Multicopper oxidase mediated single-carbon insertion for skeletal remodeling

This study presents the first biocatalytic platform utilizing multicopper oxidases to achieve sustainable, one-step skeletal remodeling of phenolic and indole derivatives into functionalized tropones and quinoline analogues via exogenous single-carbon insertion, thereby overcoming the limitations of traditional carbene-based methods and expanding bioactive compound libraries.

The Gist

Imagine you are a master architect working on a city of tiny, intricate buildings called molecules. These buildings make up the drugs we use to fight diseases. Usually, if an architect wants to add a new room to a building, they have to tear the whole thing down and rebuild it from scratch. It's expensive, messy, and takes forever.

"Skeletal editing" is like having a magic wand that lets you slip a new room right into the middle of an existing building without knocking anything down. This paper introduces a brand-new, eco-friendly way to do this magic trick, specifically adding just one single carbon atom to make the building bigger and more useful.

Here is the simple breakdown of how they did it:

1. The Problem: The Old Way is Dangerous

For the last decade, chemists have tried to add that single carbon atom using something called a carbene.

• The Analogy: Think of a carbene like a volatile, unstable stick of dynamite. It works, but it's dangerous to handle, expensive to make, and requires harsh conditions (like extreme heat or toxic chemicals) to get it to work. It's like trying to build a new room by blowing up a wall and hoping the bricks land in the right place.

2. The Solution: Nature's "Recycling Crew"

The researchers in this paper decided to swap the dangerous dynamite for a biological construction crew.

• The Tool: They used an enzyme (a tiny biological machine) called a Multicopper Oxidase (MCO). Think of this enzyme as a highly skilled, 3D-printing robot that lives inside bacteria.
• The Material: Instead of dangerous dynamite, they used nitroalkanes. These are stable, safe, and cheap chemicals (think of them as pre-made, sturdy Lego bricks).
• The Power Source: They used plain old oxygen from the air to power the process, making it very green and sustainable.

3. The Process: How the Magic Happens

The team took a specific type of starting material (phenols, which are like flat, hexagonal rings) and wanted to turn them into tropones (which are seven-sided rings, like a slightly stretched-out hexagon).

Here is the step-by-step story of what the enzyme does:

1. The Spark: The enzyme grabs the flat ring and the "Lego brick" (nitroalkane).
2. The Handshake: Instead of a violent explosion, the enzyme gently connects them using a "radical handshake" (a scientific term for a specific type of chemical bond).
3. The Stretch: Once connected, the molecule naturally wants to rearrange itself. It's like a spring-loaded toy that snaps open. The flat ring stretches out, grabs the new carbon, and snaps into a new, larger shape.
4. The Result: You end up with a brand-new, functionalized ring structure in just one step.

4. Why Does This Matter? (The "Superpower")

The researchers didn't just make pretty shapes; they made better medicine.

• They tested these new molecules against superbugs (bacteria that have learned to resist normal antibiotics).
• The Result: The original flat-ring molecules were useless against the superbugs. But after the enzyme added that single carbon atom and expanded the ring, the new molecules became powerful weapons that could kill the bacteria.
• It's like taking a dull, flat key that doesn't fit a lock, and the enzyme magically reshapes it into a complex key that opens the door.

5. The Big Picture

This is the first time scientists have used a biological machine to add a carbon atom from the outside to expand a ring structure.

• Before: You needed a chemistry lab full of hazardous chemicals and expensive equipment.
• Now: You can grow bacteria in a jar, add some safe chemicals, and let nature do the heavy lifting.

In a nutshell: This paper is about teaching bacteria to act as eco-friendly architects. They take simple, safe ingredients and, using oxygen as fuel, magically expand molecular structures to create new, life-saving medicines that are better at fighting superbugs than the original ingredients ever were. It's a cleaner, safer, and smarter way to build the drugs of the future.

Technical Summary

1. Problem Statement

Modern drug discovery requires efficient methods to generate structurally diverse compound libraries. Skeletal editing—the precise insertion, deletion, or exchange of atoms within a molecular framework—is a transformative strategy for achieving this. Specifically, single-carbon insertion into cyclic systems (e.g., converting six-membered rings to seven-membered rings) is highly valuable for accessing pharmaceutically relevant expanded-ring scaffolds like tropones and quinolines.

However, current methodologies face significant limitations:

• Reliance on Carbenes: Traditional approaches rely on carbene intermediates generated from hazardous, unstable, and potentially explosive precursors (e.g., diazo compounds, chlorodiazirines).
• Harsh Conditions: These reactions often require toxic reagents, stoichiometric oxidants, and harsh conditions, leading to low yields and poor scalability.
• Lack of Biocatalysis: While enzymatic strategies exist for single-oxygen insertion (e.g., Baeyer-Villiger oxidation), biocatalytic intermolecular single-carbon insertion for skeletal remodeling has remained entirely unexplored.

2. Methodology

The authors developed a novel multicopper oxidase (MCO)-catalyzed platform to achieve direct, one-step exogenous single-carbon insertion into phenolic and indole derivatives.

• Enzyme Engineering:
  • Starting with Bacillus freudenreichii MCO (BacFre), the team employed directed evolution via site-saturation mutagenesis around the T1 copper active site.
  • The optimized variant, BacFre-K474F/I500L, showed a threefold yield increase over the wild type.
  • Mechanistic studies identified key residues: H503 and C498 for electron transfer, E502 as a catalytic base for proton abstraction, and the K474F mutation which disrupts a salt bridge to facilitate proton abstraction, enhancing activity.
• Reaction Design:
  • Substrates: Phenol derivatives (sulfonamido phenols) and indoles.
  • Carbon Source: Stable, safe, and cost-effective nitroalkanes (e.g., nitromethane) serve as the single-carbon synthons.
  • Oxidant: Molecular oxygen ($O_2$) from air is the sole terminal oxidant.
  • Conditions: Whole-cell biocatalysis using E. coli expressing the engineered MCO in MOPS buffer (pH 9) at room temperature.
• Mechanistic Elucidation:
  • Combined experimental trapping (radical traps, intermediate isolation) and Density Functional Theory (DFT) calculations.
  • The pathway involves an MCO-mediated radical oxidative coupling between the phenol and nitroalkane to form a dienone intermediate, followed by a spontaneous tandem Michael addition and ring-expansion via a norcaradiene intermediate.

3. Key Contributions

• First Biocatalytic Platform: This work establishes the first biocatalytic system for exogenous single-carbon insertion skeletal editing, bridging a critical gap in programmable molecular editing.
• Green Chemistry: The strategy replaces hazardous carbene precursors with stable nitroalkanes and uses $O_2$ as a green oxidant, operating under mild, aqueous conditions.
• Mechanistic Insight: It elucidates a radical oxidative coupling mechanism diverging from classical MCO self-coupling pathways, providing a blueprint for future enzyme engineering for C1 insertion.

4. Results

• Substrate Scope (Phenols):
  • The engineered enzyme successfully converted diverse sulfonamido phenols into functionalized tropones (7-membered rings) with yields ranging from 20% to 86%.
  • The reaction tolerated various functional groups (aryl, heteroaryl, alkyl sulfones).
  • Limitations: Electron-donating groups at the 3-position led to byproducts (ring contraction), and 2-substituted phenols favored oxidative dimerization over ring expansion.
• Substrate Scope (Indoles & Bioactive Molecules):
  • The strategy was extended to indole derivatives, yielding quinoline analogues (4%–59% yield).
  • Late-stage functionalization: The system successfully expanded the rings of complex bioactive molecules, including antiangiogenic agents, mTORC1 inhibitors, and PI4KIII$\beta$ inhibitors, converting them into their seven-membered ring counterparts.
• Nitroalkane Scope:
  • Various nitroalkanes (e.g., nitroethane, benzyl-nitro compounds) were utilized to generate multi-substituted tropones, though yields varied based on steric and electronic factors.
• Biological Activity:
  • The synthesized tropone products exhibited superior antibacterial activity against multidrug-resistant strains (e.g., Riemerella anatipestifer and Streptococcus dysgalactiae) compared to their parent phenolic substrates, which were often inactive.
  • For example, product 3a showed potent activity against Gram-negative strains where precursor 1a was inactive.

5. Significance

This research represents a paradigm shift in synthetic chemistry and drug discovery:

1. Sustainable Scaffold Diversification: It offers a scalable, atom-economical, and environmentally benign alternative to traditional carbene chemistry for expanding molecular complexity.
2. Drug Discovery Acceleration: By enabling rapid access to expanded-ring scaffolds (tropones and quinolines) from abundant phenolic building blocks, this method significantly expands the accessible chemical space for screening novel bioactive compounds.
3. Biocatalytic Innovation: It demonstrates the power of directed evolution to unlock non-natural reactivity (C1 insertion) in enzymes, paving the way for future "skeletal editing" tools that can perform complex molecular transformations under physiological conditions.