Immobilized pH gradients for isoelectric focusing: Interaction between histones and histone-like proteins with the charged polyacrylamide matrix

Electrically immobilized enzyme reactors: bioconversion of a charged substrate. Hydrolysis Of penicillin G by penicillin G acylase

The possibility of using the multicompartment immobilized enzyme reactor (MIER) in presence of a charged substrate is here explored. Penicillin G acylase is used to convert penicillin G (a free acid, with a pK of 2.6) into two charged products: phenyl acetic acid (PAA, with a pK of 4.2) and 6-aminopenicillanic acid (6-APA, a zwitterion with a pI of 3.6). The enzyme is trapped by an isoelectric mechanism in a chamber of the electrolyzer delimited by a pI 5.0 and a pI 9.0 amphoteric, isoelectric membranes. Under normal operating conditions (continuous substrate feeding in the presence of an electric field), only a low substrate conversion can be achieved, due to rapid electrophoretic transport of unreacted penicillin G out of the reaction chamber towards the anode. Excellent conversion rates (>96%) are obtained under a "doubly-discontinuous" operation mode: a time-lapse substrate feeding, accompanied by short times (4-8 min) of electric field interruption. The product of interest (6-APA, a precursor of semisynthetic penicillins), by virtue of its amphoteric nature, is trapped in a chamber delimited by a pI 3.5 membrane and a pI 5.5 membrane, adjacent to the reaction chamber on its anodic side. The other contaminant product (PAA) first accumulates in the same chamber and then progressively vacates it to collect in the anodic reservoir, leaving behind a pure 6-APA solution. In this operation mode, vanishing amounts of unreacted substrate (penicillin G) leave the reaction chamber to contaminate the adjacent, anodic chambers. A novel class of zwitterionic buffers is additionally reported, able to cover very thoroughly any pH value along the pH 3-10 interval: polymeric, zwitterionic buffers, synthesized with the principle of the Immobiline (acrylamido weak acids and bases) chemicals. Enhanced enzyme reactivity is found in this macromolecular buffers as compared to conventional ones.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Two-dimensional electrophoresis of basic proteins with equilibrium isoelectric focusing in carrier ampholyte-pH gradients

A modified procedure for the two-dimensional electrophoretic analysis of basic polypeptides is described. This method uses isoelectric focusing with carrier ampholytes in the first dimension, and sodium dodecyl sulfate-electrophoresis in the second dimension. Counteraction of the cathodic drift is achieved by glass tube treatment (silanization), electrolyte modification (use of weak bases and acids), protection of the catholyte from carbon dioxide, and the addition of glycerol to the gel mix. Better resolution and reproducibility are obtained than with nonequilibrium pH gradient electrophoresis, since quasi equilibrium focusing can be obtained.

Isoelectric focusing of sparingly soluble proteins in immobilized pH gradients, exemplified by microvillar membrane hydrolases

A novel fractionation technique is described for analysis of membrane-bound enzymes and sparingly soluble proteins: isoelectric focusing in a mixed-type matrix, containing a primary, immobilized pH gradient with a superimposed, secondary carrier ampholyte pH gradient. Three microvilli hydrolases: dipeptidyl peptidase IV, gamma-glutamyl transferase and alkaline phosphatase exhibit an array of sharply focused, enzyme active bands in the pH 4-6.5 range. The separation pattern obtained is by far superior to any separation achieved by either technique separately.