Hydroxyl radical scavenging activity of compatible solutes

Silicon alleviates salt stress and increases antioxidant enzymes activity in leaves of salt-stressed cucumber (Cucumis sativus L.)

INTRODUCTION 369THE REQUIREMENT FOR CARBONIC ANHYDRASES IN DIFFERENTORGANISMS 370CYANOBACTERIA 371Carboxysomal Carbonic Anhydrase 373Is the Ci Pump a Membrane Carbonic Anhydrase? 374MICROALGAE 374Chloroplast Carbonic Anhydrase 375Periplasmie Carbonic Anhydrase 376Cytosolic Carbonic Anhydrase 377Induction of Carbonic Anhydrase at Low-C02 378AQUATIC MACROALGAE AND ANGIOSPERMS 379External Carbonic Anhydrose 379Internal Carbonic Anhydrase 380C4 PLANTS 381CAM PLANTS 382C3 PLANTS 382EVOLUTION OF PHOTOSYNTHETIC CARBONIC ANHYDRASES 385CONCLUSIONS 387

The Role of Carbonic Anhydrase in Photosynthesis

Carbonic anhydrases (CAs, EC 4.2.1.1) catalyse the interconversion between CO2 and bicarbonate as well as other hydrolytic reactions. Among the six genetic families known to date, the α-, β-, γ-, δ-, ζ- and η-CAs, detailed kinetic and X-ray crystallographic studies have allowed a deep understanding of the structure-function relationship in this superfamily of proteins. A metal hydroxide nucleophilic species of the enzyme, and a unique active site architecture, with half of it hydrophilic and the opposing part hydrophobic, allow these enzymes to act as some of the most effective catalysts known in Nature. The CA activation and inhibition mechanisms are also known in detail, with a large number of new inhibitor classes being described in the last years. Apart from the zinc binders, some classes of inhibitors anchor to the metal ion coordinated nucleophile, others occlude the entrance of the active site cavity and more recently, compounds binding outside the active site were described. CA inhibition has therapeutic applications for drugs acting as diuretics, antiepileptics, antiglaucoma, antiobesity and antitumour agents. Targeting such enzymes from pathogens may lead to novel anti-infectives. Successful structure-based drug design campaigns allowed the discovery of highly isoform selective CA inhibitors (CAIs), which may lead to a new generation of drugs targeting these widespread enzymes. The use of CAs in CO2 capture processes for mitigating the global temperature rise has also been investigated more recently.

Structure and function of carbonic anhydrases

Enzyme Activity Staining

Estimation of hydrogen peroxide in plant extracts using titanium(IV)

An inhibitor of catalase accumulated when leaves of chilling-sensitive species were stored in the dark at 0°C. The inhibitor could be removed from crude extracts by passing them through a column of Sephadex G-25. After this treatment, the catalase activity of extracts of chilled tissues was found to be equal to that of extracts from unchilled leaves. When chilled tissues were incubated at 20°C, the inhibitor of catalase was lost, unless the tissues had been irreversibly damaged. It specifically inhibited plant catalase, and had no effect on mammalian catalase, plant malic dehydrogenase, or plant superoxide dismutase. Despite the presence of catalase inhibitor in extracts of chilled plants, no increase in the level of H2O2 in chilled tissues was found, suggesting either that the inhibitor is compartmentalized and not in contact with catalase in vivo, or that the level of H2O2 is controlled by means other than through catalase activity. Plant tissues normally contain H2O2 which is destroyed by catalase when they are damaged. After chilling, H2O2 leaking from already injured cells would not be so readily removed by the inhibited catalase, and could contribute to further injury by acting as a source of free radical oxidants.

An Inhibitor of Catalase Induced by Cold in Chilling-Sensitive Plants

On the basis of polyacrylamide gradient gel electrophoresis of leaf extracts from 24 species of higher plants, two main forms of carbonic anhydrase (EC 4.2.1.1) were recognized; the “dicotyledon” type and the “monocotyledon” type. More than one band of enzyme was found on gels from most species, suggesting the possibility of carbonic anhydrase isoenzymes in higher plants.

Plant Carbonic Anhydrases: I. Distribution of Types among Species

Plant carbonic anhydrases (CAs) have a range of molecular weights (MW). Among flowering plants, dicotyledons with C3 photosynthesis have two isoenzymes of 140-250K each with 6 subunits, while monocotyledons have two isoenzymes of 42-45K. Plant and animal CAs have a similar amino acid content, subunit size and zinc content, suggesting they are homologous proteins, although the higher plant CAs have no esterase activity and are not strongly inhibited by sulfonamides. Algal CAs vary widely in MW and some are highly sensitive to sulfonamides like the animal enzymes. The two plant isoenzymes, from the chloroplast and cytosol, can be separated by gradient polyacrylamide gel electrophoresis and subsequently visualized by enzymic H+ ion production. In plants, CAs probably facilitate diffusion of CO2 to the site of photosynthetic fixation; they may also have a role in pH regulation, in the use of bicarbonate by aquatic plants and in concentrating inorganic carbon within the chloroplast.

Chemical Properties, Distribution, and Physiology of Plant and Algal Carbonic Anhydrases

Carbonic anhydrase (EC.4.2.1.1) was purified from leaves of the dicotyledon Pisum sativum L. (56-fold) and from leaves of the monocotyledon Tradescantia albiflora Kunth. (24-fold). The molecular weight of the Pisum enzyme was estimated to be 188,000 ± 8,000 with subunit sizes of 28,000 ± 3,000 and 56,600 ± 3,500. It contained 1 mole zinc per 32,500 ± 2,000 g protein. The molecular weight of the Tradescantia enzyme was estimated to be 42,000 ± 2,000 with a subunit size of 27,500 ± 2,200. It contained 1 mole zinc per 34,000 ± 2,000 g protein. The two enzyme preparations were different in specific activity, stability in solution, and sensitivity to sulfonamides and inorganic anions. Gel electrophoresis separated each purified preparation into two active enzyme bands.

Brian D. Patterson

Papers

Estimation of hydrogen peroxide in plant extracts using titanium(IV)

An Inhibitor of Catalase Induced by Cold in Chilling-Sensitive Plants

Plant Carbonic Anhydrases: I. Distribution of Types among Species

Chemical Properties, Distribution, and Physiology of Plant and Algal Carbonic Anhydrases

Plant Carbonic Anhydrases: II. Preparation and Some Properties of Monocotyledon and Dicotyledon Enzyme Types.