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Engineered cytochrome P450 enzymes can perform an atom-transfer radical cyclization (ATRC) that natural biology is not known to use. In the 2021 Science study by Qi Zhou, Michael Chin, Yue Fu, Peng Liu and Yang Yang, directed evolution converted P450 proteins into catalysts that turn bromide- and alkene-containing amides into lactams while controlling which stereoisomers form. Chemistry World reported best-case values of up to 97% enantioselectivity and up to 96% diastereoselectivity, including gram-scale demonstrations.
What the breakthrough actually is
Cytochrome P450 enzymes are metalloenzymes best known for selective oxidation. They use a heme iron center to activate oxygen and add oxygen to organic molecules, often with impressive control over position and three-dimensional shape. Zhou and colleagues showed that the same general protein scaffold can be redesigned for a fundamentally different job: an atom-transfer radical cyclization.
In this reaction, an amide substrate carries both a carbon–bromine bond and an alkene. The engineered catalyst initiates a carbon-centered radical, allows that radical to add across the alkene, and closes a ring. The product is a lactam, a cyclic amide found in many useful chemical structures. The transformation is described as “new-to-nature” because no natural enzyme function was known to carry out this ATRC before the work.
The result is not simply that a protein tolerates an unusual reagent. The evolved enzymes direct a radical reaction toward particular spatial arrangements. That stereocontrol is the central achievement: radicals are often valuable for bond construction but difficult to control because they can react through multiple pathways and conformations.
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The reaction in plain chemical terms
Starting material and product
The demonstrated substrates are amides designed with two reactive handles: a bromide that can participate in atom transfer and an alkene positioned to accept the resulting radical. Intramolecular addition joins atoms within one molecule, producing a ring. Because the nitrogen remains part of the new ring, the product class is a lactam rather than a simple hydrocarbon cyclization product.
What “atom-transfer radical cyclization” means
In an ATRC, a catalyst helps move a halogen atom between chemical species while generating and then terminating a radical. A simplified sequence is:
- The catalyst interacts with the carbon–bromine bond, producing a carbon-centered radical and a catalyst-bound bromine equivalent.
- The radical adds intramolecularly to the alkene, creating a new carbon–carbon bond and a ring-shaped radical intermediate.
- Atom transfer or a related termination step returns the intermediate to a closed-shell product, giving the brominated or dehalogenated lactam product specified by the reaction design.
The exact electronic steps depend on the engineered P450 and reaction conditions; the important point is that the protein environment organizes a radical pathway that is not part of the enzyme’s native oxidative cycle.
Why the ring-forming step is difficult
Several conformations can bring the bromide and alkene into proximity. Once a radical is formed, it can cyclize in more than one geometric arrangement, and newly created stereocenters can be formed in combinations that are hard to separate later. An ordinary solution catalyst may provide limited control because the reactive intermediate is short-lived and relatively insensitive to many weak interactions. An enzyme instead surrounds the substrate with a defined pocket. Mutations can alter pocket size, charge, hydrogen-bonding contacts and the orientation of the heme, changing which transition state is favored.
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How directed evolution changed a P450 catalyst
Screening for a starting scaffold
The team first screened metalloproteins for activity in the target ATRC. This is a practical way to search beyond a single enzyme family: a metal center that can exchange electrons or bind a halogen may provide a starting point even if its natural reaction is unrelated. Promising proteins were then selected for engineering.
Mutation and selection cycles
Directed evolution does not require predicting every beneficial mutation in advance. Researchers create variant genes, express the variants, test their products and retain the variants with better activity or selectivity. Repeating those cycles can combine small improvements. For this project, the selection pressure was not only whether lactam formed, but also which stereochemical outcome predominated.
The study reports stereodivergent enzymes: different evolved P450 variants can favor different stereochemical outcomes. In other words, protein engineering supplied a way to alter the reaction’s three-dimensional preference rather than accepting the single selectivity profile of the original scaffold.
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For substrate demonstrations, the researchers used intact Escherichia coli containing the evolved enzymes. Whole-cell catalysis can protect fragile proteins, provide some cofactors and simplify handling compared with purifying every enzyme variant. It also demonstrates that the chemistry can operate in a biological host, although a whole-cell experiment is not the same as a validated manufacturing process.
What the reported performance numbers mean
Chemistry World’s 2022 report describes a broad substrate range, gram-scale demonstrations, and best-case selectivities of up to 97% enantioselectivity and up to 96% diastereoselectivity. “Up to” matters: these are maxima observed for particular substrates and enzyme variants, not guaranteed values for every amide in the study.
| Metric | Reported result | How to interpret it |
|---|---|---|
| Enantioselectivity | Up to 97% | Best reported preference for one enantiomer among the tested examples; substrate-specific. |
| Diastereoselectivity | Up to 96% | Best reported preference among multiple diastereomers; not a universal value. |
| Scale | Gram-scale demonstrations | Shows that products were made on gram scale in demonstrations; it does not establish industrial throughput. |
| Substrate coverage | Wide range, with individual details in the paper | The accessible report does not provide a complete substrate-by-substrate table of selectivity values. |
Enantioselectivity describes the imbalance between mirror-image products. Diastereoselectivity describes the imbalance between products that are not mirror images, such as cis/trans or other relative configurations. A reaction can score highly on one measure and less highly on the other, so the two percentages should not be combined or averaged.
Why stereodivergence is especially useful
Many asymmetric catalysts are naturally biased toward one configuration. If a medicinal-chemistry program needs the opposite enantiomer, researchers may have to redesign the catalyst, change the substrate or use a separation step. The evolved P450 set addresses that constraint by providing variants with different stereochemical preferences.
The useful comparison is therefore not a single “best enzyme,” but a panel of catalysts:
| Comparison axis | Question to ask | Why it matters |
|---|---|---|
| Stereochemical outcome | Which enantiomer or diastereomer does each variant favor? | Determines whether the catalyst makes the structure needed for the next step. |
| Substrate scope | Which bromide/alkene amides are accepted? | A catalyst optimized for one substrate may not accept a close analogue. |
| Selectivity by substrate | What are the enantiomeric and diastereomeric ratios for that exact substrate? | Headline maxima cannot replace compound-specific data. |
| Reaction scale | Was the result analytical scale or a gram-scale demonstration? | Scale affects confidence in isolation, mixing and material balance. |
How this differs from a natural P450 reaction
Native P450 chemistry generally involves oxygen activation and oxidation of a substrate. The engineered ATRC instead uses the heme protein as a controlled environment for radical generation and ring formation. The protein has been repurposed; the reaction should not be described as a normal biological P450 function.
This distinction also explains why the work is important for enzyme engineering. It demonstrates that evolutionary optimization can move a catalyst outside the set of reactions selected by biology. The protein supplies a three-dimensional pocket and a metal center, while laboratory selection supplies the new reaction preference.
What a practical workflow would look like
The published demonstration suggests a general workflow for researchers attempting similar new-to-nature chemistry, without implying that every step is plug-and-play.
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- Identify compatible metalloproteins. Screen proteins whose metal centers can plausibly support the required electron-transfer or atom-transfer event.
- Establish a measurable assay. Detect product formation and quantify stereoisomers separately; a total-yield assay alone cannot select for stereocontrol.
- Create variant libraries. Mutate residues around the active site and other positions that influence folding, redox behavior or substrate access.
- Screen and sequence winners. Retain variants that improve conversion, enantioselectivity or diastereoselectivity, then identify the mutations responsible.
- Combine beneficial changes. Re-test combinations because mutations can reinforce or cancel one another.
- Test a substrate panel. Measure each substrate independently; selectivity often changes with steric and electronic features.
- Increase scale cautiously. Confirm that oxygen transfer, mixing, cell density, cofactor supply and heat removal remain adequate before moving from analytical experiments to gram-scale work.
Why whole-cell use does not automatically mean industrial readiness
Intact E. coli can make an evolved enzyme convenient to deploy, but manufacturing decisions require information beyond a successful reaction. The study and the cited report establish gram-scale demonstrations, not commercial readiness, cost competitiveness or robust operation at plant scale.
- Productivity: A gram-scale batch does not by itself reveal space-time yield, enzyme lifetime or the amount of cell material required.
- Raw materials: The economics of the brominated substrates, cofactors, media and downstream purification are not established by the headline selectivity figures.
- Process robustness: Scale-up can change oxygen transfer, light exposure if relevant to the setup, viscosity, mixing and heat removal.
- Purification: High selectivity reduces separation work, but residual brominated material, cells, proteins and side products still need a validated workup.
- Regulatory status: No pharmaceutical application or approved manufacturing use follows from this proof-of-concept alone.
How to read the result without overclaiming
What is established
- Cytochrome P450 scaffolds were engineered for an ATRC reaction not previously known in nature.
- The reaction converts bromide- and alkene-bearing amides into lactams.
- Directed evolution produced variants with different stereochemical preferences.
- Reported best cases reached up to 97% enantioselectivity and up to 96% diastereoselectivity, with gram-scale demonstrations.
What remains context-dependent
- The percentages are maxima across examples, not an average performance specification.
- Acceptance of a new substrate must be tested rather than inferred from the phrase “wide substrate range.”
- Gram scale is evidence of a larger demonstration, not evidence of industrial production.
- The result does not establish a particular drug, commercial enzyme product or cost advantage.
Why the finding matters beyond lactams
Lactams are valuable motifs, but the broader lesson is methodological. Enzyme engineering can combine the selectivity of a protein pocket with reaction classes traditionally associated with synthetic radical chemistry. That combination may help chemists make densely functionalized molecules while reducing the need for chiral auxiliaries, repeated protecting-group operations or difficult chromatographic separations.
The work also expands the design space for biocatalysis. Instead of asking only which natural enzyme performs a desired transformation, researchers can ask whether an existing metalloenzyme can be evolved to stabilize the transition states of a new one. The answer will depend on substrate geometry, metal reactivity and the ability to measure useful variants, but the P450 ATRC example shows that natural reaction history is not an absolute limit.
The study in context
The work was published as Qi Zhou, Michael Chin, Yue Fu, Peng Liu and Yang Yang, “Stereodivergent atom-transfer radical cyclization by engineered cytochromes P450,” Science 374 (2021), pages 1612–1616, DOI 10.1126/science.abk1603. The Yang Lab describes its wider program as expanding enzymatic machinery beyond nature’s biochemical landscape. Chemistry World’s 2022 account supplies the cited maximum selectivity and gram-scale context.
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Taken together, the result is a research demonstration of enzyme-enabled radical chemistry, not a claim that biology naturally performs this cyclization or that the method is already a universal production technology.
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