The carbonic anhydrases or carbonate dehydratases form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid i. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide. Carbonic anhydrase helps regulate pH and fluid balance.

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Carbonic Anhydrase and Zinc in Plant Physiology. Vencedores del Desierto. Carbonic anhydrase CA EC: 2. Carbonic anhydrase is a metalloenzyme that requires Zn as a cofactor and is involved in diverse biological processes including pH regulation, CO 2 transfer, ionic exchange, respiration, CO 2 photosynthetic fixation, and stomatal closure. Therefore, the review includes relevant aspects about CA morphology, oligomerization, and structural differences in the active site.

On the other hand, we consider the general characteristics of Zn, its geometry, reactions, and physiology. We then consider the CA catalysis mechanism that is carried out by the metal ion and where Zn acts as a cofactor.

Zinc deficiency can inhibit growth and protein synthesis, and there is evidence that it reduces the CA content in some plants, which is a relationship addressed in this review. In leaves, CA represents Key words: Cofactor, metal ion catalysis, metalloenzyme, zinc deficiency, CO 2 transfer. Plant physiology depends on diverse metabolic processes involving a large number of enzymes which, in turn, also depend on other elements such as cofactors or coenzymes to be activated and catalyzed.

One of the many enzymes involved in physiological processes is carbonic anhydrase CA. In plants, CA helps to elevate CO 2 concentration in the chloroplast to increase the carboxylation rate of the RuBisCO enzyme ribulose 1,5-disphosphate carboxylase. This is the reaction that incorporates CO 2 into carbohydrates during photosynthesis and can only use the C in CO 2 instead of carbonic acid or bicarbonate.

The catalytic mechanism of CA is achieved by the metal ion, and a characteristic of metal ion catalysis is manifested by CO 2 hydration through a mechanism binding Zn to hydroxide Kimber and Pai, Zinc deficiency may inhibit growth by reducing the available Zn content in the plant, which directly affects metabolism by altering the balance of other nutrients in the plants, such as iron, P, and Cu.

The effect of Zn deficiency in CA cannot be selective because it causes a general decrease in protein synthesis. Fuss, and Spinacia oleracea L. Tobin, Koch Snir, and rice Oryza sativa L. Sasaki et al. Although studies have been performed with CA, it is not being used in a practical way as a nutritional diagnostic test to detect Zn deficiency. Thus, this paper aims to differentiate the biochemical processes that exist between CA and Zn in plant physiology, as well as the theoretical elements needed to develop and implement new nutritional diagnostic techniques that establish the state of the art on this matter with the hope of benefits for technological development.

Carbonic anhydrase was found in bovine erythrocytes by Meldrum and Roughton in and Neish accidentally discovered the first p carbonic anhydrase as a component of the chloroplast of the plant leaf in Only CA families have been well characterized as catalysts in cells and tissue in all forms of life.

Based on its amino acid sequence of carbonic anhydrases CAs , it can be classified into three separate families designated as alpha, beta, and gamma Hewett-Emmett and Tashian, It should be noted that plant leaf chloroplast CA was not recognized as a different form of CA, evolutionary and structurally, until five decades later with the advent of DNA sequencing.

At that time, it was the second known evolutionarily different form of CA and was therefore designated as p. Since , many authors have shown the presence of p-CA not exclusively in photosynthetic organisms, but also in eubacteria, yeast, and archaea species Smith and Ferry, ; Moroney et al.

Beta-CA is an essential component of carboxysome, an organelle with high CO 2 concentrations in cyanobacteria. This was quickly followed by the structure of the Pisum sativum plant Kimber and Pai, and E. The Zn ion is found in a pseudo-tetrahedral environment of the Cys2His X coordination type, where X is water, an anion, or an Asp residue. This Zn ion defines the active site of the enzyme Krissinel and Henrick, Figure 1.

Ribbon diagram of carbonic anhydrase fundamental structure obtained by X-ray crystallography, which is manifested as a dimer with monomers shown in orange and green.

The Zn ions are shown as magenta spheres Kimber and Pai, Figure 2. Isoenzymes Enzymes usually have multiple forms by varying their molecular mass and metabolism regulation activities. These forms are called isoenzymes; they play a major role in the adaptation of organisms and are used as biochemical co-dominant markers to identify genotypes and establish phylogenetic relationships among different groups of taxa.

Carbonic anhydrase isoenzymes have been determined in the 24 monocot and dicot plant species Atkins et al. We talk about homo-oligomers if the subunits are identical and hetero-oligomers if they are different.

Many of these protein-protein associations are involved in diverse chain reactions in the cellular processes. The state of oligomerization seems to be driven by surface extensions or unique elaboration of the secondary structure of the p-CA core Kimber and Pai, ; Krissinel and Henrick, Figure 3. Zinc is a metal that is sometimes classified as a transition metal, although strictly speaking it is not since Zn and its type of device have full d orbitals.

This element resembles Mg and Cd in their group, but Hg is far away due to its unique physical and chemical properties lanthanide contraction and powerful relativistic effects on linking orbitals. It is a chemically active metal; it produces a blue greenish flame when burning that releases Zn oxide as smoke Barak and Helmke, It is number 23 in the list of the most abundant elements on earth and has five isotopes: 64 Zn In plants, it has been reported that there are fractions of heavy isotopes in the roots and light isotopes in the shoots Weiss et al.

Zinc forms many soluble salts including halides, sulfates, nitrates, formates, acetates, thiocyanates, perchlorates, fluorosilicates, cyanides, zincates with alkali metals, and Zn-ammonia salts. On the other hand, it also forms moderate soluble compounds including Zn-ammonium phosphate, Zn hydroxide, and Zn carbonate, along with a series of soluble and insoluble organic compounds Barak and Helmke, Zinc properties The inherent chemical potential and reactivity of Zn are not exceptional compared to other metals.

However, unlike other transition elements from the first row metals e. Therefore, the Zn ion is an ideal metal cofactor for reactions requiring a stable redox ion to function as a Lewis acid and as a catalyst in proteolysis and CO 2 hydration Butler, In all the Zn metalloenzymes studied so far, the most often observed binding geometry is a slightly distorted tetrahedral Figure 4 with the metal ion coordinated with three or four protein side chains. However, in other Zn metalloenzymes, another type of geometry has been observed McCall et al.

Figure 4. Geometry of zinc McCall et al. In the Zn protein binding sites, the Zn ion is coordinated by different combinations of the protein side chains Gregory et al. In Zn proteins, the role of the Zn ion is important and can be catalytic, co-catalytic, or structural. In a catalytic Zn site, Zn ions directly participate in the reaction.

There are several metal ions in close proximity to each other in a co-catalytic Zn site where Zn plays a catalytic role, while the other metal ions enhance the site's catalytic activity.

Finally, in structural sites, Zn ions mainly stabilize the tertiary structure of the enzyme in a manner analogous to the disulfide bonds. In all cases, eliminating Zn can lead to a loss of enzymatic activity Vallee and Auld, Reactions of Zn with water Water bound to Zn is a crucial component to activate the catalytic site since it can be ionized to form a hydroxide bridge as in CA , polarized by some base to generate nucleophilic catalysis, or displaced by the substrate Figure 5.

Figure 5. Simplified configuration of zinc reactions with water. Biochemistry and physiology of Zn The mechanism controlling Zn homeostasis is not exactly known yet Hacisalihoglu et al. However, it has long been acknowledged that Zn is essential for cell physiological processes Barak and Helmke, by influencing different processes as a micronutrient in plants Figure 6.

Figure 6. Simplified diagram of Zn as a plant micronutrient Sagardoy et al. On the other hand, in order for Zn absorption to occur, we have to consider several factors Figure 7 since micronutrient availability is a function of the "form" in which it is found in the soil that determines its "mobility" towards the roots of plants Ojeda-Barrios et al. It has also been observed that in alkaline soils with a pH ranging from 7 to 8.

This is why applying Zn to the soil is not effective and restricted only to non-calcareous soils Perea-Portillo et al. Figure 7. Simplified diagram of factors that influence soil Zn availability Alloway, After being captured, Zn is transported through the xylem where it is chelated by various small molecules Haydon and Cobbett, including organic acids such as citrate Broadley et al. When the Zn supply is high, much of this nutrient is also chelated in the cell by organic acids such as malate and citrate Kupper et al.

Otherwise, different symptoms can be observed when there is Zn deficiency Figure 8. Figure 8. A Zn atom is always bound to four or more ligands; in CA, three coordination sites are occupied by the imidazole rings of three histidine residues and an additional coordination site is occupied by a water molecule or hydroxide ion, depending on pH.

Catalysis supposes that Zn activates water. The enzyme activity is at a maximum at high pH, up to pH 8, where the reaction is near its maximum speed.

When pH decreases, the reaction speed decays. The midpoint of this transition is near pH 7; although many amino acids, mainly histidine, have pka values near 7, evidence suggests that the group responsible for this transition is not an amino acid without the zinc-bound water molecule Stryer et al. The union of a water molecule to the positively charged Zn site reduces the water molecule pka from With neutral pH, an important concentration of hydroxide ion zinc-bound is generated, which is sufficiently nucleophilic to attack the CO 2 much faster than water.

The importance of this zinc-bound hydroxide ion suggests a simple mechanism for CO 2 hydration. The reaction takes place in four stages: 1 The deprotonation of water where Zn facilitates the release of a water molecule proton to generate a hydroxide ion. Thus, the binding of water to Zn favors the formation of the transition state, leading to the formation of bicarbonate and facilitates the release of the proton through the attraction of the two reactants to a close proximity Stryer et al.

Figure 9. Carbonic anhydrase is the only photosynthetic C metabolism enzyme whose activity fluctuates in a number of species with CO 2 concentration changes in the environment. Its activity decreases rapidly into air bubbles at low CO 2 emissions and vice versa. Figure Stomatal closure In absence of the light phase of photosynthesis, energy sources to fix CO 2 get depleted, while respiration continues and increases [CO 2 ] by shifting the central reaction Figure 11 to the right which diminishes pH causing inactivation of amylase and consequently, starch and glucose are not hydrolyzed any more, resulting in a decrease in the osmotic gradient causing the occlusive cells to lose water and swelling which makes the ostiole to close Casson and Gray, Simplified diagram of the central reaction for stomatal closure.

There is evidence that Zn deficiency reduces the CA content in some plants and that there is a close relationship between CA and Zn in plant physiology. By understanding the biochemical function of Zn in this type of biological macromolecules, the connection between the detailed biochemical and physiological functions can interrelate with genotypic and phenotypic expressions.

Today, there are many available theoretical elements necessary to develop and implement new nutritional diagnostic techniques.


Anhidrasa carbònica

Carbonic Anhydrase and Zinc in Plant Physiology. Vencedores del Desierto. Carbonic anhydrase CA EC: 2. Carbonic anhydrase is a metalloenzyme that requires Zn as a cofactor and is involved in diverse biological processes including pH regulation, CO 2 transfer, ionic exchange, respiration, CO 2 photosynthetic fixation, and stomatal closure. Therefore, the review includes relevant aspects about CA morphology, oligomerization, and structural differences in the active site.


Carbonic anhydrase


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