Regulation of chloroplastic carbonic anhydrase : effect of magnesium

Thermodynamic analysis of Kex2 activity: The acylation and deacylation steps are potassium- and substrate-dependent

Kex2 is the prototype of a large family of eukaryotic subtilisin-related proprotein-processing proteases that cleave at sites containing pairs of basic residues. Here, we studied the effects of KCl on the individual rate constants of association, dissociation, acylation and deacylation and determined the thermodynamic parameters at each step of the Kex2 reaction. Potassium bound Kex2 with K_D=20.3mM. The order in which potassium entered the reaction system modified the effect of activation or inhibition, which depended on the size of the substrate. A possible allosteric potassium binding site at the S₆ subsite was involved in activation, and a distant site located between the catalytic domain and the P-domain was involved in inhibition. Potassium decreased the energetic barriers of almost all steps of catalysis. The acylation of Ac-PMYKR-AMC in the absence of potassium was the rate-limiting step. Therefore, for substrates containing a P₁-Arg, the deacylation step is not necessarily the rate-limiting event, and other residues at the P' positions may participate in controlling the acylation and deacylation steps. Thus, it is reasonable to conclude that potassium is involved in the processing of the α-mating factor that promotes Ca²⁺ mobilization by activating a high-affinity Ca²⁺-influx system to increase the cytosolic [Ca²⁺], resulting in the activation of channels that are essential for the survival of Saccharomyces cerevisiae cells.

Seasonal variations of carbonic anhydrase activity in Chongqing urban section of Jialing River and its influencing factors

Carbonic anhydrase (CA) is an enzyme in algal carbon-utilization that plays an important role in the formation of algal blooms. A year-long monitoring program in the shore area of Chongqing Urban Section of the Jialing River (JR) was launched to determine the variations in carbonic anhydrase activity (CAA) and its change mechanism in the hydro-fluctuation belt of the tributaries in the Three Gorges Reservoir (TGR) area. The variations in basic water quality parameters, different carbon forms, and CAA were investigated from November 2013 to October 2014. Results showed that the mean CAA value in JR was 0.67 ± 0.31 EU/10⁶ cells. CAA was high during the flood stage, low during the impounding stage, and peaked on April 3, 2014 during the discharging stage. No significant difference was observed in the CAA of different sampling sites in JR. However, a significant difference was observed between the CAA of JR and that of the Yangtze River. Correlation analyses showed that water temperature, pH, algal cell density, and dissoluble organic carbon were positively correlated with CAA, whereas CO₂ and dissoluble inorganic carbon were negatively correlated with CAA. A model for CAA and related parameters was built through principal component regression. The equation was expressed as follows: CAA = 0.116T + 0.00746Cells+0.0156pH-0.0157CO²-0.0150DIC+0.0135DOC+0.565. Results revealed that CAA in JR was controlled by multiple factors, which could be used for CAA monitoring. The model demonstrated a potential value in controlling algal blooms.

Significance of different carbon forms and carbonic anhydrase activity in monitoring and prediction of algal blooms in the urban section of Jialing River, Chongqing, China

The Three Gorges Dam is one of the largest hydroelectric power plants worldwide; its reservoir was preliminarily impounded in 2003 and finally impounded to 175 m in 2012. The impoundment caused some environmental problems, such as algal blooms. Carbonic anhydrase (CA) is an important biocatalyst in the carbon utilization by algae and plays an important role in algal blooms. CA has received considerable attention for its role in red tides in oceans, but less investigation has been focused on its role in algal blooms in fresh water. In this study, the seasonal variation of water quality parameters, different carbon forms, carbonic anhydrase activity (CAA), and the algal cell density of four sampling sites in the urban section of the Jialing River were investigated from November 1, 2013 to October 31, 2014. Results indicated that CAA exhibited a positive correlation with dissoluble organic carbon (DOC), pH, and temperature, but a negative correlation with CO2 and dissoluble inorganic carbon (DIC). Algal cell density exhibited a positive correlation with flow velocity (V), pH, particulate organic carbon (POC), and CAA, a negative correlation with CO2, and a negative partial correlation with DIC. The relationship between CAA and algal cell density for the entire year can be described as cells = 23.278CAA - 42.666POC + 139.547pH - 1057.106. The algal bloom prediction model for the key control period can be described as cells = -45.895CAA + 776.103V- 29.523DOC + 14.219PIC + 35.060POC + 19.181 (2 weeks in advance) and cells = 69.200CAA + 203.213V + 4.184CO2 + 38.911DOC + 40.770POC - 189.567 (4 weeks in advance). The findings in this study demonstrate that the carbon utilization by algae is conducted by CA and provide a new method of monitoring algal cell density and predicting algal blooms.

Association of Carbonic Anhydrase Activity with Carboxysomes Isolated from the Cyanobacterium Synechococcus PCC7942

The development of a simple method for the isolation of purified carboxysomes from the cyanobacterium Synechococcus PCC7942 has made it possible to identify a specific and inducible, intracellular carbonic anhydrase (CA) activity that is strongly associated with carboxysomes. This was shown, in part, through enzyme recovery experiments that indicated that a clear majority of a CA activity that is sensitive to the CA inhibitor ethoxyzolamide (I(50) = 4 mum) copurifies with a majority of the cell's ribulose-1,5-bisphosphate carboxylase/oxygenase activity in a highly purified pelletable fraction. Electron microscopy of this pelletable fraction revealed the presence of carboxysomes that were physically intact. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of carboxysome proteins showed that the large and small subunits of ribulose-1,5-bisphosphate carbosylase/oxygenase were clearly prominent and that several other minor proteins could be distinguished. The specific location of this carboxysomal CA activity is further reinforced by the finding that a previously isolated high CO(2)-requiring mutant, Type II/No. 68 (G.D. Price, M.R. Badger [1989] Plant Physiol 91: 514-525), displayed a 30-fold reduction in carboxysome-associated CA activity when tested under optimal conditions. Carboxysomal CA has the unusual property of being inactivated by dithiothreitol. The enzyme also requires 20 mm Mg(2+) (as MgSO(4)) for near maximum activity; other divalent cations, such as Ca(2+) and Mn(2+), also stimulate carboxysomal CA activity, but to a lesser extent than Mg(2+). Results are discussed in relation to the role of carboxysomes in the CO(2)-concentrating mechanism in cyanobacteria and the role that carboxysomal CA activity appears to play in this process.

Acclimation to High CO(2) in Bean : Carbonic Anhydrase and Ribulose Bisphosphate Carboxylase

Young bean plants (Phaseolus vulgaris L. cv Seafarer) grew faster in air enriched with CO(2) (1200 microliters per liter) than in ambient CO(2) (330 microliters per liter). However, by 7 days when increases in overall growth (dry weight, leaf area) were visible, there was a significant decline (about 25%) in the leaf mineral content (N, P, K, Ca, Mg) and a drop in the activity of two enzymes of carbon fixation, carbonic anhydrase and ribulose 1,5-bisphosphate (RuBP) carboxylase under high CO(2). Although the activity of neither enzyme was altered in young, expanding leaves during the acclimation period, in mature leaves the activity of carbonic anhydrase was reduced 95% compared with a decline of 50% in ambient CO(2). The drop in RuBP carboxylase was less extreme with 40% of the initial activity retained in the high CO(2) compared with 50% in the ambient atmosphere. While CO(2) enrichment might alter the flow of carbon into the glycolate pathway by modifying the activities of carbonic anhydrase or RuBP carboxylase, there is no early change in the ability of photosynthetic tissue to oxidize glycolate to CO(2).