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Levansucrase is a secretary enzyme of Acetobacter nitrogenifigens strain RG1. The enzyme shows enhanced activity in the presence of Hg 2+ in spite of being inhibited by other heavy metal ion Cd 2+ . In this study the structural characterization of levansucrase in native state as well as in the presence of Hg 2+ and Cd 2+ by CD spectroscopy is done. The secondary structures of the native enzyme and the enzyme treated with Hg 2+ and Cd 2+ on comparison by their CD spectra revealed that their spectra showed no significant difference indicating that both Hg 2+ as well as Cd 2+ had no effect on the overall secondary structure of the protein. The respective CD spectra on analysis revealed that they have almost identical percentage of secondary structural elements. The interaction of levansucrase with Hg 2+ as well as Cd 2+ was studied further by tryptophan fluorescence spectroscopy which on analysis revealed static quenching indicating protein-heavy metal complex formation. A blue shift in the tryptophan fluorescence spectra of Hg 2+ treated protein indicated that the tryptophan residues have moved to a more hydrophobic environment in the protein away from aqueous phase. The mechanism of interaction of enzyme with mercury and cadmium was determined from their tryptophan fluorescence spectra., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The purification and characterization of an extracellular levansucrase enzyme produced by novel nitrogen-fixer Acetobacter nitrogenifigens strain RG1(T) is described. Culture conditions were optimized for maximum levansucrase production. Levansucrase purified to homogeneity by tenfold purification has a molecular weight of 65 kDa, contained four cysteine residues, polymerized raffinose and was stable for 21 days at pH 6.0 when stored at 4 °C or -20 °C but was vulnerable to DTT and β-mercaptoethanol. Interestingly, this enzyme showed enhanced hydrolytic and polymerization activity in the presence of mercuric ion which, to our knowledge, is the first report for any levansucrase enzyme characterized so far. Evidences obtained from Native PAGE, tryptophan fluorescence study and activity measurements at different temperatures and in the presence of thiol modifying agents, show that mercuric ion stabilizes the enzyme. Levan, synthesized by the enzyme, has a molecular weight of 7,080 kDa and was shown to be a homopolymer of fructose.

Aims: Mercury compounds are highly toxic to all types of living cells. Isolated yeast strains of Rhodotorula rubra showed high and low resistance pattern towards mercury and organomercurial compounds. To investigate the basis of differential sensitivity of these two types of strains, glucose utilization was measured in the presence of mercury compounds., Methods and Results: Glucose utilization process remained unaffected in resting cells of highly Hg(2+)-resistant strain in the presence of HgCl(2) but not in the presence of phenylmercuric acetate and thimerosal. However, HgCl(2) significantly affected glucose utilization in the case of low-resistant cells. The Hg-retaining ability of the cell wall of highly Hg(2+)-resistant yeast strain was greater than that of the weakly Hg(2+)-resistant strain. The spheroplast-bound Hg(2+) was also significantly less in the highly Hg(2+)-resistant strain than in the weakly Hg(2+)-resistant strain., Conclusions: Glucose uptake machinery was not affected in the presence of toxic metal ions in the case of high-resistant strains. But in the case of low Hg(2+)-resistant strain, glucose transport system may be affected either by inactivation of sensor proteins containing -SH group associated with glucose uptake., Significance and Impact of the Study: Cell wall of mercury-resistant yeast cells may play an important role in heavy metal bioremediation process.

Flavobacterium rigense strain PR2, a broad-spectrum mercury-resistant bacterium abundantly present in soil exhibited multiple metal resistance properties. Mercury resistance was due to the sequential action of two mercury-detoxicating enzymes, organomercurial lyase and mercuric reductase. The levels of these enzyme activities were determined using different mercury compounds as inducers and substrates. Mercuric reductase was partially purified from the bacterium and the physicochemical properties of the enzyme were studied. The effect of several enzyme inhibitors and heavy metal ions on the enzyme activity was also studied.

A broad-spectrum Hg-resistant strain of B. pasteurii DR2 utilized phenylmercuric acetate (PMA) as sole source of carbon. This bacterial strain contained a constitutive organomercurial lyase which specifically degraded PMA but not other organo-mercurials. This PMA-lyase activity was also stimulated to different extents when this bacterial strain was grown in presence of different organic compounds as sole source of carbon.