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Researchers engineered an artificial metalloenzyme that catalysed an enantioselective Friedel–Crafts alkylation of indole derivatives. The catalyst combined a bacterial protein scaffold, LmrR, with a metal-binding unnatural amino acid and copper. Its strongest reported results were for 2-methylindole; other indole substrates performed much worse, making this a laboratory proof of concept rather than evidence of a broadly useful or industrial catalyst.
How the researchers built the artificial metalloenzyme
The work, led by University of Groningen researcher Gerard Roelfes, was reported in a 2014 Royal Society of Chemistry blog post and published as a 2015 Chemical Science paper, “Novel artificial metalloenzymes by in vivo incorporation of metal-binding unnatural amino acids.” The blog described the reaction as a catalytic asymmetric Friedel–Crafts alkylation.
The researchers used amber stop-codon suppression to incorporate a non-proteinogenic, metal-binding amino acid into LmrR, a protein scaffold. They then combined the modified protein with copper. In this constructed catalyst, copper provided reaction capabilities while the protein supplied a chiral, hydrophobic environment around the reaction site. It was an engineered metalloenzyme, not a naturally evolved enzyme.
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What reaction it catalysed
The study examined vinylogous Friedel–Crafts alkylation using indole derivatives. In this type of reaction, an indole-derived substrate forms a carbon–carbon bond with an electrophilic partner. “Vinylogous” describes reaction at a position connected through a conjugated system rather than directly adjacent to the reactive group. The researchers were interested not just in whether the reaction occurred, but in whether the chiral protein environment could favor one product enantiomer over the other.
Enantiomeric excess (ee) measures that imbalance: a higher ee means a greater proportion of one enantiomer relative to its mirror-image partner. Conversion measures how much starting substrate was transformed. These are separate outcomes; a reaction can be selective among product formed while converting only a small fraction of its starting material.
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Reported results depended strongly on the indole substrate
The paper’s reported data show that performance was not uniform across the tested indoles. The table values below are experimental results from the study, not general guarantees for other substrates or conditions.
| Indole substrate | Catalyst variant | Conversion | Enantiomeric excess (ee) |
|---|---|---|---|
| 2-methylindole | LmrR_LM_M89X_Cu(II) | 92 ± 4% | 80 ± 2% |
| 2-methylindole | LmrR_LM_M89X_F93W_Cu(II) | 94 ± 8% | 83 ± 0% |
| 5-chloroindole | Three listed variants | 2–5% | 21–50% |
| Another tested indole | Variants reported in the paper’s table | 11–16% | 49–55% |
The authors identified 2-methylindole as especially compatible with the protein pocket and 5-chloroindole as a poor substrate. They noted that this kind of substrate specificity is unattractive when broad substrate scope is the goal. In other words, the best result for one indole should not be read as evidence that the catalyst works equally well across indole derivatives.
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Conditions and limits of the experiment
The paper’s indexed experimental notes give typical conditions as 9 mol% Cu(H₂O)₆(NO₃)₂ (90 μM), 1.25 equivalents of LmrR variant measured in monomer, 20 mM MOPS buffer at pH 7.0, and 150 mM NaCl. Reactions ran for three days at 4 °C. The reported table values were averages of two independent experiments, each performed in duplicate.
Those conditions describe a controlled laboratory experiment, not a manufacturing process. The work establishes that a deliberately assembled protein–metal catalyst can promote an asymmetric reaction for particular substrates under specified conditions. It does not establish scale-up, broad utility, commercial availability, or use in industrial production.
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How later studies fit in
Subsequent research explored related artificial metalloenzymes, but those findings are distinct from the LmrR results above. A 2020 study used copper(II) with TetR-family proteins CgmR, RamR, and QacR, without an external ligand, for enantioselective vinylogous Friedel–Crafts alkylation. It reported up to 75% ee and proposed that electrostatic and π-stacking interactions in the proteins’ second coordination sphere help bind the copper–substrate complex. The 2020 report is evidence of continued investigation into protein scaffolds, not a result from the 2015 LmrR paper.
Another 2020 LmrR study examined how the position of an abiological metal cofactor relates to catalytic preference. It considered Friedel–Crafts alkylation of indoles with β-substituted enones and a tandem alkylation/enantioselective protonation using α-substituted enones, reporting that a single protein mutation could specialize the artificial metalloenzyme toward one reaction type. That study likewise represents later work, rather than expanded performance data for the title-matching experiment.
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What the result means
The central result is a design demonstration: genetic-code expansion let researchers place a metal-binding unnatural amino acid in LmrR, add copper, and use the resulting protein environment to catalyse an enantioselective carbon–carbon bond-forming reaction. The measured performance varied substantially by substrate, with strong conversion and ee for 2-methylindole but very low conversion for 5-chloroindole. The study therefore shows both the potential of engineered metalloenzymes and the challenge of obtaining broad substrate compatibility.
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