Circularly permuted dihydrofolate reductase of E. coli has functional activity and a destabilized tertiary structure

Three circularly permuted variants of Escherichia coli dihydrofolate reductase genes were constructed. Linkers coding tri- and pentapeptides were used to connect the natural 5'- and 3'-terminal ends. Only one variant of circularly permuted protein with tripeptide linker and the cleavage of the peptide bond between 107 and 108 amino acid residues was produced in a good yield. The expressed protein was insoluble in the cells, but at pH 8.0 and higher the isolated protein was soluble. Enzymatic assay and physical studies have shown that permuted dihydrofolate reductase has a destabilized tertiary structure. Only the addition of the natural substrates or inhibitors lead to the protein with the native-like structure and functional activity.