Enhanced recovery and purification of Aspergillus glucoamylase from Saccharomyces cerevisiae by the addition of poly(aspartic acid) tails

Global screening of protein chromatographic behavior on ion exchangers from a complex cell proteome. Towards in silico downstream processing of bioproducts

Protein separation during ion-exchange chromatography implies complex physicochemical events. This work has evaluated the chromatographic behaviour of a complex cell proteome on commercial agarose-based adsorbents. Various ligand types in the cation- and anion-exchange mode were studied. ANX-Sepharose, a weak anion exchanger, performed similarly to the strong anion exchanger-type materials. Proteomic tools were applied in order to understand protein separation. Experimental evidence showed a correlation between apparent isoelectric point distributions and the mobile phase conductivity. Molecular weight distributions were unaffected by the elution position. On the basis of two-dimensional electrophoresis, operational windows were described having typical minor contaminants. These could be annotated for future implementation of in silico downstream processing.

Host selection as a downstream strategy: polyelectrolyte precipitation of beta-glucuronidase from plant extracts

Host selection can be a strategy to simplify downstream processing for protein recovery. Advancing capabilities for using plants as hosts offers new host opportunities that have received only limited attention from a downstream processing perspective. Here, we investigated the potential of using a polycationic precipitating agent (polyethylenimine; PEI) to precipitate an acidic model protein (beta-glucuronidase; GUS) from aqueous plant extracts. To assess the potential of host selection to enhance the ease of recovery, the same procedure was applied to oilseed extracts of canola, corn (germ), and soy. For comparison, PEI precipitation of GUS was also evaluated from a crude bacterial fermentation broth. Two versions of the target protein were investigated--the wild-type enzyme (WTGUS) and a genetically engineered version containing 10 additional aspartates on each of the enzyme's four homologous subunits (GUSD10). It was found that canola was the most compatible expression host for use with this purification technique. GUS was completely precipitated from canola with the lowest dosage of PEI (30 mg PEI/g total protein), and over 80% of the initial WTGUS activity was recovered with 18-fold purification. Precipitation from soy gave yields over 90% for WTGUS but only 1.3-fold enrichment. Corn, although requiring the most PEI relative to total protein to precipitate (210 mg PEI/g total protein for 100% precipitation), gave intermediate results, with 81% recovery of WTGUS activity and a purification factor of 2.6. The addition of aspartate residues to the target protein did not enhance the selectivity of PEI precipitation in any of the systems tested. In fact, the additional charge reduced the ability to recover GUSD10 from the precipitate, resulting in lower yields and enrichment ratios compared to WTGUS. Compared to the bacterial host, plant systems provided lower polymer dosage requirements, higher yields of recoverable activity and greater purification factors.

Polyionic fusion peptides function as specific dimerization motifs

The de novo design of a molecular adapter for directed association and covalent linkage of two polypeptides is presented. Using peptides containing charged amino acid residues and an additional cysteine residue (AlaCysLys(8) and AlaCysGlu(8)) we demonstrate that the electrostatic interaction promotes the association of two synthetic peptides and, subsequently, disulfide bond formation. The reaction depends on both the redox potential and on the ionic strength of the buffer. Varying the redox potential, the interaction of the peptides was quantified by a Delta G(0') of 6.6 +/- 0.2 kcal/mol. Heterodimerization of the peptides is highly specific, a competition of association by other cysteine containing compounds could not be observed. Two proteins comprising cysteine-containing polyionic fusion peptides, a modified Fab fragment and an alpha-glucosidase fusion, could be specifically conjugated by directed association and subsequent disulfide bond formation. Both proteins retain their functional characteristics within the bifunctional conjugate: enzymatic activity of the alpha-glucosidase and antigen-binding capacity of the Fab fragment are equivalent to the non-conjugated components.

Glucoamylase: structure/function relationships, and protein engineering

Glucoamylases are inverting exo-acting starch hydrolases releasing beta-glucose from the non-reducing ends of starch and related substrates. The majority of glucoamylases are multidomain enzymes consisting of a catalytic domain connected to a starch-binding domain by an O-glycosylated linker region. Three-dimensional structures have been determined of free and inhibitor complexed glucoamylases from Aspergillus awamori var. X100, Aspergillus niger, and Saccharomycopsis fibuligera. The catalytic domain folds as a twisted (alpha/alpha)(6)-barrel with a central funnel-shaped active site, while the starch-binding domain folds as an antiparallel beta-barrel and has two binding sites for starch or beta-cyclodextrin. Certain glucoamylases are widely applied industrially in the manufacture of glucose and fructose syrups. For more than a decade mutational investigations of glucoamylase have addressed fundamental structure/function relationships in the binding and catalytic mechanisms. In parallel, issues of relevance for application have been pursued using protein engineering to improve the industrial properties. The present review focuses on recent findings on the catalytic site, mechanism of action, substrate recognition, the linker region, the multidomain architecture, the engineering of specificity and stability, and roles of individual substrate binding subsites.

Role of hydrodynamic shear on activity and structure of proteins

Proteins are important products used in industry. They may be enzymes which catalyze different reactions or they may be required for their biological activities as hormones, growth factors or therapeutics. During production and recovery, proteins are subjected to fluid forces which arise due to operations such as stirring, pumping and centrifugation. The resulting hydrodynamic shear forces may cause damage to the large molecular weight proteins, resulting in denaturation and inactivation of the protein. This is a major concern as it affects the overall efficiency of protein recovery and final yield of the product. A considerable amount of research has been devoted to studying the effects of hydrodynamic shear stress on proteins, especially with respect to the enzymes. Enzymes are subjected to shear stresses during their production in fermentors, during isolation and purification steps in downstream operations and also during their use in enzyme reactors, especially if stirred reactors are employed to perform enzyme catalysed reactions. The present review discusses the effects of fluid shear stress on proteins including enzymes. A brief description on deactivation has been included in order to understand the effect of shear on the deactivation kinetics of proteins. The model systems used to subject proteins to shear and some unit operations during protein processing or use wherein they are exposed to shear stresses have also been presented. The significance of shear effects in designing bioprocesses involving shear sensitive biocatalysts as well as suggestions for future work have also been given.

Expression in Pichia pastoris and purification of Aspergillus awamori glucoamylase catalytic domain

In this paper we report the expression in Pichia pastoris, purification, and characterization of the Aspergillus awamori glucoamylase catalytical domain (GAc). Pichia pastoris produced GAc to the level of 0.4 g per liter medium. This production level is about the same level as that gained for recombinant GA from Aspergillus and about 100-fold more than previously achieved by Saccharomyces cerevisiae. The GAc expressed in Pichia pastoris was purified by two independent chromatographic methods employing ion exchange or affinity chromatography to apparent homogeneity. The purified protein has a molecular weight of about 75,000 and specific activity of 78 units per milligram protein. The propeptide present in the glucoamylase N terminus was found to be removed correctly by P. pastoris. Glucoamylase produced by P. pastoris is N- and O-glycosylated, with 23% carbohydrate content. The N-linked oligosaccharides appear to be larger than in invertase, another glycoprotein heterologously expressed in P. pastoris. O-glycosides (studied to our knowledge for the first time in P. pastoris in this report) contribute about half of the total carbohydrate content in GAc. Purified GAc appears as multiple hands on isoelectric focusing with p1 values around 3.5, a value that is little higher than that for GAc produced in S. cerevisiae. GAc could be used as a versatile tool in studying protein expression in P. pastoris: as an affinity handle for other secreted proteins produced in P. pastoris, as a reporter gene when studying gene expression, and as a model protein in studying protein secretion and processing in P. pastoris.

Large-scale processing of macromolecules

Over the past year, a number of advances have been made in the large-scale purification of macromolecules, particularly proteins. Although refinements to individual unit operations have occurred, especially in improving the speed of operation and performance of large-scale chromatographic media, a major research thrust has been the development of processes in which steps are combined or eliminated to improve operability and reduce cost.