Summary Overview

Recently, the research team led by Prof. Huilai Yu at East China University of Science and Technology, together with Prof. Yilei Zhao’s team at Shanghai Jiao Tong University, published their latest findings in the international chemistry journal Angewandte Chemie International Edition. The paper, titled “Flipping of a Non-productive Substrate Binding Conformation Facilitates Hydroxynitrile Lyase Catalyzed Hydrocyanation,” reveals a conformational flipping mechanism of hydroxynitrile lyase (PcHNL5) during the catalytic hydrocyanation reaction.

Through crystal structure analysis and molecular dynamics simulations, the team found that substrate binding in the PcHNL5_L331A mutant occurs in a nonproductive conformation, requiring a “flip” to enter the catalytic pathway. To resolve this, the researchers rationally introduced S333V and P340L mutations to weaken the incorrect hydrogen bonding and enhance active-site flexibility, thereby facilitating substrate flipping. The resulting triple mutant PcHNL5_L331A/S333V/P340L exhibited an ~8-fold increase in catalytic efficiency and >99% enantioselectivity, enabling gram-scale synthesis. This work introduces a new concept—destabilizing nonproductive binding states to enhance enzyme activity, providing theoretical insights for dynamics-driven enzyme design.

Background

Efficient enzyme catalysis is often attributed to precise substrate recognition and transition state stabilization. However, emerging studies have shown that dynamic interconversion of substrate conformations within the enzyme is also critical for catalytic efficiency. Hydroxynitrile lyases (HNLs) catalyze the reversible addition of hydrogen cyanide to carbonyl compounds (aldehydes or ketones) to form chiral cyanohydrins, and are widely used in pharmaceutical biocatalysis—particularly for producing precursors of β₂ adrenergic receptor agonists such as salmeterol and vilanterol. Nevertheless, natural HNLs usually exhibit low activity toward structurally complex or bulky substrates.

Overcoming this “substrate selectivity bottleneck” through structural engineering is a major challenge in biocatalyst development.

As early as 2020, the research team addressed this challenge by analyzing the crystal structure of the sweet almond (Prunus communis)–derived FAD-dependent PcHNL5 in complex with its natural substrate benzaldehyde. Rational mutations targeting the substrate tunnel revealed that the L331A mutation widens the tunnel, allowing turnover of bulky non-native substrates such as 1,3-dioxane benzaldehyde (1d). However, the catalytic efficiency of mutant PcHNL5_L331A was only 1.8 s⁻¹·mM⁻¹, far lower than the wild type PcHNL5 toward benzaldehyde (76 s⁻¹·mM⁻¹).

Structural Findings: The Substrate Is “Sitting Backwards”

To further enhance activity toward non-native substrates, the team solved the crystal structure of PcHNL5_L331A in complex with substrate 1d (PDB 8JM0, 1.8 Å). Surprisingly, the substrate binds in a reversed orientation: the aldehyde group no longer points toward the canonical catalytic triad (Tyr458–His460–His498) but instead forms hydrogen bonds with Ser333 and His358 (O···O 3.0 Å, O···N 3.1 Å). Meanwhile, the 1,3-dioxane group is surrounded by hydrophobic residues such as Phe72, Ala73, Phe336, and Pro340, effectively locking it in the incorrect orientation. This finding reveals for the first time that although L331A widens the tunnel, the substrate still binds nonproductively, limiting the reaction.

Molecular Dynamics Reveal a Three-State Conformational Flipping Mechanism

To determine whether the substrate can “flip” into the productive conformation, the researchers performed accelerated molecular dynamics simulations (500 ns) starting from the crystal structure to track the behavior of substrate 1d within the PcHNL5_L331A active site.

The simulations revealed three key conformational states:

The trajectory shows that the substrate must overcome a significant energy barrier and rotate approximately 170° to transition from the nonproductive to the productive state. This flipping step represents the rate-limiting process in the mutant.

Rational Mutation Design: Weakening Incorrect Binding

Based on structural and simulation data, the team proposed destabilizing the nonproductive conformation to boost activity. Analyses showed that Ser333 and His358 stabilize the incorrect substrate binding via hydrogen bonding, while hydrophobic residues such as Phe72, Ala73, Val317, Val329, Phe336, and Pro340 interact with the aromatic ring.

Thus, they designed a two-step mutation strategy:

Remove hydrogen bonds: Saturation mutagenesis of Ser333 and His358

→ Replacing serine’s hydrophilic hydroxyl group with a hydrophobic isopropyl group significantly improved activity.

Weaken hydrophobic interactions around the phenyl ring

→ The triple mutant PcHNL5_L331A/S333V/P340L further improved activity toward substrate 1d.

Experimental Validation: Structural and Functional Evidence

Crystal structures showed that in the triple-mutant–1d complex, both nonproductive and productive substrate conformations can be captured. The hydrogen bond between Ser333 and the aldehyde group disappears in the nonproductive-like state, and in the productive-like state, the aldehyde forms hydrogen bonds with Tyr458 and His498. Additionally, the triple mutant bound with substrate 2d adopts a conformation similar to that of the natural enzyme with 2a, indicating consistent catalytic behavior.

Kinetic analysis demonstrated that the triple mutant improves kcat/Km by 8.1-fold compared to the single mutant, with ee > 99% and TON > 3.6 × 10⁴ (~900× that of the wild type). The mutant enabled gram-scale synthesis of (R)-cyanohydrin, which could be reduced by LiAlH₄ to obtain high-purity (R)-β-amino alcohol.

Molecular Mechanism of Substrate Flipping

Molecular dynamics simulations showed that Phe72 undergoes pronounced conformational changes synchronized with substrate flipping from nonproductive to productive states. In the triple mutant, the Phe72 side chain can rotate downward to make room for the substrate; P340L disrupts stabilizing interactions between Leu76 and Leu340, further increasing the mobility of Phe72. Principal component analysis also confirmed Phe72’s critical role in substrate flipping.

Umbrella sampling simulations quantified these effects:

• In the L331A single mutant, flipping requires crossing a 5.3 kcal/mol energy barrier, and the productive conformation is still 5.2 kcal/mol higher in energy than the nonproductive one, meaning the enzyme favors the incorrect state.
• In the triple mutant, the flipping barrier drops to 3.6 kcal/mol, and the productive state becomes more stable, reducing the energy gap to 2.2 kcal/mol.

Thus, by weakening incorrect hydrogen bonds and increasing local flexibility, the triple mutant accelerates substrate flipping, resulting in significantly enhanced catalytic efficiency.

Research Insights and Significance

The study uncovers a substrate conformational flipping mechanism in PcHNL5 during hydrocyanation and, through structural analysis, molecular simulations, and rational design, successfully engineered a triple mutant (L331A/S333V/P340L) that weakens incorrect binding and enhances conformational flexibility—achieving an eightfold efficiency improvement and >99% enantioselectivity.

This work highlights several key concepts for biocatalyst engineering:

Shifting from static structure to dynamic mechanisms

Traditional enzyme engineering focuses on static substrate binding. This study shows that substrate dynamics and active-site flexibility are equally crucial and should be incorporated in future design strategies.

Destabilization can enhance catalysis

Instead of strengthening interactions, the researchers improved catalysis by weakening nonproductive binding. Controlled destabilization may thus serve as a universal principle for improving turnover and substrate compatibility.

Dynamics-driven enzyme design

The work demonstrates that optimizing the conformational energy landscape directly impacts catalytic efficiency. Future design should target dynamic—not just static—optimization along the reaction pathway.

Implications for drug-intermediate synthesis

The engineered PcHNL5 can efficiently and stereoselectively catalyze bulky aromatic substrates, enabling greener biocatalytic routes for synthesizing β₂-agonist precursors. The dynamic-control strategy can be applied to other difficult transformations.

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