The impact of heteroaggregation between nZVI and SNPs on the co-transport of Cd(II) in saturated sand columns.

This study investigated the co-transport behaviors of nano zero-valent iron (nZVI) and Cd(II) in the presence of soil nanoparticles (SNPs) under various SNPs/nZVI mass ratios. It was illustrated that the mobility of colloidal Cd(II) was highly dependent on the nZVI-SNPs heteroaggregation behavior. In the case of 40 mg/L nZVI with SNPs/nZVI mass ratios > 1, the formation of stable SNPs-nZVI heteroaggregates with hydrodynamic diameters (Dh) < 500 nm facilitated the nZVI and colloidal Cd(II) transport at their effluent mass recoveries of 34.76-37.82 % and 9.81-17.17 %, respectively. However, in the case of 100 mg/L nZVI with SNPs/nZVI mass ratios of 0.4-2, the interception of nZVI-SNPs heteroaggregates with Dh > 1500 nm by quartz sands led to almost complete retention of nZVI and colloidal Cd(II) in the columns. Combined with analytical results of zeta potentials and XRD spectrum, it was revealed that the Cd(II) ions could accelerate nZVI corrosion. The positively charged Fe3O4 and γ-FeOOH on corroded nZVI surface could facilitate the heteroaggregation of nZVI-SNPs by the patch-charge attraction, which further reduced the environmental risk of colloidal Cd(II) transport. These findings revealed the important effects of heteroaggregation between nZVI and SNPs on the transport risk of Cd(II) in groundwater.

Enzymatic Characterization and Coenzyme Specificity Conversion of a Novel Dimeric Malate Dehydrogenase from Bacillus subtilis.

Malate is an important material to various industrials and clinical applications. Bacillus subtilis is a widely used biocatalyst tool for chemical production. However, the specific enzymatic properties of malate dehydrogenase from Bacillus subtilis (BsMDH) remain largely unknown. In the present study, BsMDH was cloned, recombinantly expressed and purified to test its enzymatic properties. The molecular weight of single unit of BsMDH was 34,869.7 Da. Matrix-Assisted Laser-Desorption Ionization-Time-of-Flight Mass Spectrometry and gel filtration analysis indicated that the recombinant BsMDH could form dimers. The kcat/Km values of oxaloacetate and NADH were higher than those of malate and NAD+, respectively, indicating a better catalysis in the direction of malate synthesis than the reverse. Furthermore, six BsMDH mutants were constructed with the substitution of amino acids at the coenzyme binding site. Among them, BsMDH-T7 showed a greatly higher affinity and catalysis efficiency to NADPH than NADH with the degree of alteration of 2039, suggesting the shift of the coenzyme dependence from NADH to NADPH. In addition, BsMDH-T7 showed a relatively lower Km value, but a higher kcat and kcat/Km than NADPH-dependent MDHs from Thermus flavus and Corynebacterium glutamicum. Overall, these results indicated that BsMDH and BsMDH-T7 mutant might be promising enzymes for malate production.

Heteroexpression and biochemical characterization of thermostable citrate synthase from the cyanobacteria Anabaena sp. PCC7120.

The present study recombinantly expressed a citrate synthase from cyanobacteria Anabaena sp. PCC7120 (AnCS) in Escherichia coli and characterized its enzymatic activity. The molecular mass of native AnCS was 88,533.1 Da containing two 44,162.7 Da subunits. Recombinant AnCS revealed the highest activity at pH 9.0 and 25 °C. AnCS displayed high thermal stability with a half-life time (t1/2) of approximately 6.5 h at 60 °C, which was more thermostable than most CS from general organisms, but less than those from hyperthermophilic bacteria. The Km values of oxaloacetate and acetyl-CoA were 138.50 and 18.15 µM respectively, suggesting a higher affinity to acetyl-CoA than oxaloacetate. Our inhibition assays showed that AnCS activity was not severely affected by most metal ions, but was strongly inhibited by Cu2+ and Zn2+. Treatments with ATP, ADP, AMP, NADH, and DTT depressed the AnCS activity. Overall, our results provide information on the enzymatic properties of AnCS, which contributes to the basic knowledge on CS selection for industrial utilizations.

Expression and characterization of a thermostable citrate synthase from Microcystis aeruginosa PCC7806.

Citrate synthase (CS) is an important enzyme in energy conversion and material circulation, participating in many important biochemical processes. In the present study, CS from Microcystis aeruginosa PCC7806 (MaCS) was cloned and expressed in Escherichia coli Rosetta (DE3). The recombinant MaCS was purified and its enzymological properties were characterized. The results showed that MaCS formed dimers in native status. The optimum temperature and pH of MaCS was 30°C and 8.2, respectively. MaCS displayed relative high thermal stability. Treatment at 50°C for 20 min only decreased 11.30% activity of MaCS and the half-life of MaCS was approximately 35 min at 55°C. The kcat and Km of acetyl-CoA and oxaloacetic acid were 17.133 s-1 (kcat) and 11.62 µM (Km), 24.502 s-1 and 103.00 µM, respectively. MaCS activity was not drastically inhibited by monovalent ions and NADH but depressed by divalent ions and some small molecular compounds, especially Mg2+, Zn2+, Co2+ and DTT. Overall, these data contributed to further understanding of energy metabolism in cyanobacteria and also provided basic information for industrial application of CS.