Structure of human aldehyde oxidase under tris(2-carboxyethyl)phosphine-reducing conditions

This study demonstrates that replacing the inactivating reducing agent dithiothreitol (DTT) with tris(2-carboxyethyl)phosphine (TCEP) enables the crystallization of active human aldehyde oxidase in a new orthorhombic form, thereby overcoming previous limitations and facilitating future time-resolved crystallographic applications.

The Gist

Imagine your body is a bustling city, and inside every cell, there are tiny, specialized factories called enzymes. One of the most important workers in this city is a machine called Human Aldehyde Oxidase (hAOX1). Its job is to process drugs and chemicals, acting like a chemical recycling plant that breaks down or modifies substances so your body can use or eliminate them.

For scientists to understand how this machine works, they need to take a "photograph" of it in extreme detail. To do this, they use a technique called X-ray crystallography, which requires turning the enzyme into a solid, perfectly ordered crystal—like turning a pile of loose Lego bricks into a rigid, frozen sculpture.

The Problem: The "Poisonous" Lubricant

For years, scientists had a major problem. To get the enzyme to form these perfect crystals, they had to add a chemical called DTT (dithiothreitol). Think of DTT as a special lubricant that stops the enzyme's sticky parts from clumping together, allowing them to line up neatly.

However, there was a catch: DTT was actually poisoning the machine.
While DTT helped the enzyme form a crystal, it also broke the engine. It removed a crucial part of the enzyme's core (a sulfur atom), effectively turning the machine off. So, scientists were stuck in a dilemma:

• If they used DTT, they could take a picture, but the picture would be of a broken, inactive machine.
• If they didn't use DTT, the machine wouldn't form a crystal, and they couldn't take a picture at all.

The Solution: A New, Safe Lubricant

The researchers in this paper decided to swap out the "poisonous" lubricant for a new one called TCEP.

• The Analogy: Imagine you are trying to photograph a delicate, sensitive camera. The old method required you to spray it with a cleaning fluid that worked great for focusing the lens but permanently fogged up the sensor. The new method uses a different, sulfur-free cleaning fluid that keeps the lens clear without fogging the sensor.

TCEP is a "sulfur-free" reducing agent. It does the same job as DTT (keeping the enzyme stable and preventing clumps) but doesn't attack the enzyme's core.

What Happened When They Tried It?

1. Better Crystals: When they used TCEP, the enzyme formed beautiful, flat, plate-like crystals (instead of the messy, star-shaped ones they got with DTT). These new crystals were sharper and allowed for a much higher-resolution photo (2.3 Ångströms).
2. The Machine Was Still Running: When they tested the enzyme after using TCEP, they found that the machine was still working. Unlike the DTT version, which was dead, the TCEP version could still process chemicals.
3. Reversible vs. Irreversible:
   • DTT was like a permanent lock; once it broke the enzyme, washing it off didn't fix it.
   • TCEP was like a temporary pause button. It slowed the enzyme down a bit while it was there, but once they washed the TCEP away, the enzyme "woke up" and started working at full speed again.

Why This Matters

This discovery is a game-changer for drug development.

• Before: Scientists were studying a "broken" version of the enzyme, which made it hard to predict how real drugs would behave in the human body.
• Now: They can study the enzyme in its natural, active state.

Think of it like this: Previously, scientists were trying to fix a car by looking at a photo of the engine with the spark plugs removed. Now, they can take a photo of the engine with the spark plugs still in, allowing them to see exactly how the car runs.

The Bottom Line

By swapping a toxic chemical (DTT) for a safe one (TCEP), the researchers unlocked a new way to see human aldehyde oxidase in action. This opens the door for:

• Designing better drugs that work more effectively.
• Understanding how the body processes medicines.
• Performing "time-lapse" experiments to watch the enzyme work in real-time.

In short, they found a way to freeze the machine without breaking it, finally letting us see how it really works.

Technical Summary

1. Problem Statement

Human aldehyde oxidase (hAOX1) is a critical enzyme in drug metabolism (Phase I), yet its structural and functional characterization has been hindered by crystallization challenges.

• The DTT Paradox: Previous successful crystallization of hAOX1 relied on dithiothreitol (DTT) to prevent unspecific intermolecular interactions (specifically disulfide bond formation) and facilitate crystal growth.
• Enzyme Inactivation: Recent studies revealed that DTT irreversibly inactivates hAOX1 by removing the essential sulfido ligand ($Mo=S$) from the molybdenum cofactor (Moco).
• Consequence: Using DTT for crystallization compromises the enzyme's active state, making it unsuitable for studying catalytic mechanisms, trapping intermediates, or performing time-resolved crystallography.

2. Methodology

The authors developed a new crystallization strategy to bypass DTT-induced inactivation.

• Protein Engineering:
  • A variant of hAOX1, named hAOX1_6A, was created via site-directed mutagenesis.
  • Six surface-exposed cysteines (C161, C165, C170, C171, C179, C180) located in a flexible 71-residue loop were mutated to alanine to reduce the need for strong reducing agents.
• Crystallization Conditions:
  • Reducing Agent: Replaced DTT with Tris(2-carboxyethyl)phosphine (TCEP), a sulfur-free, oxidation-stable reducing agent.
  • Conditions: Hanging-drop vapor diffusion at 4°C and 20°C.
  • Optimization: Tested protein concentrations (5 and 10 mg/mL) and pH levels (4.5–6.0). The optimal condition for hAOX1_6A was found at pH 4.7 with 12% PEG 3350 and 100 mM sodium malonate.
• Data Collection & Structure Solution:
  • X-ray diffraction data were collected at the ESRF (Beamline ID30A-3) at 100 K.
  • Structure solved by Molecular Replacement (MR) using the wild-type (wt) hAOX1 structure (PDB: 4UHW) as a search model.
  • Refinement performed using Coot, REFMAC, and PDB_REDO.
• Activity Assays:
  • Kinetic assays compared the effects of DTT vs. TCEP on wt hAOX1 activity.
  • Enzymes were incubated with reducing agents for 30 min, 4 hours, and overnight (O/N).
  • Post-O/N incubation, samples underwent buffer exchange (desalting) to remove reducing agents and assess reversibility.
  • Substrate: Phenanthridine; Product monitored: Phenanthridinone at 321 nm.

3. Key Contributions & Results

A. Structural Findings

• New Crystal Form: Unlike the star-shaped crystals obtained with DTT (tetragonal space group $P4_22_12$), TCEP-grown crystals (both wt and 6A variant) formed plate-shaped crystals in the orthorhombic space group $P2_12_12_1$.
• Resolution: The hAOX1_6A variant diffracted to 2.3 Å, an improvement over the previous 2.8 Å limit for wt crystals grown with DTT.
• Asymmetric Unit: Contains two molecules forming the biological dimer via non-crystallographic symmetry (NCS), whereas the DTT-grown structure had one molecule per asymmetric unit.
• Conformational Changes:
  • The overall fold is highly similar to the DTT-grown structure (RMSD ~0.535 Å for the dimer).
  • However, the dimer exhibits a "relaxed" conformation with increased distance between protomers in the TCEP-grown form.
  • Gate 1 Ordering: A previously disordered region near the substrate pocket (Gate 1: residues C654–K661) is now ordered and visible in the electron density maps, providing new insights into substrate access.
• Active Site Integrity:
  • The Moco center retains the sulfido ligand ($Mo=S$), confirming the enzyme is not chemically altered by TCEP.
  • High B-factors in the Moco region suggest some flexibility or partial occupancy but no removal of the sulfur atom.
  • No TCEP molecules were found bound in the active site or near the cofactors.

B. Functional Findings (Activity Assays)

• DTT Effect: Confirmed irreversible inactivation. After O/N incubation, activity dropped to ~10% and did not recover after buffer exchange.
• TCEP Effect:
  • TCEP caused a reversible decrease in activity (approx. 40% reduction after 30 min/4 hours).
  • Crucially, full enzymatic activity was restored after overnight incubation followed by buffer exchange to remove TCEP.
  • This demonstrates that TCEP acts as a reversible inhibitor rather than an inactivator.

4. Significance

• Methodological Breakthrough: This study establishes TCEP as a viable, superior alternative to DTT for crystallizing hAOX1. It resolves the conflict between obtaining high-quality crystals and maintaining enzyme activity.
• Structural Insights: The new crystal form provides higher resolution data and reveals ordered regions (Gate 1) previously invisible, which are critical for understanding substrate specificity and inhibitor binding.
• Future Applications: By preserving the active state of the enzyme, this strategy enables:
  • Time-resolved crystallography to capture catalytic intermediates.
  • Accurate structure-based drug design (SBDD) for hAOX1 inhibitors.
  • Mechanistic studies of the enzyme's promiscuous substrate binding without the artifact of DTT-induced inactivation.

Conclusion

The replacement of DTT with TCEP in the crystallization of human aldehyde oxidase allows for the determination of high-resolution structures of the active enzyme. This approach circumvents irreversible inactivation, offering a robust platform for future mechanistic investigations and pharmaceutical applications involving hAOX1. The atomic coordinates for the new structure are deposited in the PDB under accession code 28MB.