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1. CARBONIC ANHYDRASE AND ZINC IN PLANTPHYSIOLOGY...............................................................

11 March 2015

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CARBONIC ANHYDRASE AND ZINC IN PLANT PHYSIOLOGY
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Resumen: 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 physiologicalprocesses is carbonic anhydrase (CA). In plants, CA helps
to elevate CO2 concentration in the chloroplast to increase the carboxylation rate of the RuBisCO enzyme
(ribulose 1,5-disphosphate carboxylase). This is the reaction that incorporates CO2 into carbohydrates during
photosynthesis and can only use the C in CO2 instead of carbonic acid or bicarbonate. There are several forms
of CA in nature;in the case of plants, we find β carbonic anhydrase, which is a metalloenzyme that requires
Zn2+ for its activity. The catalytic mechanism of CA is achieved by the metal ion, and a characteristic of metal
ion catalysis is manifested by CO2 hydration through a mechanism binding Zn to hydroxide (Kimber and Pai,
2000). Zinc deficiency may inhibit growth by reducing the available Zn content in theplant, 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. However, the
association of Zn with CA has been reported years ago in crops such as Pisum sativum L., Lactuca sativa L.,
Petroselinum crispum (Mill.) Fuss, andSpinacia oleracea L. (Tobin, 1970). Subsequent studies also report such
associations of Zn with CA in pecan Carya illinoinensis (Wangenh.) K. Koch (Snir, 1983) and rice (Oryza sativa
L.) (Sasaki et al., 1998). 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 thebiochemical
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.
The active site which binds the enzyme contains a Zn ion (Zn2+) that reduces pKa and allows the nucleophilic
attack on CO2.In biological systems, Zn is found only in the +2 oxidation state. 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). The global burden of the Zn (His)3 complex remains as +2 since all the moleculesthat occupy the
coordination sites are neutral. Catalysis supposes that Zn activates water. It was determined by observing the
pH profile in enzymatically catalyzed CO2 hydration where changes in pH alter the hydration speed of CO2
catalyzed by β-AC. 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 speeddecays. 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., 2008).
The union of a water molecule to the positively charged Zn site reduces the water molecule pka from 15.7 to 7.
With neutral pH, an...