Examination of the binding behaviour of several proteins with the immobilized copper(II) complexes of o-, m- and p-xylylene bridged bis(1,4,7-triazacyclononane) macrocycles

Promiscuity of host cell proteins in the purification of histidine tagged recombinant xylanase A by IMAC procedures: A case study with a Ni2+-tacn-based IMAC system

Determination of the extent of host cell protein (HCP) contamination is an essential pre-requisite to validate the chromatographic purification of recombinant proteins. This study explores how different experimental conditions affect the HCP profiles generated during the immobilised metal ion affinity chromatographic (IMAC) purification with a Ni²⁺-1,4,7-triaza-cyclononane (tacn) Sepharose FF™ sorbent of the Bacillus halodurans N- and C-terminal His₆-tagged xylanase A, expressed by Escherichia coli BL21(DE3) cells, and captured directly from cell lysates. Comparative studies were also carried out under identical loading, wash and elution conditions using nitrilotriacetic acid (NTA), also immobilised onto an agarose support and complexed with Ni²⁺ ions. High-resolution tandem mass spectrometry of the tryptic peptides derived from the proteins present in the IMAC flow-through, wash and elution fractions confirmed that the E. coli BL21(DE3) HCP profiles were dependent on the choice of adsorbent. With feedstocks containing the N- or C-terminal His₆-tagged xylanase A, in several instances the same E. coli BL21(DE3) HCPs were found to co-elute with the tagged protein from either adsorbent, indicating a preferential ability of some HCPs to bind to both the IMAC resin and to the recombinant protein. This promiscuous behaviour has been found to be due to factors other than just the presence of histidine-rich motifs within the amino acid sequences of these HCPs. This case study demonstrates that the choice of protein expression and separation conditions impact on the levels of HCP contamination when different IMAC systems are employed.

Studies with an immobilized metal affinity chromatography cassette system involving binuclear triazacyclononane-derived ligands: automation of batch adsorption measurements with tagged recombinant proteins

This study describes the determination of the adsorption isotherms and binding kinetics of tagged recombinant proteins using a recently developed IMAC cassette system and employing automated robotic liquid handling procedures for IMAC resin screening. These results confirm that these new IMAC resins, generated from a variety of different metal-charged binuclear 1,4,7-triaza-cyclononane (tacn) ligands, interact with recombinant proteins containing a novel N-terminal metal binding tag, NT1A, with static binding capacities similar to those obtained with conventional hexa-His tagged proteins, but with significantly increased association constants. In addition, higher kinetic binding rates were observed with these new IMAC systems, an attribute that can be positively exploited to increase process productivity. The results from this investigation demonstrate that enhancements in binding capacities and affinities were achieved with these new IMAC resins and chosen NT1A tagged protein. Further, differences in the binding performances of the bis(tacn) xylenyl-bridged ligands were consistent with the distance between the metal binding centres of the two tacn moieties, the flexibility of the ligand and the potential contribution from the aromatic ring of the xylenyl group to undergo π/π stacking interactions with the tagged proteins.

An AIL/IL-based liquid/liquid extraction system for the purification of His-tagged proteins

A sorbent based on affinity ionic liquid (AIL), triazacyclononane-ionic liquid, was synthesized, characterized, and applied to the extraction of histidine (His)-tagged proteins from aqueous buffer to ionic liquid (IL) phase. The adsorbed His-tagged proteins could be back-extracted from the IL phase to the aqueous buffer with an imidazole solution. The specific binding of His-tagged proteins with AIL/IL could be affected by a few factors including the ionic strength and coordinated metal ions. In the case of His-tagged enhanced green fluorescent protein (EGFP), the maximum binding capacity of Cu(2+)-AIL/IL reached 2.58 μg/μmol under the optimized adsorption conditions. The eluted His-tagged EGFP kept fluorescent and remained active through the purification process. Moreover, a tandem extraction process successively using Cu(2+)-AIL/IL and Zn(2+)-AIL/IL systems was developed, which was proven very efficient to obtain the ultimate protein with a purity of about 90 %. An effective reclamation method for the AIL/IL extraction system was further established. The sorbent could be easily regenerated by removing metal ions with EDTA and the followed reimmobilization of metal ions. Easy handling of the presented M(2+)-AIL/IL system and highly specific ability to absorb His-tagged proteins make it attractive and potentially applicable in biomolecular separation.

Use of phage display methods to identify heptapeptide sequences for use as affinity purification 'tags' with novel chelating ligands in immobilized metal ion affinity chromatography

This study describes the screening of a peptide phage display library for amino acid sequences that bind with different affinities to a novel class of chelating ligands complexed with Ni²+ ions. These chelating ligands are based on the 1,4,7-triazacyclononane (TACN) structure and have been chosen to allow enhanced efficiency in protein capture and decreased propensity for metal ion leakage in the immobilized metal ion affinity chromatographic (IMAC) purification of recombinant proteins. Utilising high stringency screening conditions, various peptide sequences containing multiple histidine, tryptophan, and/or tyrosine residues were identified amongst the different phage peptide sequences isolated. The structures, and particularly the conserved locations of these key amino acid residues within the selected heptapeptides, form a basis to design specific peptide tags for use with these novel TACN ligands as a new mode of IMAC purification of recombinant proteins.