Potential health risks from radioactive contamination of saltmarshes in NW England.

The present study focuses on the detection of Sellafield-derived (137)Cs and (241)Am in contaminated saltmarshes from North-West England, UK, with a view to assessing the radiological impacts from radioactivity stored within the sediment record. The surface activities from these radionuclides were found in the range between 73 and 851 Bq kg(-1) whereas peak activities ranging from 383 to 12690 Bq kg(-1) were found below the surface of the upper marsh at a depth of approximately 5-20 cm. Potential radioactive exposure to humans from these highly active radionuclides comes mainly from direct exposure and resuspended dust inhalation for different saltmarsh users, which may be exacerbated by the remobilisation of radionuclides resulting from saltmarsh erosion. The total annual minimum, maximum and 'best estimate' doses ranging from 11 to 972 µSv y(-1), fall below the ICRP-recommended annual dose limit, but the highest estimated total effective dose (972 µSv y(-1)) for a marsh user falls within 97% of the recommended dose limit and the highest 'best estimate' total annual doses of 110 and 307 µSv y(-1) for Dee estuary and Biggar marshes, respectively, are almost 3 and 4 times higher than the estimated doses that are based on existing surface activities.

Assessing CZT detector performance for environmental radioactivity investigations.

Cadmium zinc telluride (CdZnTe or CZT) is the leading semiconductor detector for gamma spectroscopy at room temperature. In the present study, a coplanar-grid CZT detector was used for gamma-ray measurements of environmental radioactivity on a contaminated saltmarsh sediment core in comparison with results from a coaxial high-purity germanium detector to assess their comparative performance. The findings reveal that the CZT performs well for both (241)Am and (137)Cs measurements over a broad range of activities, despite limited detector efficiency, and can be used to good effect in the study of environmental radioactivity in contaminated estuarine settings.

Records of pan (floodplain wetland) sedimentation as an approach for post-hoc investigation of the hydrological impacts of dam impoundment: The Pongolo river, KwaZulu-Natal.

River impoundment by dams has far-reaching consequences for downstream floodplains in terms of hydrology, water quality, geomorphology, ecology and ecosystem services. With the imperative of economic development, there is the danger that potential environmental impacts are not assessed adequately or monitored appropriately. Here, an investigation of sediment composition of two pans (floodplain wetlands) in the Pongolo River floodplain, KwaZulu-Natal, downstream of the Pongolapoort dam constructed in 1974, is considered as a method for post-hoc assessment of the impacts on river hydrology, sediment supply and water quality. Bumbe and Sokhunti pans have contrasting hydrological regimes in terms of their connection to the main Pongolo channel - Bumbe is a shallow ephemeral pan and Sokhunti is a deep, perennial water body. The results of X-ray fluorescence (XRF) geochemical analysis of their sediment records over a depth of >1 m show that whilst the two pans exhibit similar sediment composition and variability in their lower part, Bumbe pan exhibits a shift toward increased fine-grained mineral supply and associated nutrient influx at a depth of c. 45 cm whilst Sokhunti pan is characterised by increased biogenic productivity at a depth of c. 26 cm due to enhanced nutrient status. The underlying cause is interpreted as a shift in hydrology to a 'post-dam' flow regime of reduced flood frequencies with more regular baseline flows which reduce the average flow velocity. In addition, Sokhunti shows a greater sensitivity to soil influx during flood events due to the nature of its 'background' of autochthonous biogenic sedimentation. The timing of the overall shift in sediment composition and the dates of the mineral inwash events are not well defined, but the potential for these wetlands as sensitive recorders of dam-induced changes in floodplain hydrology, especially those with a similar setting to Sokhunti pan, is clearly demonstrated.

Modelling approaches for coastal simulation based on cellular automata: the need and potential.

The paper summarizes the theoretical and practical needs for cellular automata (CA)-type models in coastal simulation, and describes early steps in the development of a CA-based model for estuarine sedimentation. It describes the key approaches and formulae used for tidal, wave and sediment processes in a prototype integrated cellular model for coastal simulation designed to simulate estuary sedimentary responses during the tidal cycle in the short-term and climate driven changes in sea-level in the long-term. Results of simple model testing for both one-dimensional and two-dimensional models, and a preliminary parameterization for the Blackwater Estuary, UK, are shown. These reveal a good degree of success in using a CA-type model for water and sediment transport as a function of water level and wave height, but tidal current vectors are not effectively simulated in the approach used. The research confirms that a CA-type model for the estuarine sediment system is feasible, with a real prospect for coupling to existing catchment and nearshore beach/cliff models to produce integrated coastal simulators of sediment response to climate, sea-level change and human actions.

A functional role for a flexible loop containing Glu182 in the class II fructose-1,6-bisphosphate aldolase from Escherichia coli.

Class II fructose 1,6-bisphosphate aldolases (FBP-aldolases) catalyse the zinc-dependent, reversible aldol condensation of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P) to form fructose 1,6-bisphosphate (FBP). Analysis of the structure of the enzyme from Escherichia coli in complex with a transition state analogue (phosphoglycolohydroxamate, PGH) suggested that substrate binding caused a conformational change in the beta5-alpha7 loop of the enzyme and that this caused the relocation of two glutamate residues (Glu181 and Glu182) into the proximity of the active site. Site-directed mutagenesis of these two glutamate residues (E181A and E182A) along with another active site glutamate (Glu174) was carried out and the mutant enzymes characterised using steady-state kinetics. Mutation of Glu174 (E174A) resulted in an enzyme which was severely crippled in catalysis, in agreement with its position as a zinc ligand in the enzyme's structure. The E181A mutant showed the same properties as the wild-type enzyme indicating that the residue played no major role in substrate binding or enzyme catalysis. In contrast, mutation of Glu182 (E182A) demonstrated that Glu182 is important in the catalytic cycle of the enzyme. Furthermore, the measurement of deuterium kinetic isotope effects using [1(S)-(2)H]DHAP showed that, for the wild-type enzyme, proton abstraction was not the rate determining step, whereas in the case of the E182A mutant this step had become rate limiting, providing evidence for the role of Glu182 in abstraction of the C1 proton from DHAP in the condensation direction of the reaction. Glu182 lies in a loop of polypeptide which contains four glycine residues (Gly176, Gly179, Gly180 and Gly184) and a quadruple mutant (where each glycine was converted to alanine) showed that flexibility of this loop was important for the correct functioning of the enzyme, probably to change the microenvironment of Glu182 in order to perturb its pK(a) to a value suitable for its role in proton abstraction. These results highlight the need for further studies of the dynamics of the enzyme in order to fully understand the complexities of loop closure and catalysis in this enzyme.

Conserved residues in the mechanism of the E. coli Class II FBP-aldolase.

The two classes of fructose-1,6-bisphosphate aldolase both catalyse the reversible cleavage of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. The Class I aldolases use Schiff base formation as part of their catalytic mechanism, whereas the Class II enzymes are zinc-containing metalloproteins. The mechanism of the Class II enzymes is less well understood than their Class I counterparts. We have combined sequence alignments of the Class II family of enzymes with examination of the crystal structure of the enzyme to highlight potentially important aspartate and asparagine residues in the enzyme mechanism. Asp109, Asp144, Asp288, Asp290, Asp329 and Asn286 were targeted for site-directed mutagenesis and the resulting proteins purified and characterised by steady-state kinetics using either a coupled assay system to study the overall cleavage reaction or using the hexacyanoferrate (III) oxidation of the enzyme bound intermediate carbanion to investigate partial reactions. The results showed only minor changes in the kinetic parameters for the Asp144, Asp288, Asp290 and Asp329 mutants, suggesting that these residues play only minor or indirect roles in catalysis. By contrast, mutation of Asp109 or Asn286 caused 3000-fold and 8000-fold decreases in the kcat of the reaction, respectively. Coupled with the kinetics measured for the partial reactions the results clearly demonstrate a role for Asn286 in catalysis and in binding the ketonic end of the substrate. Fourier transform infra-red spectroscopy of the wild-type and mutant enzymes has further delineated the role of Asp109 as being critically involved in the polarisation of the carbonyl group of glyceraldehyde 3-phosphate.

The crystal structure of a class II fructose-1,6-bisphosphate aldolase shows a novel binuclear metal-binding active site embedded in a familiar fold.

BACKGROUND: [corrected] Aldolases catalyze a variety of condensation and cleavage reactions, with exquisite control on the stereochemistry. These enzymes, therefore, are attractive catalysts for synthetic chemistry. There are two classes of aldolase: class I aldolases utilize Schiff base formation with an active-site lysine whilst class II enzymes require a divalent metal ion, in particular zinc. Fructose-1,6-bisphosphate aldolase (FBP-aldolase) is used in gluconeogenesis and glycolysis; the enzyme controls the condensation of dihydroxyacetone phosphate with glyceraldehyde-3-phosphate to yield fructose-1,6-bisphosphate. Structures are available for class I FBP-aldolases but there is a paucity of detail on the class II enzymes. Characterization is sought to enable a dissection of structure/activity relationships which may assist the construction of designed aldolases for use as biocatalysts in synthetic chemistry. RESULTS: The structure of the dimeric class II FBP-aldolase from Escherichia coli has been determined using data to 2.5 A resolution. The asymmetric unit is one subunit which presents a familiar fold, the (alpha/beta)8 barrel. The active centre, at the C-terminal end of the barrel, contains a novel bimetallic-binding site with two metal ions 6.2 A apart. One ion, the identity of which is not certain, is buried and may play a structural or activating role. The other metal ion is zinc and is positioned at the surface of the barrel to participate in catalysis. CONCLUSIONS: Comparison of the structure with a class II fuculose aldolase suggests that these enzymes may share a common mechanism. Nevertheless, the class II enzymes should be subdivided into two categories on consideration of subunit size and fold, quaternary structure and metal-ion binding sites.