Mohamed Ali Borgi

Centre of Biotechnology of Sfax (CBS), Şafāqis, Şafāqis, Tunisia

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Publications (7)12.02 Total impact

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    ABSTRACT: The phyL gene encoding phytase from the industrial strain Bacillus licheniformis ATCC 14580 (PhyL) was cloned, sequenced, and overexpressed in Escherichia coli. Biochemical characterization demonstrated that the recombinant enzyme has an apparent molecular weight of nearly 42 kDa. Interestingly, this enzyme was optimally active at 70-75 °C and pH 6.5-7.0. This enzyme is distinguishable by the fact that it preserved more than 40 % of its activity at wide range of temperatures from 4 to 85 °C. This new phytase displayed also a high specific activity of 316 U/mg. For its maximal activity and thermostability, this biocatalyst required only 0.6 mM of Ca(2+) ion and exhibited high catalytic efficiency of 8.3 s(-1) μM(-1) towards phytic acid.
    Applied Microbiology and Biotechnology 12/2013; · 3.81 Impact Factor
  • Mohamed A Borgi, Moez Rhimi, Adel Kadri
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    ABSTRACT: The crystallization behaviour of the highly thermostable glucose isomerase from the Streptomyces sp. strain isolated from Tunisian soil was investigated using ammonium sulfate as a precipitating agent. We established phase diagrams at different temperatures and protein concentrations. It was found that the solubility increased with increasing temperature and decreased with increasing salt concentration. The temperature-dependent solubility was used to characterize the thermodynamic parameters of crystallization such as enthalpy, entropy and free energy.
    General Physiology and Biophysics 10/2013; · 0.85 Impact Factor
  • Mohamed Ali Borgi, Moez Rhimi, Samir Bejar
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    ABSTRACT: The Ala103 to Gly mutation, introduced within the glucose isomerase from Streptomyces sp. SK (SKGI) decreased its catalytic efficiency (k(cat)/K(m)) toward D-glucose from 7.1 to 3 mM(-1) min(-1). The reverse counterpart replacement Gly103Ala introduced into the glucose isomerase of Streptomyces olivochromogenes (SOGI) considerably improved its catalytic efficiency to be 6.7 instead of 3.2 mM(-1) min(-1). This later mutation also increased the half-life time of the enzyme from 70 to 95 min at 80 degrees C and mainly modified its pH profile. These results provide evidence that the residue Ala103 plays an essential role in the kinetic and physicochemical properties of glucose isomerases from Streptomyces species.
    Biotechnology Journal 03/2007; 2(2):254-9. · 3.71 Impact Factor
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    ABSTRACT: To develop a feasible enzymatic process for the concomitant d-tagatose and d-fructose production, the thermostable l-arabinose isomerase of Bacillus stearothermophilus US100 (l-AI US100) and the mutant d-glucose isomerase obtained from that of Streptomyces SK (SKGI-A103G) were successfully co-expressed in Escherichia coli HB101 strain. The recombinant cells were immobilized in alginate beads and showed, similarly to the free cells, optimal temperatures for d-galactose and d-glucose isomerisation of 80 and 85 °C, respectively. The two isomerases were optimally active at pH 7.5. Cell entrapment significantly enhanced the acidotolerance of the two isomerases, as well as their stability at high temperatures. To perform simultaneous isomerisation of d-galactose and d-glucose at 65 °C and pH 7.5 in packed-bed bioreactor, cells concentration, dilution rate, productivity and bioconversion rate were optimized to be 32 g/l, 2.6 h−1, 3 g/l h and 30%, respectively.
    Enzyme and Microbial Technology. 01/2007;
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    ABSTRACT: In order to over express the xylA gene of Streptomyces sp. SK strain, it was cloned under the control of the constitutive ermE-up promoter. This construct was integrated through site-specific recombination process into the chromosome of a Streptomyces violaceoniger glucose isomerase deficient strain using the non-replicative vector pTS55. The resulting CBS4 strain shows a perfect stability in the absence of selection pressure. Its glucose isomerase activity was about four and nine-fold greater, than that obtained from Streptomyces sp. SK, respectively fully induced or not by xylose.
    Enzyme and Microbial Technology. 01/2005;
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    ABSTRACT: The glucose isomerase gene (xylA) from the Streptomyces sp. SK strain encodes a 386-amino-acid protein (42.7 kDa) showing extensive identities with many other bacterial glucose isomerases. We have shown by gel filtration chromatography and SDS-PAGE analysis that the purified recombinant glucose isomerase (SKGI) is a 180 kDa tetramer of four 43 kDa subunits. Sequence inspection revealed that this protein, present some special characteristics like the abundance of hydrophobic residues and some original amino-acid substitutions, which distinguish SKGI from the other GIs previously reported. The presence of an Ala residue at position 103 in SKGI is especially remarkable, since the same amino-acid was found at the equivalent position in the extremely thermostable GIs from Thermus thermophilus and Thermotoga neapolitana; whereas a Gly was found in the majority of less thermostable GIs from Streptomyces. The Ala103Gly mutation, introduced in SKGI, significantly decreases the half-life time at 90 degrees C from 80 to 50 min and also shifts the optimum pH from 6.5 to 7.5. This confirms the implication of the Ala103 residue on SKGI thermostability and activity at low pH. A homology model of SKGI based on the SOGI (that of Streptomyces olivochromogenes) crystal structure has been constructed in order to understand the mutational effects on a molecular scale. Hence, the Ala103Gly mutation, affecting enzyme properties, is presumed to increase molecular flexibility and to destabilize, in particular at elevated temperature, the 91-109 loop that includes the important catalytic residue, Phe94.
    Biochimie 09/2004; 86(8):561-8. · 3.14 Impact Factor
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    ABSTRACT: The implication of the original alanine 63 (Ala63) and the unique cysteine 306 (Cys306) residues in the thermostability of the Streptomyces sp. SK glucose isomerase (SKGI) were investigated by site-directed mutagenesis and homology modelling. The Cys306 to Ala mutation within SKGI dramatically affected its thermal stability by decreasing the half-life from 80 to 15 min at 90°C while the Ala63 to Ser replacement shifted this half-life to 65 min. The electrophoretic analysis proves that the residue Cys306 participates in oligomerization of the SKGI. Its stabilizing role is materialized by hydrogen bonds established with arginines at positions 284 and 259, as deduced from the constructed three-dimensional model. We have also shown that the presence of an Ala63 instead of Ser63 seems to be more suitable for enzyme thermostability by maintaining hydrophobic pocket that contributes to the protection of the enzyme active site.
    Biologia 64(5):845-851. · 0.51 Impact Factor