The effects of carbonic anhydrase inhibitors formate, bicarbonate, acetazolamide, and imidazole on photosystem II in maize chloroplasts

High and unique carbonic anhydrase activity of Photosystem II from Pisum sativum: Measurements by a new and very sensitive fluorescence method

Carbonic anhydrase (CA) activity, associated with Photosystem II (PSII), has been shown to enhance water oxidation. However, CA activity was thought to be a side effect or even a "contamination" of other CAs because of the relatively low rates of CA reactions in PSII measured previously. Here, by using 8-hydroxy-pyrene-1,3,6-trisulfonate (pyranine), a fluorescent dye, as a pH indicator, we show that PSII preparations (∗∗BBYs) from Pisum sativum have a high CA activity (as measured by HCO3- dehydration), which is close to that of highly active CAs. This fluorescence method is new for BBYs giving at least ten times higher activity than the other methods used earlier, as well as being highly sensitive and, thus, more convenient to use for BBYs than any other approach. We show here that the pH range of 5.0-7.5 is optimum for the pyranine measuring system, in general, and this pH range is suitable not only for the CA in BBYs but also for other CAs. Further, the CA activity of BBYs has the following unique properties: (1) low sensitivity to some known, and otherwise, effective CA inhibitors; (2) an opposite pH profile of HCO3- dehydration than observed in other known CAs. These findings indicate that the high CA activity, we have observed, belongs to BBYs, i.e., free of other CAs. At pH 6.5, CA activity of BBYs is shown to be directly correlated with that of photosynthetic O2 evolution. We propose that the CA activity may accelerate the removal of H+s during water oxidation. # Celebrating 80th birthday of Alan James Stemler, a pioneer on the role of bicarbonate on the electron donor side of Photosystem II. S.G. Vaklinova & associates (1982), and A.J. Stemler (1986) were the first who have measured carbonic anhydrase activity in Photosystem II preparations. ∗∗ BBYs stand for Photosystem II samples made by the procedure of Berthold (B), Babcock (B) and Yocum (Y); see Berthold et al. (1981).

Single and combined effects of the drugs salicylic acid and acetazolamide: Adverse changes in physiological parameters of the freshwater macrophyte, Lemna gibba

Pharmaceutical drugs are among the most used chemicals, for human and veterinary medicines, aquaculture and agriculture. Pharmaceuticals are biologically active molecules, having also environmental persistence, thereby exerting biological effects on non-target species. Among the most used pharmaceuticals, one may find salicylic acid (SA), a non-steroid anti-inflammatory drugs (NSAIDs), and acetazolamide (ACZ), a diuretic drug that acts by inhibiting the activity of carbonic anhydrase (CA). In this work, single and combined effects of SA and ACZ were assessed in the aquatic macrophyte Lemna gibba L., focusing on physiological parameters, namely photosynthetic pigments, (chlorophyll a, b and total (Chl a, b and TChl) as well as carotenoids (Car)). In addition, chemical biomarkers, namely, glutathione S-transferases (GSTs), catalase (CAT) and carbonic anhydrase (CA) activities, were also determined. The highest concentrations of ACZ, caused a decrease in the contents of all chlorophylls; this effect was however reverted by SA exposure. Both ACZ and SA levels caused a decrease in CA activity. Nevertheless, when in combination, this inhibition was not observed in plants exposed to the lowest concentration of these drugs. In conclusion, both pharmaceuticals have the capacity to cause alterations in L. gibba enzymatic activity and photosynthetic pigments content. Additionally, SA seems to exert a protective effect on this species against deleterious effects caused by ACZ.

Emerging roles for carbonic anhydrase in mesophyll conductance and photosynthesis

Carbonic anhydrase (CA) is an abundant protein in most photosynthesizing organisms and higher plants. This review paper considers the physiological importance of the more abundant CA isoforms in photosynthesis, through their effects on CO₂ diffusion and other processes in photosynthetic organisms. In plants, CA has multiple isoforms in three different families (α, β and γ) and is mainly known to catalyze the CO₂↔HCO3- equilibrium. This reversible conversion has a clear role in photosynthesis, primarily through sustaining the CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Despite showing the same major reaction mechanism, the three main CA families are evolutionarily distinct. For different CA isoforms, cellular localization and total gene expression as a function of developmental stage are predicted to determine the role of each family in relation to the net assimilation rate. Reaction-diffusion modeling and observational evidence support a role for CA activity in reducing resistance to CO₂ diffusion inside mesophyll cells by facilitating CO₂ transfer in both gas and liquid phases. In addition, physical and/or biochemical interactions between CAs and other membrane-bound compartments, for example aquaporins, are suggested to trigger a CO₂ -sensing response by stomatal movement. In response to environmental stresses, changes in the expression level of CAs and/or stimulated deactivation of CAs may correspond with lower photosynthetic capacity. We suggest that further studies should focus on the dynamics of the relationship between the activity of CAs (with different subcellular localization, abundance and gene expression) and limitations due to CO₂ diffusivity through the mesophyll and supply of CO₂ to photosynthetic reactions.

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.

Substantial role for carbonic anhydrase in latitudinal variation in mesophyll conductance of Populus trichocarpa Torr. & Gray

In Populus trichocarpa (black cottonwood), net photosynthesis (A_n ) varies with latitude and, in northern genotypes, is supported by higher stomatal conductance (g_s ). We report here a parallel cline in mesophyll conductance (g_m ) and link this variation to carbonic anhydrase (CA) activity. Using concurrent carbon isotope discrimination and chlorophyll fluorescence methods, we examined the effects of acetazolamide, an inhibitor of CA, on g_m in six representative genotypes (three from either end of the north-south cline). Acetazolamide reduced CA activity, g_m , g_s , chloroplast CO₂ concentration (C_c ) and A_n at normal CO₂ (400 μmol mol^-1 ), the latter being reversible at saturating CO₂ . Absolute reductions in A_n , g_m and CA activity were greater in northern genotypes than in southern genotypes (P < 0.025) but percent reductions were similar. In contrast, northern genotypes showed lower percent reduction in C_c compared to southern genotypes (P < 0.025). The northern genotypes had greater CA activity relative to both leaf area (two-fold) and mass (1.8-fold) (P < 0.016). The relationship between CA activity and g_m was similar whether the variation was inherent or inhibitor induced. We suggest that greater CA activity contributes to higher g_m in northern P. trichocarpa genotypes, but other diffusion pathway components may also be involved.

Carbonic anhydrases in photosynthetic cells of higher plants

This review presents information about carbonic anhydrases, enzymes catalyzing the reversible hydration of carbon dioxide in aqueous solutions. The families of carbonic anhydrases are described, and data concerning the presence of their representatives in organisms of different classes, and especially in the higher plants, are considered. Proven and hypothetical functions of carbonic anhydrases in living organisms are listed. Particular attention is given to those functions of the enzyme that are relevant to photosynthetic reactions. These functions in algae are briefly described. Data about probable functions of carbonic anhydrases in plasma membrane, mitochondria, and chloroplast stroma of higher plants are discussed. Update concerning carbonic anhydrases in chloroplast thylakoids of higher plants, i.e. their quantity and possible participation in photosynthetic reactions, is given in detail.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

A carbonic anhydrase inhibitor induces bicarbonate-reversible suppression of electron transfer in pea photosystem 2 membrane fragments

The effects of suppression of the carbonic anhydrase (CA) activity by a CA-inhibitor, acetazolamide (AA), on the photosynthetic activities of photosystem II (PS II) particles from higher plants were investigated. AA along with CA-activity inhibits the PS II photosynthetic electron transfer and the AA-induced suppression is totally reversed by the addition of bicarbonate (3-5 mM). Similar effect of recovery in the PS II photosynthetic activity was also revealed upon the addition of known artificial electron donors (potassium ferrocyanide and TMPD). Significance and possible functions of CA for the PS II donor side are discussed.

A pea mutant (costata) expressing higher activity in thylakoid membrane-bound carbonic anhydrase alters PSII downregulation mechanisms

The interaction between photosynthetic electron transport and the activities of the thylakoid associated carbonic anhydrase (tCA), estimated as combined tCA activity in pea plants (Pisum sativum L. Borek cv., WT) and mutant form (costata 2/125) that differ in chlorophyll content have been compared. Chlorophyll a fluorescence changes after the inhibition of tCA by ethoxyzolamide (EZ), estimating possible role of tCA in PSII downregulation were investigated. Costata expresses higher tCA activity and higher O2 evolution in comparison to WT. Inhibition of tCA by EZ decreased effective PSII photochemistry that coincided with an enhancement in thermal dissipation, while maximal PSII quantum yield (F(v)/F(m)) did not significantly change. Ethoxyzolamide induced changes in fluorescence parameters that were more strongly expressed in costata 2/125. The results show that tCA is involved in the regulation of the proton gradient across thylakoid membranes and thus limits PSII downregulation.

Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II

Since the end of the 1950s hydrogencarbonate ('bicarbonate') is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn(4)O(x)Ca cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (~0.3) HCO(3)(-) per PSII upon addition of formate. The same amount of HCO(3)(-) release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO(3)(-) from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO(3)(-) is corroborated by showing that a reductive destruction of the Mn(4)O(x)Ca cluster inside the MIMS cell by NH(2)OH addition does not lead to any CO(2)/HCO(3)(-) release. We note that even after an essentially complete HCO(3)(-)/CO(2) removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain > or =75% of their initial flash-induced O(2)-evolving capacity. We therefore conclude that HCO(3)(-) has only 'indirect' effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.

A novel alpha-type carbonic anhydrase associated with the thylakoid membrane in Chlamydomonas reinhardtii is required for growth at ambient CO2

A 29.5 kDa intracellular alpha-type carbonic anhydrase, designated Cah3, from the unicellular green alga Chlamydomonas reinhardtii is the first of this type discovered inside a photosynthetic eukaryote cell. We describe the cloning of a cDNA which encodes the protein. Immunoblot studies with specific antibodies raised against Cah3 demonstrate that the polypeptide is associated exclusively with the thylakoid membrane. The putative transit peptide suggests that Cah3 is directed to the thylakoid lumen, which is confirmed further by the presence of mature sized Cah3 after thermolysin treatment of intact thylakoids. Complementation of the high inorganic carbon concentration-requiring mutant, cia-3, with a subcloned cosmid containing the cah3 gene yielded transformants that grew on atmospheric levels of CO2 (0.035%) and contained an active 29.5 kDa alpha-type carbonic anhydrase. Although, cia-3 has reduced internal carbonic anhydrase activity, unexpectedly the level of Cah3 was similar to that of the wild-type, suggesting that the mutant accumulates an inactive Cah3 polypeptide. Genomic sequence analysis of the mutant revealed two amino acid changes in the transit peptide. Results from photosynthesis and chlorophyll a fluorescence parameter measurements show that the cia-3 mutant is photosynthetically impaired. Our results indicate that the carbonic anhydrase, extrinsically located within the chloroplast thylakoid lumen, is essential for growth of C.reinhardtii at ambient levels of CO2, and that at these CO2 concentrations the enzyme is required for optimal photosystem II photochemistry.

Oxidation-reduction potential dependence of photosystem II carbonic anhydrase in maize thylakoids

In characterizing the carbonic anhydrase (CA) found in maize thylakoid membranes, it was observed that the enzyme's activity was inhibited somewhat when the Hill oxidant, ferricyanide, was given in the dark [Stemler, A. (1986) Biochim. Biophys. Acta 850, 97-107]. In the present work, a redox titration of this effect shows that the CA activity is mediated by a component that has a midpoint potential (Em) of about 485 mV at pH 6.5 and a pH dependence of 60 mV/pH. These redox titration characteristics are identical to those of the redox mediator "D480", which modulates formate and bicarbonate binding affinity to photosystem II (PS II). Bicarbonate binds to PS II more readily, and CA activity is higher, when D480 is reduced, whereas both bicarbonate binding and thylakoid-bound CA activity are low when D480 is oxidized in the dark by ferricyanide. Both the low bicarbonate binding affinity and the low CA activity induced by the presence of ferricyanide are reversed by a single saturating flash of light. In contrast, the activity of soluble CA, which is extracted from maize mesophyll cytosol, does not exhibit any redox dependence in the range 400-550 mV. Furthermore, thylakoid-bound CA activity is inhibited by 5 mM ZnCl2 by as much as 75%, whereas the activity of soluble CA shows no significant decrease induced by ZnCl2. Also, at a medium potential of 400 mV, ferricyanide (1 mM) inhibits soluble CA activity by 88% and thylakoid-bound CA activity by only 18%. It is concluded from these results that CA activity observed in thylakoids arises from CA inherent to PS II and is not some form of contamination by soluble CA. Possible roles of CA in PS II reaction mechanisms are discussed.

Characterization of the complex interaction between the electron acceptor silicomolybdate and Photosystem II

Silicomolybdate (SiMo) and its effects on thylakoids have been characterized to evaluate its use as a probe for Photosystem II (PS II). It can accept electrons at two places in the electron transport chain: one at PS II and the other at PS I. In the presence of 1 μM 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) only the site at PS II is available. It is suggested that SiMo must disp;ace bicarbonate from its binding site to be able to function as an electron acceptor. This displacement is non-competitive. The binding of SiMo is inhibited differentially by PS II inhibitors: dinoseb>ioxynil> diuron. This difference is determined by the different positions of the inhibitors within the QB binding niche and their interaction with bicarbonate. The experimental results show that the SiMo-binding niche is located between the parallel helices of the D1 and D2 proteins of PS II, close to the non-heme iron. We conclude that SiMo is an electron acceptor with unique characteristics useful as a probe of the acceptor side of PS II.

Bicarbonate effects in leaf discs from spinach

In this paper, we show the unique role of bicarbonate ion in stimulating the electron transfer of photosystem II (PS II) in formate-treated leaf discs from spinach. This is referred to as the "bicarbonate effect" and is independent of the role of CO2 in CO2 fixation. It is shown to have two sites of action: (1) the first, described here for the first time, stimulates the electron flow between the hydroxylamine donation site ("Z" or "D") and QA, the first plastoquinone electron acceptor and (2) the other accelerates the electron flow beyond QA, perhaps at the QA QB complex, where QB is the second plastoquinone electron acceptor. The first site of inhibition by formate-treatment is detected by the decrease of the rate of oxygen evolution and the simultaneous quenching of the variable chlorophyll a (Chl a) fluorescence of leaf discs infiltrated with 100 mM formate for about 10 s followed by storage for 10 min in dark. This is referred to as short-term formate treatment. Addition of bicarbonate reverses this short-term formate effect and restores fully both Chl a fluorescence and oxygen evolution rate. Reversible quenching of variable Chl a fluorescence of heated and short-term formate treated leaf discs, in the presence of hydroxylamine as an artificial electron donor to PS II, is also observed. This suggests that the first site of action of the anion effect is indeed between the site of donation of hydroxylamine to PS II (i.e. "Z" or "D") and QA. The second site of the effect, where bicarbonate depletion has its most dramatic effect, as well known in thylakoids, is shown by an increase of Chl a fluorescence of leaf discs infiltrated with 100 mM formate for about 10 min followed by storage for 10 min in dark. This is referred to as the long-term formate treatment. Addition of bicarbonate fully restores the variable Chl a fluorescence of these leaf discs. Chl a fluorescence transient of DCMU-infiltrated (10 min) leaf discs is similar to that of long-term formate-treated one suggesting that the absence of bicarbonate, like the presence of DCMU, inhibits the electron flow beyond QA.

Multiple anion effects on photosystem II in chloroplast membranes

We investigated the activity of several anions at various sites on photosystem II, in particular those associated with the Cl(-) effect (anion binding-site I) and the HCO3 (-) effect (anion binding-site II). Chlorophyll a fluorescence changes were used to monitor partial photosystem II reactions either in the oxygen-evolving mechanism or involving endogenous quinone electron acceptors. We find that anions such as NO3 (-), HCO3 (-), HCO2 (-), F(-), NO2 (-), and acetate can, depending on conditions, bind to either anion binding-site I, anion binding-site II, or both sites simultaneously. The anions N3 (-) and Au(CN)2 (-) are exceptions. In their presence, oxygen-consumption reactions are enhanced. The results demonstrate that an exclusive site or mode of action of an anion on photosystem II cannot be determined by measuring the Hill reaction alone. Anion interactions with photosystem II are shown to be very complex and, therefore, caution is advisable in interpreting related experiments. Carbonic anhydrase associated with photosystem II was also investigated as a possible target for some anion effects. In Cl(-)-depleted thylakoids, NO3 (-), stimulated both electron transport and carbonic anhydrase activity at low concentrations, while higher concentrations inhibited both. However, carbonic anhydrase was more sensitive to inhibition by NO3 (-) than was electron flow. Possible interpretations are discussed; the electron transport and carbonic anhydrase activity appear not to be functionally linked.

Electron transfer through photosystem II acceptors: Interaction with anions

We present an overview of anionic interactions with the oxidation-reduction reactions of photosystem II (PSII) acceptors. In section 1, a framework is laid for the electron acceptor side of PSII: the overview begins with a current scheme of the electron transport pathway and of the localization of components in the thylakoid membrane, which is followed by a brief description of the electron acceptor Q or QA and the various heterogeneities associated with it. In section 2, we review briefly the nature of the active species of the bicarbonate (HCO3 (-)) effect, the location of the site of action of HCO3 (-), and its relationship to interactions with other anions. In section 3, we review data on the anion effects on the reoxidation of QA (-) and on the various reactions involved in the two-electron gate mechanism of PSII, and provide a hypothesis as to the action of HCO3 (-) on the protonation reactions. New data obtained by one of us (G) in collaboration with J.J.S. van Rensen, J.F.H Snel and W. Tonk for HCO3 (-)-depleted thylakoids, demonstrating the abolition of the binary oscillations contained within the periodicity of 4 observed for proton release, are also reviewed. In section 4, we comment on the measured binding constant of HCO3 (-) at the anion binding site. And, in section 5, we review our current concept of the mechanism of the HCO3 (-) effect on the electron acceptor side of PSII, and comment on the possible physiological roles for HCO3 (-). Measurements of HCO3 (-) reversible anionic inhibition in intact cells of a green alga Scenedesmus are also reviewed.

Regulation of photosynthetic electron transport by bicarbonate formate and herbicides in isolated broken and intact chloroplasts

In this paper, we have presented a minireview on the interaction of bicarbonate, formate and herbicides with the thylakoid membranes.The regulation of photosynthetic electron transport by bicarbonate, formate and herbicides is described. Bicarbonate, formate, and many herbicides act between the primary quinone electron acceptor QA and the plastoquinone pool. Many herbicides like the ureas, triazines and the phenol-type herbicides act, probably, by the displacement of the secondary quinone electron acceptor QB from its binding site on a QB-binding protein located at the acceptor side of Photosystem II. Formate appears to be an inhibitor of electron transport; this inhibition can be removed by the addition of bicarbonate. There appears to be an interaction of the herbicides with bicarbonate and/or It has been suggested that both the binding of a herbicide and the absence of bicarbonate may cause a conformational alteration of the environment of the QB-binding site. The alteration brought about by a herbicide decreases the affinity for another herbicide or for bicarbonate; the change caused by the absence of bicarbonate decreases the affinity for herbicides. Moreover, this change in conformation causes an inhibition of electron transport. A bicarbonate-effect in isolated intact chloroplasts is demonstrated.