Evolved enzymes for cancer therapeutics and orthogonal systems
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Directed evolution has been explored for a long time. Various ideas, methods, have been shown to be feasible and successful in the enzyme field. We were interested in evolving enzymes for applications. Therefore, we evolved human cystathionine gamma-lyase (hCGL) and E. coli biotin ligase for therapeutic and biotechnology applications. Wild-type human cystathionine gamma-lyase does not have any methionine-degrading activity, unlike the high methionine-degrading abilities of bacterial methionine gamma-lyase (MGL) found in Pseudomonas putida. The ability to engineer hCGL to breakdown methionine can be a potential cancer treatment by targeting the methionine-dependent cancer cells. However, the methionine-degrading activity of previously engineered hCGL has only shown 1% activity compared to MGL, too low to be useful in practical cancer therapeutics. By using a combination of protein design and phylogenetic analysis, we further evolved hCGL to achieve a higher methionine-degrading activity, with one variant displaying as much as 7% activity compared to bacterial MGL, making it a more likely candidate in cancer treatment.In addition, it has been shown that new orthogonal pairs of biotin protein ligase and biotin have many biotechnology applications. Therefore, we have developed selection scheme for directing the evolution of E. coli biotin protein ligase (BPL, gene: BirA) via in vitro compartmentalization, and have altered the substrate specificity of BPL towards the utilization of the biotin analogue desthiobiotin. Following just 6 rounds of selection and amplification several variants that demonstrated higher activity with desthiobiotin were identified. The best variants from Round 6, BirA₆₋₄₀ and BirA₆₋₄₇, showed 17-fold and 10-fold higher activity, respectively, their abilities to use desthiobiotin as a substrate. Further characterization of BirA₆₋₄₀ and the single substitution variant BirA[subscript M157T] revealed that they had 2- to 3-fold higher kcat values for desthiobiotin, and 3- to 4-fold higher K[subscript M] values. The k[subscript cat]/K[subscript M] values for these enzymes were around 0.7-fold that of BirA[subscript wt-]. It is interesting the selections did not lower the K[subscript M] for desthiobiotin and actually led to a less efficient enzyme. This is an example of how "you get what you select for". Because peptide:DNA conjugates were distributed such that there was on average one template or less per emulsion compartment there was selection only for the catalytic rate (k[subscript cat]) of desthiobiotinylation and not for turnover. Given these conditions, it might be anticipated that the peptide substrate, rather than desthiobiotin, should be bound better by the winning variants, and in fact BirA₆₋₄₀ showed a reduced K[subscript M] value for BAP.