Recombinant Pongo abelii Dihydroorotate dehydrogenase (quinone), mitochondrial (DHODH)

What is Dihydroorotate dehydrogenase (DHODH) and what is its biochemical role?

Dihydroorotate dehydrogenase is a flavoenzyme that catalyzes the oxidation of dihydroorotate to orotate in the de novo pyrimidine-biosynthesis pathway . This represents the fourth step and only redox reaction in the biosynthesis of pyrimidine nucleotides . The enzyme plays a critical role in cellular metabolism by facilitating the production of essential precursors for DNA and RNA synthesis. Without this enzymatic reaction, cells cannot effectively synthesize the nucleotides required for growth and division, making DHODH a potential therapeutic target for various diseases .

How does DHODH inhibition affect cellular metabolism?

When DHODH is inhibited, it rapidly leads to the depletion of intracellular pyrimidine pools, forcing cells to rely on extracellular salvage pathways . In the absence of sufficient salvage, this intracellular nucleotide starvation results in inhibition of DNA and RNA synthesis, cell cycle arrest, and ultimately, cell death . This mechanism explains why rapidly dividing cells such as T lymphoblasts appear to be exquisitely sensitive to nucleotide starvation following DHODH inhibition . The metabolic dependency on DHODH varies between cell types, with certain cancer cells demonstrating particular vulnerability to its inhibition .

How are DHODHs classified and how does Pongo abelii DHODH fit into this classification?

DHODHs are classified based on sequence, structural, and kinetic features. They are broadly divided into two classes: Class 1 and Class 2. Class 1 is further subdivided into Class 1A and 1B based on their quaternary structure and electron acceptors. For example, analysis of Leishmania braziliensis DHODH classifies it as a member of the class 1A DHODHs .

While the search results don't explicitly classify Pongo abelii DHODH, its mitochondrial localization (as indicated by "mitochondrial" in its full name) suggests it belongs to Class 2 DHODHs, which are typically membrane-bound and use ubiquinone as an electron acceptor. This is consistent with other mammalian DHODHs, which are generally Class 2 enzymes.

How does substrate binding occur in DHODH enzymes?

Studies on Lactococcus lactis DHODA provide insights into substrate binding mechanisms likely conserved across DHODHs. When orotate (the reaction product) binds, it displaces water molecules from the active site and stacks above the DHODA flavin isoalloxazine ring, causing relatively minor movements of surrounding protein residues .

The orotate is completely buried beneath the protein surface, and its binding significantly reduces the mobility of the active site loop . Specifically, the substrate is bound by four conserved asparagine side chains (Asn 67, Asn 127, Asn 132, and Asn 193), the side chains of Lys 43 and Ser 194, and the main chain NH groups of Met 69, Gly 70, and Leu 71 . Notably, the Lys 43 side chain forms hydrogen bonds with both the flavin isoalloxazine ring and the carboxylate group of orotate, playing a crucial role in substrate recognition and catalysis .

How does Pongo abelii DHODH compare structurally to DHODH from other species?

While the search results don't provide direct structural comparisons between Pongo abelii DHODH and other species, comparative analysis of DHODHs from different organisms reveals both conservation and divergence. For example, Leishmania braziliensis DHODH shows high structural conservation with orthologous trypanosomatid enzymes despite sequence differences .

What are the optimal conditions for expressing and purifying recombinant Pongo abelii DHODH?

Recombinant Pongo abelii DHODH can be expressed using E. coli expression systems . For optimal production:

• Expression construct: The full-length protein (amino acids 1-395) is typically expressed with an N-terminal 10xHis-tag to facilitate purification .

• Buffer conditions: After purification, the protein is typically stored in a Tris-based buffer with 50% glycerol, optimized for this specific protein .

• Lyophilization parameters: If provided as a lyophilized powder, it is typically prepared from Tris/PBS-based buffer with 6% Trehalose at pH 8.0 .

This reproducible protocol ensures the production of active recombinant enzyme suitable for various research applications .

What are the recommended storage conditions for maintaining Pongo abelii DHODH stability?

For optimal stability of recombinant Pongo abelii DHODH, the following storage conditions are recommended:

• Temperature: Store at -20°C, and for extended storage, conserve at -20°C or -80°C .

• Working conditions: For short-term use, store working aliquots at 4°C for up to one week .

• Freeze-thaw cycles: Repeated freezing and thawing is not recommended as it can compromise protein stability and activity .

• Shelf life: The shelf life varies depending on the formulation. Generally, liquid forms maintain stability for approximately 6 months at -20°C/-80°C, while lyophilized forms can remain stable for up to 12 months at the same temperatures .

These storage recommendations help ensure that the recombinant protein maintains its structural integrity and enzymatic activity for experimental use.

How can researchers assess the enzymatic activity of recombinant DHODH in vitro?

A fluorescence-based enzymatic activity assay is commonly used to assess DHODH activity. The protocol includes:

• Reaction components: Prepare aqueous buffer at pH 7.0 containing 100 mM HEPES, 150 mM NaCl, 0.3% CHAPS, 0.5 mg/mL BSA, 0.1 μM FMN, and 1% DMSO .

• Enzyme and substrate preparation: Add 5 nM DHODH, 25 μM dihydroorotate (DHO), and 60 μM resazurin to the reaction buffer .

• Two-step assay procedure:

  • First, incubate the mixture for 1 hour at 25°C to allow DHODH-catalyzed oxidation of DHO to orotate (ORO) and the concurrent conversion of resazurin to resorufin .

  • Second, add 5 mM of stop mixture (100 mM HEPES and 10 mM ORO) and measure the fluorescent signal from resorufin .

This assay can be used to determine enzymatic parameters and to evaluate the effects of potential inhibitors on DHODH activity .

How is DHODH being investigated as a therapeutic target for disease treatment?

DHODH has emerged as a promising therapeutic target for several diseases:

These diverse applications highlight the broad therapeutic potential of targeting DHODH in multiple disease contexts.

What combination therapies involving DHODH inhibition have shown promise in preclinical studies?

Combination therapies targeting DHODH have demonstrated significant promise in preclinical models:

• Neuroblastoma: A combination of the DHODH inhibitor brequinar and the standard chemotherapeutic agent temozolomide was curative in the majority of transgenic TH-MYCN neuroblastoma mice . This synergistic effect suggests that combining DHODH inhibition with conventional chemotherapy could be a highly effective approach for treating aggressive neuroblastoma .

• Cancer treatment mechanisms: DHODH inhibition was shown to reduce the expression of MYC targets in three neuroblastoma models in vivo . This suggests that DHODH inhibitors may exert their anti-cancer effects not only through nucleotide depletion but also by modulating oncogenic signaling pathways.

These findings provide a strong rationale for clinical testing of DHODH inhibitors in combination with established therapies for various cancers.

How does DHODH inhibition affect different cell types and what are the implications for therapy?

Different cell types show varying sensitivities to DHODH inhibition:

• T lymphoblasts: These cells appear to be "exquisitely sensitive" to nucleotide starvation after DHODH inhibition . This selective vulnerability makes DHODH inhibition particularly promising for T-cell malignancies.

• Neuroblastoma cells: High DHODH expression correlates with aggressive disease and poor survival in neuroblastoma . Targeting DHODH in these cells not only inhibits tumor growth but also affects MYC-dependent signaling pathways .

• Cell-specific mechanisms: While some cells are highly dependent on de novo pyrimidine synthesis catalyzed by DHODH, others may rely more on salvage pathways for nucleotide acquisition . Understanding these cell-type specific metabolic dependencies is crucial for predicting which cancers will respond best to DHODH inhibition.

These differential effects on various cell types suggest that patient selection and biomarker development will be important considerations for the clinical application of DHODH inhibitors.

What are the current challenges in developing selective DHODH inhibitors?

Development of selective and effective DHODH inhibitors faces several challenges:

• Clinical translation: Despite the development of high-potency inhibitors that have proven safe in humans, their efficacy as single agents in myeloid malignancies remains unproven . This highlights the gap between preclinical promise and clinical success.

• Mechanism understanding: The mechanisms by which DHODH inhibition induces differentiation of malignant cells remain largely unknown . Elucidating these mechanisms would facilitate more rational drug design and development.

• Novel chemistry approaches: In the case of acute myeloid leukemia, no previously discovered DHODH inhibitors have succeeded in clinical applications, indicating a need for new chemical scaffolds as alternatives to current compounds .

• Structure-based design: While structure-based drug discovery approaches have identified novel inhibitors of human DHODH, optimizing these compounds for specific disease contexts remains challenging .

Addressing these challenges requires integrated approaches combining structural biology, medicinal chemistry, and disease-specific understanding of DHODH's role.

How can evolutionary conservation of DHODH inform drug development strategies?

Evolutionary conservation analysis of DHODH provides valuable insights for drug development:

• Species-specific targeting: Understanding structural differences between parasite/pathogen DHODHs and host enzymes can guide the development of selective inhibitors. For example, the structural and functional differences between Leishmania braziliensis DHODH and human DHODH could be exploited for antileishmanicidal drug development .

• Conserved binding sites: Crystal structures of DHODHs from different species reveal conserved elements in the active site, such as the four asparagine residues involved in substrate binding in Lactococcus lactis DHODH . These conserved features can serve as starting points for designing broad-spectrum inhibitors.

• Primate comparisons: The availability of recombinant Pongo abelii DHODH provides opportunities for comparative studies with human DHODH . Such comparisons could reveal subtle structural differences that might be exploited for developing more selective human DHODH inhibitors.

This evolutionary perspective enhances our ability to design drugs with optimal selectivity profiles for different therapeutic applications.

What novel methodological approaches are emerging for studying DHODH function and inhibition?

Recent advances in studying DHODH function and inhibition include:

• Structure-based drug discovery: Researchers have implemented systematic approaches for identifying DHODH inhibitors, including prefiltering steps to omit PAINS (Pan-Assay Interference Compounds) and Lipinski violators at early stages to enrich compounds with favorable oral druggability .

• Multi-omics integration: A pancancer, multiomic approach has been applied to elucidate metabolic dependencies in cancer, identifying critical roles of DHODH both as a prognostic marker and as a mediator of tumor cell survival in neuroblastoma .

• In vivo models: Transgenic and xenograft mouse models have been developed to evaluate DHODH inhibitors, allowing assessment of not only tumor growth inhibition but also effects on signaling pathways such as MYC targets .

• Combination screening: Systematic testing of DHODH inhibitors in combination with standard chemotherapeutics has revealed synergistic interactions, such as the combination of brequinar and temozolomide in neuroblastoma models .

These methodological innovations are accelerating the development and evaluation of DHODH-targeting therapeutic strategies across multiple disease areas.