Structural analyses of Trichomonas vaginalis pyrophosphate-dependent phosphofructokinase (TvPPi-PFK)

This study presents the structural analysis of three tetrameric crystal structures of *Trichomonas vaginalis* pyrophosphate-dependent phosphofructokinase (TvPPi-PFK), revealing conserved catalytic motifs, unexpected ATP and sugar phosphate binding sites at the dimer interface, and validating the enzyme's potential as a drug target for trichomoniasis.

The Gist

The Big Picture: A Tiny Thief and a New Lockpick

Imagine a microscopic thief called Trichomonas vaginalis. This tiny parasite causes a very common sexually transmitted infection called trichomoniasis. It's a big problem because it's hard to get rid of, and the current drugs (like antibiotics) are starting to fail, much like a lock that the old keys no longer fit.

Scientists need a new way to stop this thief. To do that, they decided to study the thief's most important tool: a machine inside the parasite that helps it eat and survive. This machine is called TvPPi-PFK.

Think of this machine as the parasite's power plant. Without it, the parasite starves. The goal of this study was to take a high-resolution "3D photograph" of this power plant to see exactly how it works, hoping to design a new drug that jams the gears.

The Mission: Taking a 3D Snapshot

The scientists did three main things:

1. Built a Model: They made millions of copies of this parasite machine in a lab (like cloning a specific Lego set).
2. Frozen it in Time: They grew crystals out of these machines. Think of this as freezing the machines in a block of ice so they can be studied under a super-powerful microscope (X-ray crystallography).
3. Took the Photos: They took three different "snapshots" of the machine in different states.

The Surprise: The Machine Has a Secret Door

Here is where the story gets exciting. Scientists knew this machine usually runs on a specific fuel called PPi (pyrophosphate). They expected to see the machine holding onto PPi.

But they found something weird.

When they soaked the crystals in a solution containing ATP (a different fuel that humans use), the machine didn't just sit there. It actually grabbed the ATP and broke it apart, turning it into AMP (a spent battery).

The Analogy:
Imagine you have a car designed to run only on diesel (PPi). You expect it to ignore gasoline (ATP). But when you pour gasoline into the tank, the engine doesn't just sit there; it actually starts sipping the gasoline and converting it into something else.

This was a huge surprise because:

• Humans use ATP for this job.
• Most parasites use PPi for this job.
• This parasite seems to be able to do both, or at least has a secret pocket that can grab ATP.

The "Dimer Interface": The Secret Meeting Spot

The machine is made of four parts stuck together (a tetramer). The scientists found that the "ATP-busting" activity happens at the seam where two of these parts meet.

Think of the machine as a four-person rowing boat. The scientists found a hidden compartment right in the middle of the boat where the rowers shake hands. In this hidden compartment, the machine can grab ATP and break it down. This spot is usually blocked in other parasites, but in Trichomonas, the door is wide open.

Why This Matters: The New Key to the Lock

Why should you care about a parasite's power plant?

1. It's a Weak Spot: Because this machine is different from the ones in humans, we can potentially build a drug that jams the parasite's machine without hurting the human's machine.
2. New Strategy: Since the machine might be able to use ATP (like humans do), maybe we can trick it. If we can find a drug that blocks this "secret door," we might be able to starve the parasite.
3. Drug Repurposing: The scientists found that this machine looks a lot like machines in other diseases (like sleeping sickness). This means drugs already being tested for those other diseases might actually work on trichomoniasis too.

The Bottom Line

The scientists successfully built a 3D map of the parasite's power plant. They discovered that this machine is more versatile than we thought—it can grab and break down ATP, a fuel it wasn't supposed to touch.

This discovery is like finding a secret backdoor in the thief's house. Now, instead of just trying to break down the front door, scientists can try to lock that backdoor, potentially leading to a new cure for a very common and stubborn infection.

Technical Summary

1. Problem Statement

Trichomonas vaginalis is the causative agent of trichomoniasis, the most common non-viral sexually transmitted disease globally. Current treatments rely on nitroimidazoles (e.g., metronidazole), but resistance has been documented for over 60 years, and treatment failure is common due to reinfection or side effects. There is an urgent need for novel therapeutic targets.

• Target: The study focuses on T. vaginalis pyrophosphate-dependent phosphofructokinase (TvPPi-PFK). This enzyme catalyzes the first committed step of glycolysis using pyrophosphate (PPi) rather than ATP as the phosphoryl donor.
• Rationale: Unlike mammals, which rely solely on ATP-dependent PFK, T. vaginalis utilizes PPi-PFK for the majority of its glycolytic flux. The enzyme is absent in humans, making it an attractive target for structure-based drug discovery. However, high-resolution structural data for TvPPi-PFK was lacking, hindering rational drug design.

2. Methodology

The researchers employed a comprehensive structural biology workflow to characterize TvPPi-PFK:

• Macromolecule Production:
  • The full-length gene (amino acids 1–426) of TvPPi-PFK was cloned into an expression vector (BG1861) with an N-terminal hexa-histidine tag.
  • The protein was expressed in E. coli BL21(DE3) using auto-induction media.
  • Purification involved a two-step protocol: Immobilized Metal Affinity Chromatography (Ni-NTA) followed by Size-Exclusion Chromatography (SEC).
• Crystallization:
  • Three distinct crystal forms were grown using sitting-drop vapor diffusion with the Morpheus screen.
  • Soaking Experiments: Crystals were soaked in solutions containing various ligands to probe binding sites:
    • Structure 1 (9MDT): Soaked with ATP and PPi.
    • Structure 2 (9MF6): Soaked with ADP and $\alpha$-D-Ribose 5-phosphate (R5P).
    • Structure 3 (9MED): Soaked with AMP and $\beta$-D-Glucose 6-phosphate (G6P).
• Data Collection & Structure Determination:
  • X-ray diffraction data were collected at NSLS-II (Beamline 19-ID) at 100 K.
  • Structures were solved via molecular replacement using Phaser (search model: 9DQM) and refined using PHENIX and COOT.
  • Resolutions ranged from 2.16 Å to 2.65 Å.
• Biophysical Validation:
  • Mass Photometry: Used to determine the oligomeric state of the protein in near-physiological conditions (with and without ligands).
  • Comparative Analysis: Structural alignment was performed using ENDScript and DALI servers against known PFK structures (both PPi- and ATP-dependent).

3. Key Results

A. Oligomeric State and Overall Architecture

• Tetrameric Assembly: All three crystal structures revealed a homotetramer in the asymmetric unit, consistent with the canonical PPi-PFK fold. Mass photometry confirmed that recombinant TvPPi-PFK exists primarily as a tetramer in solution under near-physiological conditions, with monomer/dimer populations diminishing in the presence of ATP, PPi, and MgCl₂.
• Topology: Each protomer adopts a two-domain structure (N-terminal and C-terminal) with a central substrate-binding cavity. The secondary structure composition is ~41% $\alpha$-helix and ~15% $\beta$-strand.
• Interfaces: The tetramer is formed by two tightly packed dimers. The larger dimer interface (chains A-D) has a buried surface area of ~2230 Ų, while the smaller interface (chains A-C) is ~1270 Ų.

B. Ligand Binding and Catalytic Motifs

• PPi Binding Site: The structures confirmed the presence of the canonical PPi-binding motifs, including the GGDD motif (residues 114–115) and the "KTID" motif (containing catalytic Lys139). Mg²⁺ ions were observed coordinating with PPi (forming MgPPi²⁻), interacting with Asp114, Asp115, and Asp144.
• Novel ATP/AMP Binding: Unexpectedly, ATP or AMP molecules were found bound at the dimer interface in all three structures.
  • In structure 9MDT, 3 ATP and 1 AMP were bound.
  • In 9MED and 9MF6, 4 AMP molecules were bound.
  • This binding site is distinct from the PPi site and involves residues from two opposing monomers (e.g., Trp386 forming $\pi$-stacking with the adenine ring).
• Sugar Phosphate Binding: Structures 9MF6 and 9MED revealed binding of R5P and G6P, respectively, at the dimer interface near the ATP/AMP site. These sugar phosphates occupy positions similar to the phosphoryl group of ATP in ATP-PFKs.

C. Catalytic Activity Observation

• Dephosphorylation: The crystals exhibited unexpected catalytic activity. Soaking crystals with ATP/PPi resulted in the observation of AMP and inorganic phosphate (Pi), suggesting the enzyme dephosphorylated ATP to AMP. Similarly, soaking with ADP/R5P yielded AMP and PPi.
• Implication: This suggests TvPPi-PFK may possess a latent or alternative ATP-dependent activity, or at least a novel ATP-binding capability not previously reported for PPi-PFKs.

D. Structural Comparisons

• Similarity to PPi-PFKs: TvPPi-PFK shares ~29% sequence identity with Candidatus Prometheoarchaeum syntrophicum PPi-PFK and Borrelia burgdorferi PPi-PFK. While the overall topology is conserved, the ATP-binding region in these other PPi-PFKs is obstructed by protein chains, preventing ATP binding. In TvPPi-PFK, this region is accessible.
• Similarity to ATP-PFKs: Surprisingly, DALI analysis showed topological similarity to Trypanosoma brucei ATP-PFK and Geobacillus stearothermophilus ATP-PFK, despite low sequence identity. This suggests a structural convergence or shared ancestry that allows for potential dual-substrate utilization.

4. Key Contributions

1. First High-Resolution Structures: Provides the first three crystal structures of TvPPi-PFK (PDB: 9MDT, 9MF6, 9MED), validating its PPi-dependent catalytic mechanism.
2. Discovery of a Novel Binding Site: Identifies a previously unreported ATP/AMP binding site at the dimer interface, a feature typical of ATP-PFKs but unexpected in PPi-PFKs.
3. Evidence of Dual Potential: Demonstrates that TvPPi-PFK crystals can catalyze the dephosphorylation of ATP to AMP, challenging the strict classification of the enzyme as solely PPi-dependent and suggesting potential plasticity in its substrate specificity.
4. Oligomeric Validation: Confirms the tetrameric state of the enzyme in solution using mass photometry, resolving discrepancies between SEC (which suggested monomers) and biological reality.

5. Significance

• Drug Target Validation: The structural data confirms TvPPi-PFK as a viable, unique drug target for trichomoniasis, distinct from human enzymes.
• Rational Drug Design: The identification of the novel ATP-binding cavity and the sugar phosphate binding site provides new pockets for structure-based drug design. Known inhibitors of ATP-PFKs (e.g., those targeting T. brucei) may be repurposed or optimized to target TvPPi-PFK.
• Evolutionary Insight: The findings expand the understanding of the PFK superfamily, suggesting that some PPi-PFKs may retain structural features allowing for ATP interaction, potentially playing a role in metabolic flexibility or regulation in anaerobic parasites.
• Future Directions: The study lays the groundwork for mutagenesis studies to confirm the physiological relevance of the ATP-binding site and for screening existing PFK inhibitors against TvPPi-PFK to develop new therapeutics for trichomoniasis.