A Hidden Binding Pocket in the β- ketoacyl-ACP Synthase FabB

This study reveals that the bacterial enzyme FabB possesses a hidden secondary binding pocket that accommodates medium-to-long acyl chains in an alternate conformation, a structural feature absent in its homolog FabF that allows FabB to maintain catalytic function despite mutations that would otherwise disrupt substrate binding.

The Gist

Imagine a factory assembly line inside a tiny cell. This factory's job is to build fatty acids, which are like the building blocks for fats and oils. The workers on this line are enzymes, and two of the most important workers are named FabB and FabF.

These two workers are almost identical twins. They both have a specific job: grabbing a chain of carbon atoms (the "cargo") and making it longer. Usually, they are very picky about how long the cargo chain is and where it fits in their hands.

The Mystery: The "Blocking" Accident

Scientists decided to play a trick on these workers. They took a tiny piece of the worker's hand (a specific spot in the pocket where the cargo fits) and put a "block" there.

• Worker FabF: When they blocked its hand, it got confused. It could only handle very short chains. If you tried to give it a medium or long chain, it just dropped it. It stopped working.
• Worker FabB: When they blocked its hand, something weird happened. It didn't stop. It still managed to build long chains, though it made a slightly different mix of products than usual.

The Big Question: How did FabB keep working when its main hand was blocked, while FabF gave up?

The Discovery: The Secret Back Door

To solve this, the scientists built a high-resolution 3D model (like a super-powerful X-ray) of the blocked FabB worker holding a cargo chain.

They found the answer: FabB has a secret back door.

• The Main Door (Pocket A): This is the normal spot where the cargo chain usually sits. The "block" the scientists added made this door too small for long chains.
• The Secret Door (Pocket B): When the main door was blocked, the cargo chain didn't just give up. Instead, it bent and slipped into a second, hidden pocket that nobody knew existed before. It's like a car that can't fit in the main garage, so it pulls into a hidden driveway in the backyard.

This secret pocket allowed FabB to keep working even when its main hand was injured.

Why Didn't FabF Have This?

The scientists then looked at the twin, FabF. They realized that while FabF looks like it has the same layout, its "backyard" is messy. The furniture (other parts of the protein) is arranged differently, making that secret driveway unstable or impossible to use. When FabF's main hand was blocked, it had nowhere to go, so it stopped working.

The "Shape-Shifting" Magic

The paper explains that proteins aren't rigid statues; they are more like playdough. They can wiggle and change shape.

• FabB is like a flexible playdough figure that can quickly reshape itself to find a new spot for the cargo when the main spot is blocked.
• FabF is like a stiffer figure that can't reshape, so if you block its main spot, it's stuck.

Why Does This Matter?

This discovery is a big deal for a few reasons:

1. Evolutionary Safety Net: It shows that nature builds "backup plans" into its machines. If the main way of doing something gets broken, the protein can sometimes find a clever workaround using a hidden pocket.
2. Designing New Drugs: If we want to stop bacteria from making fats (which kills them), we usually try to block their main hand. But if they have a secret back door, they might survive. Now that we know about this "back door," we can design better drugs that block both the main hand and the secret pocket.
3. Customizing Factories: Scientists who want to engineer bacteria to make specific oils (like for biofuels or plastics) can now understand that simply changing one part of the enzyme might not work as expected because of these hidden backup modes.

In short: The scientists found that a bacterial enzyme (FabB) has a hidden "Plan B" pocket that lets it keep working even when its main job site is broken. Its twin (FabF) doesn't have this trick, which explains why they react so differently to the same injury. It's a great example of how nature's machines are surprisingly flexible and full of hidden surprises.

Technical Summary

1. Problem Statement

Assembly-line enzymes, such as Fatty Acid Synthases (FASs) and Polyketide Synthases (PKSs), utilize a limited set of catalytic motifs to synthesize diverse metabolites. A major limitation in engineering these systems is the poor understanding of how structurally similar enzymes achieve distinct substrate specificities.

• Specific Context: The study focuses on Escherichia coli FabB and FabF, two homologous $\beta$-ketoacyl-ACP synthases (KSs) responsible for fatty acid elongation.
• The Anomaly: Previous work showed that the G107M mutation in FabB (located in the acyl binding pocket) disrupts the binding of acyl chains longer than 8 carbons (C8) but surprisingly still allows the production of C10–C18 fatty acids. In contrast, the analogous mutation in FabF (I108M) completely abolishes the production of chains longer than C8.
• The Question: Why does FabB tolerate this mutation and maintain activity on longer chains, while FabF does not, despite their high structural similarity? Standard X-ray structures of wild-type enzymes could not explain this discrepancy.

2. Methodology

The authors employed a multi-disciplinary approach combining structural biology, biochemistry, and advanced computational simulations:

• Crosslinking Strategy: To capture transient enzyme-substrate complexes that are difficult to crystallize, the team used a catalytic cycle-based crosslinking strategy.
  • They utilized modified Acyl Carrier Proteins (ACPs) containing $\alpha$-bromopantetheine amide (to trap the transacylation state) and chloroacrylate (to trap the condensation state).
  • These mimics represented acyl chains of varying lengths (C2 to C16).
• X-ray Crystallography:
  • Generated and solved crystal structures of the FabB$^{G107M}$ mutant crosslinked with C8 (crosslinker 4) and C12 (crosslinker 5) substrate mimics.
  • Resolutions achieved were 2.14 Å and 2.25 Å, respectively.
• Molecular Dynamics (MD) Simulations:
  • Utilized Multiple Topology Replica Exchange of Expanded Ensembles (MT-REXEE), an enhanced sampling method allowing acyl chains to grow and shrink (in 2-carbon increments) during simulation.
  • Simulated both non-covalent enzyme-ACP complexes and covalent acyl-enzyme intermediates for Wild-Type (WT) and Mutant (G107M/I108M) forms of FabB and FabF.
  • Calculated relative binding free energies and tracked the population of catalytically competent conformations (defined as the distance between Cys163 and the catalytic carbonyl < 4 Å).

3. Key Contributions

• Discovery of a "Hidden" Binding Pocket (Pocket B): The study identifies a previously unobserved secondary binding site in FabB, termed Pocket B, which accommodates medium-to-long acyl chains (≥ C8) in an alternate conformation when the primary pocket (Pocket A) is obstructed.
• Mechanism of Mutational Buffering: The paper elucidates how homologous enzymes diverge not just in primary substrate preference, but in their biomolecular plasticity—specifically, the availability of alternative, energetically accessible binding modes that buffer against mutational perturbations.
• Structural Basis for Specificity: It reveals that the difference between FabB and FabF lies in subtle stabilizing interactions (hydrogen bonds and transient contacts) involving the phosphopantetheine (Ppant) linker, rather than major structural obstructions.

4. Key Results

• Crystallographic Findings:
  • FabB$^{G107M}$ + C8: The acyl chain adopts two conformations: one in the truncated primary tunnel (Pocket A) and one bending away into a novel region (Pocket B).
  • FabB$^{G107M}$ + C12: The acyl chain exclusively adopts the Pocket B conformation. Pocket A shows no electron density, indicating the mutation effectively blocks long chains from entering the primary tunnel.
  • Pocket B Structure: Formed by residues His298–Thr302 and Gly305–Val307 on FabB.
• Simulation Insights:
  • Pocket Accessibility: In WT FabB and FabF, acyl chains preferentially sample Pocket A (40–70% of trajectory). However, in the FabB$^{G107M}$ mutant, sampling of Pocket B increases 3- to 10-fold compared to FabF mutants.
  • Energetic Penalties: The G107M mutation imposes a 2–3 kcal/mol penalty on extending chains from C6 to C12 within Pocket A. Crucially, there is no significant free energy penalty for extending chains into Pocket B in FabB, whereas the I108M mutation in FabF destabilizes the complex, preventing catalytic competency.
  • Stabilization Mechanism: Pocket B is stabilized in FabB by specific interactions (Met206 and Lys310 forming H-bonds with the Ppant linker) that are absent in FabF (where analogous residues are Ala204 and Ala312).
• Catalytic Cycle Implications:
  • The study proposes a cycle where Pocket C is a transient delivery site, Pocket A is the primary elongation site, and Pocket B acts as a regulatory/tuning site.
  • The G107M mutation shifts longer chains from Pocket A to Pocket B, maintaining catalytic activity. In FabF, the mutation prevents entry into Pocket B, leading to a loss of function for longer chains.
  • The study also confirms the role of the "E-Q gate" (Glu200-Gln113) in negative cooperativity within the FabB homodimer, which Pocket B may help alleviate.

5. Significance

• Functional Diversification: The findings demonstrate that homologous enzymes can diverge functionally through the availability of "hidden" conformational landscapes (alternative binding modes) rather than just changes in the primary active site.
• Engineering Implications: Understanding these alternative binding modes is critical for tuning the product profiles of assembly-line enzymes. It suggests that mutations which appear to block a primary site may not necessarily abolish activity if alternative, cryptic pockets exist and are accessible.
• Biomolecular Plasticity: The work highlights how protein plasticity allows biological systems to buffer against genetic mutations and environmental changes, offering a new perspective on enzyme evolution and design.
• Future Applications: The discovery of Pocket B opens new avenues for investigating unsaturated fatty acid biosynthesis (e.g., FabB's preference for cis-dec-3-enoyl-ACP) and designing synthetic pathways with tailored chain lengths.