low CO₂ in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO₂. Although COS had less effect on HCO₃⁻ accumulation at pH 9 by a HCO₃⁻ pump in Scenedesmus, COS reduced O₂ evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO₂ and H₂S, its inhibition of the CO₂ pump activity and photosynthesis is not accurate, when measured by O₂ evolution, by NaH¹⁴CO₃ accumulation, or by ¹⁴CO₂ fixation.     

Carboxysomal carbonic anhydrases: Structure and role in microbial CO2 fixation

Cyanobacteria and some chemoautotrophic bacteria are able to grow in environments with limiting CO₂ concentrations by employing a CO₂-concentrating mechanism (CCM) that allows them to accumulate inorganic carbon in their cytoplasm to concentrations several orders of magnitude higher than that on the outside. The final step of this process takes place in polyhedral protein microcompartments known as carboxysomes, which contain the majority of the CO₂-fixing enzyme, RubisCO. The efficiency of CO₂ fixation by the sequestered RubisCO is enhanced by co-localization with a specialized carbonic anhydrase that catalyzes dehydration of the cytoplasmic bicarbonate and ensures saturation of RubisCO with its substrate, CO₂. There are two genetically distinct carboxysome types that differ in their protein composition and in the carbonic anhydrase(s) they employ. Here we review the existing information concerning the genomics, structure and enzymology of these uniquely adapted carbonic anhydrases, which are of fundamental importance in the global carbon cycle.     

Identification and localization of a thylakoid-bound carbonic anhydrase from the green algae Tetraedron minimum (Chlorophyta) and Chlamydomonas noctigama (Chlorophyta)

In order to broaden our understanding of the eukaryotic CO₂-concentrating mechanism the occurrence and localization of a thylakoid-associated carbonic anhydrase (EC 4.2.1.1) were studied in the green algae Tetraedron minimum and Chlamydomonas noctigama. Both algae induce a CO₂-concentrating mechanism when grown under limiting CO₂ conditions. Using mass-spectrometric measurements of ¹⁸O exchange from doubly labelled CO₂, the presence of a thylakoid-associated carbonic anhydrase was confirmed for both species. From purified thylakoid membranes, photosystem I (PSI), photosystem II (PSII) and the light-harvesting complex of the photosynthetic apparatus were isolated by mild detergent gel. The protein fractions were identified by 77 K fluorescence spectroscopy and immunological studies. A polypeptide was found to immunoreact with an antibody raised against thylakoid carbonic anhydrase (CAH3) from Chlamydomonas reinhardtii. It was found that this polypeptide was mainly associated with PSII, although a certain proportion was also connected to light harvesting complex II. This was confirmed by activity measurements of carbonic anhydrase in isolated bands extracted from the mild detergent gel. The thylakoid carbonic anhydrase isolated from T. minimum had an isoelectric point between 5.4 and 4.8. Together the results are consistent with the hypothesis that thylakoid carbonic anhydrase resides within the lumen where it is associated with the PSII complex. Received: 13 May 2000 / Accepted: 16 August 2000     

The roles of carbonic anhydrases in photosynthetic CO2 concentrating mechanisms

Cyanobacteria, algae, aquatic angiosperms and higher plants have all developed their own unique versions of photosynthetic CO₂ concentrating mechanisms (CCMs) to aid Rubisco in efficient CO₂ capture. An important aspect of all CCMs is the critical roles that the specialised location and function that various carbonic anhydrase enzymes play in the overall process, participating the interconversion of CO₂ and HCO₃ ⁻ species both inside and outside the cell. This review examines what we currently understand about the nature of the carbonic anhydrase enzymes, their localisation and roles in the various CCMs that have been studied in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heterogeneous origin of carbonic anhydrase activity of thylakoid membranes

Carbonic anhydrase activities of pea thylakoids as well as thylakoid fragments enriched either in Photosystem 1 (PS1-membranes) or Photosystem 2 (PS2-membranes) were studied. The activity of PS1-membranes if calculated on chlorophyll basis was much higher than the activity of PS2-membranes. Acetazolamide, a non-permeable inhibitor of carbonic anhydrases, increased carbonic anhydrase activity of PS2-membranes at concentrations lower than 10⁻⁶ M and suppressed this activity only at higher concentrations. A lipophilic inhibitor of carbonic anhydrases, ethoxyzolamide, effectively suppressed the carbonic anhydrase activity of PS2-membranes (I ₅₀ = 10⁻⁹ M). Carbonic anhydrase activity of PS1-membranes was suppressed alike by both inhibitors (I ₅₀ = 10⁻⁶ M). In the course of the electrophoresis of PS2-membranes treated with n-dodecyl-β-maltoside “high-molecular-mass” carbonic anhydrase activity was revealed in the region corresponding to core-complex of this photosystem. Besides, carbonic anhydrase activity in the region of low-molecular-mass proteins was discovered in the course of such an electrophoresis of both PS2-and PS1-membranes. These low-molecular-mass carbonic anhydrases eluted from corresponding gels differed in sensitivity to specific carbonic anhydrase inhibitors just the same as PS1-membranes versus PS2-membranes. The results are considered as evidence for the presence in the thylakoid membranes of three carriers of carbonic anhydrase activity. Published in Russian in Biokhimiya, 2006, Vol. 71, No. 5, pp. 651–659.     

Basic muscle protein,a third genetic locus isoenzyme of carbonic anhydrase?

Rabbit muscle cytosol extract contains a basic protein which represents about 2% of the total cytosol protein. It contains zinc in a 1:1 stoichiometric ratio, based on a molecular weight of 30,000, and it catalyzes the hydration of CO₂. It is immunochemically distinct from the high and low activity forms of rabbit blood carbonic anhydrase. It has comparatively poor activity as an esterase, and about 20% of the CO₂ hydratase activity of the rabbit blood low activity carbonic anhydrase. This CO₂ hydratase activity is not inhibited by acetazolamide at concentrations which totally inhibit the activity of the blood carbonic anhydrases. The evidence obtained to date, though circumstantial, suggests that this basic metalloprotein is a carbonic anhydrase derived from a third genetic locus with properties considerably different from those of the mammalian carbonic anhydrases heretofore identified.     

The p-nitrophenyl phosphatase activity of muscle carbonic anhydrase

Carbonic anhydrase III from rabbit muscle, a newly discovered major isoenzyme of carbonic anhydrase, has been found to be also a p-nitrophenyl phosphatase, an activity which is not associated with carbonic anhydrases I and II. The p-nitrophenyl phosphatase activity has been shown to chromatograph with the CO₂ hydratase activity; both activities are associated with each of its sulfhydryl oxidation subforms; and both activities follow the same pattern of pH stability. This phosphomonoesterase activity of carbonic anhydrase III has an acidic pH optimum (<5.3); its true substrate appears to be the phosphomonoanion with a K_m of 2.8 mm. It is competitively inhibited by the typical acid phosphatase inhibitors phosphate (K_i = 1.22 × 10^?3M), arsenate (K_i = 1.17 × 10^?3M), and molybdate (K_i = 1.34 × 10^?7M), with these inhibitors having no effect on the CO₂ hydratase or the p-nitrophenyl acetate esterase activities of carbonic anhydrase III. The p-nitrophenyl acetate esterase activity of carbonic anhydrase III, on the other hand, has the sigmoidal pH profile with an inflection at neutral pH, typical of carbonic anhydrases for all of their substrates, and is inhibitable by acetazolamide (a highly specific carbonic anhydrase inhibitor) to the same degree as the CO₂ hydratase activity. The acid phosphatase-like activity of carbonic anhydrase III is slightly inhibited by acetazolamide at acidic pH, and inhibited to nearly the same degree at neutral pH. These data are taken to suggest that the phosphatase activity follows a mechanism different from that of the CO₂ hydratase and p-nitrophenyl acetate esterase activities and that there is some overlap of the binding sites.     

Advances in understanding the physiological role and locations of carbonic anhydrases in C3 plant cells

The review describes the structures of plant carbonic anhydrases (CAs), enzymes catalyzing the interconversion of inorganic carbon forms and belonging to different families, as well as the interaction of inhibitors and activators of CA activity with the active sites of CAs in representatives of these families. We outline the data that shed light on the location of CAs in green cells of C3 plants, algae and angiosperms, with the emphasis on the recently obtained data. The proven and proposed functions of CAs in these organisms are listed. The possibility of the involvement of several chloroplast CAs in acceleration of the conversion of bicarbonate to CO₂ and in supply of CO₂ for fixation by Rubisco is particularly considered. Special attention is paid to CAs in various parts of thylakoids and to discussion about current knowledge of their possible physiological roles. The review states that, despite the significant progress in application of the mutants with suppressed CAs synthesis, the approach based on the use of the inhibitors of CA activity in some cases remains quite effective. Combination of these two approaches, namely determining the effect of CA activity inhibitors in plants with certain knocked-out CA genes, turns out to be very useful for understanding the functions of other CAs.

     

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.     

INDUCIBLE MECHANISMS FOR HCO3– UTILIZATION AND REPRESSION OF PHOTORESPIRATION IN PROTOPLASTS AND THALLI OF THREE SPECIES OF ULVA (CHLOROPHYTA)1

Thalli of Ulva reticulata Forskaal, Ulva rigida C. Ag., and Ulva pulchra Jaasund were incubated at different concentrations of dissolved CO₂. Incubation at a high CO₂ concentration resulted in decreased oxygen evolution rate and lower affinity for inorganic carbon at high pH conditions, i.e. the ability to use HCO₃^– as a carbon source was reduced. This effect was reversible, and plants regained this HCO₃^– uptake capacity when transferred to air concentrations of CO₂. The phytosynthetic oxygen evolution rate of plants grown at high CO₂ concentration was reduced by high O₂ concentrations, whereas thalli and protoplasts from cultures grown at air concentration were not affected. This is interpreted as a deactivation of the carbon-concentrating mechanism during conditions of high CO₂ resulting in high photorespiration when plants are exposed to high O₂ concentrations. Protoplasts were not affected by high O₂ to the same extent and were not able to utilize HCO₃^– from the medium. The algae were able to grow at very low CO₂ concentrations, but growth was suppressed when an inhibitor of external carbonic anhydrase was present. Assay of carbonic anhydrase activities showed that external and internal CA activities were lower in plants grown at a high CO₂ concentration compared to plants grown at a low concentration of CO₂. Possible mechanisms for HCO₃^– utilization in these Ulva species are discussed.     

Carbonic anhydrase activity of Prochloron sp. isolated from an ascidian host

The prokaryotic algal symbiont of ascidians, Prochloron sp., was found to exhibit carbonic anhydrase activity which is largely associated with the cell surface. This extracellular carbonic anhydrase activity was inhibited, while the intracellular activity was not affected, by chloride or bromide. Acetazolamide and ethoxyzolamide inhibited carbonic anhydrase activity with I₅₀ values of 7×10^-4 and 3×10^-4M, respectively. These I₅₀ values are similar to those observed for intracellular carbonic anhydrases of Synechococcus sp. PCC7942, Chlamydomonas reinhardii and spinach.Abbreviations AZA acetazolamide - CA carbonic anhydrase - chl chlorophyll - EZA ethozyzolamide - I₅₀ concentration of an inhibitor required to cause 50% inhibition - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - U unit     

Introducing an algal carbon‐concentrating mechanism into higher plants: location and incorporation of key components

Many eukaryotic green algae possess biophysical carbon‐concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO₂ concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H¹⁴CO₃^‐ uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild‐type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.     

Arabidopsis thaliana carbonic anhydrase: cDNA sequence and effect of CO2 on mRNA levels

A full-length cDNA clone encoding carbonic anhydrase was isolated from an Arabidopsis thaliana (Columbia) leaf library. Comparison of the derived amino acid sequence obtained from this clone with those of pea and spinach reveals a considerable degree of identity. The carbonic anhydrase cDNA was used to probe the level of RNA encoding this protein in the leaves of plants grown in elevated CO₂ (660 ppm). We have found that under these conditions the steady-state level of carbonic anhydrase mRNA was increased in comparison with control plants grown in normal atmospheric concentrations of CO₂ (330 ppm). This raises the intruiging possibility that there exists in higher plants a mechanism for perceiving and responding to changes in environmental CO₂ concentrations at the genetic level.     

Structure and catalytic mechanism of the β-carbonic anhydrases

The β-carbonic anhydrases (β-CAs) are a diverse but structurally related group of zinc-metalloenzymes found in eubacteria, plant chloroplasts, red and green algae, and in the Archaea. The enzyme catalyzes the rapid interconversion of CO₂ and H₂O to HCO₃⁻ and H⁺, and is believed to be associated with metabolic enzymes that consume or produce CO₂ or HCO₃⁻. For many organisms, β-CA is essential for growth at atmospheric concentrations of CO₂. Of the five evolutionarily distinct classes of carbonic anhydrase, β-CA is the only one known to exhibit allosterism. Here we review the structure and catalytic mechanism of β-CA, including the structural basis for allosteric regulation.     

Carbon Concentration Mechanism(s) in Unicellular Green Algae and Cyanobacteria

Unicellular green algae and cyanobacteria have mechanism(s) to actively concentrate dissolved inorganic carbon (DIC) into the cells, only if they are grown with air levels of CO₂. The DIC concentration mechanisms are environmental adaptations to actively transport and accumulate inorganic carbon into the chloroplasts of green algae or into the carboxysomes of cyanobacteria. The current working model of cyanobacterial carbon concentration mechanism consists of at least two basic components: an active C_i transport system and a Rubisco-rich polyhedral carboxysome. In case of unicellular green algae, the working model for DIC concentration mechanism includes several isoforms of carbonic anhydrase (CA), and ATPase driven active bicarbonate transporters at the plasmalemma and at the inner chloroplast envelopes. In the past twenty years, significant progress has been made in isolating and characterizing the isoforms of carbonic anhydrase. However, active transporters are yet to be characterized. This mini-review summarizes the current status of research on DIC-pumps including its significance and possible application to increase the productivity of plants of economic importance.     

Effect of Carbonic Anhydrase Inhibitors on Inorganic Carbon Accumulation by Chlamydomonas reinhardtii

Membrane-permeable and impermeable inhibitors of carbonic anhydrase have been used to assess the roles of extracellular and intracellular carbonic anhydrase on the inorganic carbon concentrating system in Chlamydomonas reinhardtii. Acetazolamide, ethoxzolamide, and a membrane-impermeable, dextran-bound sulfonamide were potent inhibitors of extracellular carbonic anhydrase measured with intact cells. At pH 5.1, where CO₂ is the predominant species of inorganic carbon, both acetazolamide and the dextran-bound sulfonamide had no effect on the concentration of CO₂ required for the half-maximal rate of photosynthetic O₂ evolution (K_0.5[CO₂]) or inorganic carbon accumulation. However, a more permeable inhibitor, ethoxzolamide, inhibited CO₂ fixation but increased the accumulation of inorganic carbon as compared with untreated cells. At pH 8, the K_0.5(CO₂) was increased from 0.6 micromolar to about 2 to 3 micromolar with both acetazolamide and the dextran-bound sulfonamide, but to a higher value of 60 micromolar with ethoxzolamide. These results are consistent with the hypothesis that CO₂ is the species of inorganic carbon which crosses the plasmalemma and that extracellular carbonic anhydrase is required to replenish CO₂ from HCO₃⁻ at high pH. These data also implicate a role for intracellular carbonic anhydrase in the inorganic carbon accumulating system, and indicate that both acetazolamide and the dextran-bound sulfonamide inhibit only the extracellular enzyme. It is suggested that HCO₃⁻ transport for internal accumulation might occur at the level of the chloroplast envelope.     

Purification and properties of the carbonic anhydrase of Rhodospirillum rubrum

The carbonic anhydrase (EC 4.2.1.1) of Rhodospirillum rubrum has been purified to apparent homogeneity and some of its properties have been determined. The enzyme was cytoplasmic and was found only in photosynthetically grown cells. It had a molecular weight of about 28,000, and was apparently composed of two equal subunits. The amino acid composition was similar to that of other reported carbonic anhydrases except that the R. rubrum enzyme contained no arginine. The isoelectric point of the enzyme was 6.2 and the pH optimum was 7.5. It required Zn(II) for stability and enzymatic activity. The K _m(CO₂) was 80 mM. Typical carbonic anhydrase inhibition patterns were found with the R. rubrum enzyme. Strong acetazolamide and sulfanilamide inhibition confirmed the importance of Zn(II) for enzymatic activity as did the anionic inhibitors iodide, and azide. Other inhibitors indicated that histidine, sulfhydryl, lysine and serine residues were important for enzymatic activity.Abbreviation CA carbonic anhydrase In memory of R. Y. Stanier     

Chloroplast pH values and buffer capacities in darkened leaves as revealed by CO2 solubilization in vivo

Leaves of the C₃ plants Brassica oleracea L., Datura suaveolens Humb. & Bonpl. ex Willd., Helianthus annuus L. and Nicotiana tabacum L. with open stomata were exposed in a leaf chamber in the dark to CO₂ concentrations varying from 1 to 20% in air. When they were transferred back into CO₂-free air, CO₂ was rapidly released. It originated from dissolved CO₂ and from the bicarbonate in the chloroplast stroma, since vacuoles are acidic and chloroplasts contain carbonic anhydrase which rapidly liberates CO₂ from bicarbonate. The data were fitted to a model which accounts for the CO₂/bicarbonate equilibrium in buffers with different CO₂ concentrations and initial pH values. From this, pH values and CO₂-dependent pH changes in the chloroplast stroma were calculated. The full range of external CO₂ concentration caused acidic shifts up to 1 pH unit. The best fits of the data points were obtained with stromal buffer concentrations ranging from 45 to 65 mM and stromal pH values at low CO₂ between 7.5 and 7.9. Calculated buffer capacities ranged from 23 to 31 mM H⁺ per pH unit. The work shows that measurements of solubilized CO₂ are useful to investigate proton buffering and pH regulation in the chloroplast stroma of intact leaves.Abbreviation Chl chlorophyll This work was supported by Sonderforschungsbereich 251 of the University of Würzburg and the Volkswagenstiftung grant I-67762. We are very grateful to Dr. V. Oja for helpful advice.