Carbonic anhydrases of higher plants: an overview

Carbonic anhydrase and the molecular evolution of C4 photosynthesis

C(4) photosynthesis, a biochemical CO(2)-concentrating mechanism (CCM), evolved more than 60 times within the angiosperms from C(3) ancestors. The genus Flaveria, which contains species demonstrating C(3), C(3)-C(4), C(4)-like or C(4) photosynthesis, is a model for examining the molecular evolution of the C(4) pathway. Work with carbonic anhydrase (CA), and C(3) and C(4) Flaveria congeners has added significantly to the understanding of this process. The C(4) form of CA3, a β-CA, which catalyses the first reaction in the C(4) pathway by hydrating atmospheric CO(2) to bicarbonate in the cytosol of mesophyll cells (mcs), evolved from a chloroplastic C(3) ancestor. The molecular modifications to the ancestral CA3 gene included the loss of the sequence encoding the chloroplast transit peptide, and mutations in regulatory regions that resulted in high levels of expression in the C(4) mesophyll. Analyses of the CA3 proteins and regulatory elements from Flaveria photosynthetic intermediates indicated C(4) biochemistry very likely evolved in a specific, stepwise manner in this genus. The details of the mechanisms involved in the molecular evolution of other C(4) plant β-CAs are unknown; however, comparative genetics indicate gene duplication and neofunctionalization played significant roles as they did in Flaveria.

A 160 kDa protein with carbonic anhydrase activity is complexed with rubisco on the outer surface of thylakoids

This study provides evidence that, in the soluble fraction from buffer-washed pea thylakoids, one form of soluble carbonic anhydrase (CA) is associated with rubisco in a stromal protein complex. On native-PAGE gels, it is present as a protein band with MW approximately 160 kDa. On SDS-PAGE gels, it is resolved as a single 25-kDa polypeptide. Analysis of Western blots developed with polyclonal antibodies to barley rubisco and to soluble pea CA shows that a 160-kDa protein with CA activity is associated with rubisco in a protein complex localized on the outer surface of thylakoid membranes.

Carbonic anhydrase is essential for growth of Ralstonia eutropha at ambient CO(2) concentrations

Mutant strain 25-1 of the facultative chemoautotroph Ralstonia eutropha H16 had previously been shown to exhibit an obligately high-CO(2)-requiring (HCR) phenotype. Although the requirement varied with the carbon and energy sources utilized, none of these conditions allowed growth at the air concentration of CO(2). In the present study, a gene designated can and encoding a beta-carbonic anhydrase (CA) was identified as the site altered in strain 25-1. The mutation caused a replacement of the highly conserved glycine residue 98 by aspartate in Can. A can deletion introduced into wild-type strain H16 generated mutant HB1, which showed the same HCR phenotype as mutant 25-1. Overexpression of can in Escherichia coli and mass spectrometric determination of CA activity demonstrated that can encodes a functional CA. The enzyme is inhibited by ethoxyzolamide and requires 40 mM MgSO(4) for maximal activity. Low but significant CA activities were detected in wild-type H16 but not in mutant HB1, strongly suggesting that the CA activity of Can is essential for growth of the wild type in the presence of low CO(2) concentrations. The HCR phenotype of HB1 was overcome by complementation with heterologous CA genes, indicating that growth of the organism at low CO(2) concentrations requires sufficient CA activity rather than the specific function of Can. The metabolic function(s) depending on CA activity remains to be identified.

Extrinsic photosystem II carbonic anhydrase in maize mesophyll chloroplasts

One form of carbonic anhydrase (CA) has been observed in maize (Zea mays) thylakoids and photosystem II (PSII)-enriched membranes. Here, we show that an antibody produced against a thylakoid lumen-targeted CA found in Chlamydomonas reinhardtii reacts with a single 33-kD polypeptide in maize thylakoids. With immunoblot analysis, we found that this single polypeptide could be identified only in mesophyll thylakoids and derived PSII membranes, but not in bundle sheath thylakoids. Likewise, a CA activity assay confirmed a large amount of activity in mesophyll, but not in bundle sheath membranes. Immunoblot analysis and CA activity assay showed that the maximum CA can be obtained in the supernatant of the PSII-enriched membranes washed with 1 M CaCl(2), the same procedure used to remove all extrinsic lumenal proteins from PSII. Because this CA reacts with an antibody to lumen-directed CA in C. reinhardtii, and because it can be removed with 1 M CaCl(2) wash, we refer to it tentatively as extrinsic CA. This is to distinguish it from another form of CA activity tightly bound to PSII membranes that remains after CaCl(2) wash, which has been described previously. The function of extrinsic CA is not clear. It is unlikely to have the same function as the cytoplasmic CA, which has been proposed to increase the HCO(-)(3) concentration for phosphoenolpyruvate carboxylase and the C(4) pathway. We suggest that because the extrinsic CA is associated only with thylakoids doing linear electron flow, it could function to produce the CO(2) or HCO(-)(3) needed for PSII activity.