Partition of isomeric dipeptides in poly(ethylene glycol)/magnesium sulfate aqueous two-phase systems

Partial characterization of Bacillus pumilus catalase partitioned in poly(ethylene glycol)/sodium sulfate aqueous two-phase systems

Aqueous two-phase partitioning system (ATPS) was used to extract and purify catalase from Bacillus pumilus. The system parameters for effective purification of catalase were optimized. The best catalase recovery (123%) with a 4.6-fold purification was obtained in the bottom phase of ATPS including the mixture of 15% (w/w) PEG4000, 10% (w/w) Na₂SO₄ and 3% (w/w) NaCl at pH 5.0. The purified enzyme was characterized regarding its activity and stability. The highest enzyme activity was observed at pH 7.0 and 37 °C on hydrogen peroxide. The enzyme was quite stable at temperatures between 30 and 55 °C and a pH range of 7.0-9.0. The K_m and V_max values were determined from Lineweaver-Burk plot as 11 mM and 1667 µmole ml^-1min^-1, respectively. Overall, it can be said that ATPS is a rapid, reasonable, straightforward and cost-effective process for catalase purification in comparison to the chromatographic methods.

Interactions between PEG and type I soluble tumor necrosis factor receptor: modulation by pH and by PEGylation at the N terminus

The effects of polyethylene glycol (PEG) on protein structure and the molecular details that regulate its association to polypeptides are largely unknown. These issues were addressed using type I soluble tumor necrosis factor receptor (sTNF-RI) as a model system. Changes in solution viscosity established that a truncated form of sTNF-RI bound free PEG in a pH-dependent manner. Above pH 5.3, the viscosity escalated as the pH increased, while no effect occurred below pH 5.0. Conjugation of 2 kD, 5 kD, or 20 kD PEG to the N terminus attenuated the viscosity at the higher pH values. Tryptophan phosphorescence spectroscopy correlated changes in the protein structure about Trp-107, at the C terminus, with the pH-dependent and PEGylation-dependent attenuation of the viscosity. The results indicate that specific interactions between PEG and the truncated form of sTNF-RI are elicited by an increased flexibility of the truncated protein combined perhaps with removal of steric or charge barriers. Covalently bound PEG at the N terminus reduced the protein affinity for the free polymer and induced a more rigid and polar configuration around Trp-107. Deprotonation of His-105, which is perpendicular to Trp-107, was integral to the binding mechanism producing a pH-dependent switching mechanism. These findings stress the importance of surface charge and structural plasticity in determining macromolecular binding affinities and demonstrate the ability of conjugated PEG to modify the localized surface structure in proteins away from the site of conjugation.

Partition coefficients of alpha-amylase in aqueous two-phase systems PEG + MgSO4.7H2O + H2O at 298 K

Partition coefficients of alpha-amylase have been determined in a polyethylene glycol (average molecular mass 8000)/MgSO4.7H2O aqueous two-phase system at 298 K and the influence of polymer, salt and initial enzyme concentration on partition was investigated. Correlations are proposed which relates the partition coefficient to the initial enzyme concentration and the concentration difference between phases of polymer and salt. The free volume change, related to the densities of the separate phases, has a direct dependence with such polymer and salt concentration differences, respectively, and is then used to facilitate future estimations. Thus, the partition coefficient is a function of this physical parameter and the initial enzyme concentration employed (1.25, 2.50 and 5.00 g dm-3).