The Haemophilus influenzae carbonic anhy- drase (HICA) is important in converting carbon dioxide to bicarbonate in bacteria. Endogenous cellular proteins, like Escherichia coli carbonic anhydrase (ECCA), have been observed to bind to a Ni-NTA column, which can be used as a means of protein purification. The possibility ex- ists that proteins that do not normally bind to Ni-NTA, like HICA, can be engineered using site directed mutagenesis to introduce histidine resi- dues that would give the protein the capability to bind, allowing for a one-step purification method. Site-directed mutagenesis was used to introduce the double mutation of E56H + E59H to H. influenzae carbonic anhydrase encoded on a plasmid. This protein was overexpressed in competent cells. A cell lysate was run on a Ni- NTA column to see how well the recombinant protein was able to bind to the column. We found that the E56H + E59H recombinant en- zyme bound loosely to the column, and while most of the protein eluted in 10 and 25 mM washes with imidazole, some of the protein re- mained bound and was eluted with concentra- tions of imidazole between 25 and 250 mM. Therefore, the arrangement of surface histidine residues in the E56H + E59H mutant allowed some of the recombinant enzyme to bind loosely to Ni-NTA.
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