Recombinant Kluyveromyces lactis Dihydroorotate dehydrogenase (quinone), mitochondrial (URA9)

What is the biochemical function and structure of K. lactis URA9?

K. lactis URA9 (dihydroorotate dehydrogenase) is a mitochondrial enzyme that catalyzes the fourth step in de novo pyrimidine biosynthesis, specifically the oxidation of dihydroorotate to orotate. This FMN-dependent enzyme plays a rate-limiting role in the pathway essential for nucleic acid synthesis.

Structurally, the mature K. lactis URA9 protein consists of amino acids 17-445, typically containing a large C-terminal α/β barrel domain and a smaller N-terminal helical domain . When expressed recombinantly, it is often produced with a His-tag to facilitate purification. The protein requires FMN as a cofactor for catalytic activity, which is consistent with its classification as a flavoprotein .

The enzyme's catalytic mechanism involves electron transfer from dihydroorotate through FMN to its final electron acceptor, which varies depending on the DHODH class. For K. lactis URA9, as a mitochondrial enzyme, the electron acceptor is likely ubiquinone (coenzyme Q) .

How does URA9 differ across yeast species and from human DHODH?

K. lactis URA9 differs from S. cerevisiae DHODH (URA1) primarily in subcellular localization and electron acceptor preferences. While S. cerevisiae DHODH is a cytosolic Class 1A enzyme using fumarate as an electron acceptor, K. lactis URA9 is a mitochondrial enzyme that likely functions more similarly to human DHODH (Class 2) .

This distinction is important for researchers, as K. lactis provides a better model for studying aspects of human DHODH function due to its respiratory metabolism and mitochondrial localization pattern. Both K. lactis and humans predominantly rely on respiratory metabolism, whereas S. cerevisiae is primarily fermentative .

The table below summarizes key differences between DHODH classes:

These differences affect experimental design considerations when using K. lactis URA9 as a model for human DHODH in drug development or fundamental research .

What are the expression patterns and regulation mechanisms of URA9?

Unlike S. cerevisiae, which has duplicate genes differentially expressed under aerobic and hypoxic conditions, K. lactis typically has single copies of genes that are regulated by oxygen availability . The expression of URA9 is affected by oxygen levels, with hypoxic conditions altering expression patterns.

K. lactis, while unable to grow in strictly anoxic conditions (likely due to limitations in sterol import mechanisms), can grow in hypoxic environments (defined as oxygen availability below 1% of fully aerobic levels) through fermentation . Under these conditions, URA9 expression is regulated as part of the cellular adaptation to reduced oxygen.

How can I express and purify recombinant K. lactis URA9?

To express recombinant K. lactis URA9, E. coli expression systems have proven effective, as evidenced by commercially available proteins . The methodology involves:

• Vector Design: Create a construct containing the URA9 gene (coding for amino acids 17-445 of the mature protein) with an N-terminal His-tag for purification .

• Expression Conditions: Transform into an E. coli expression strain (typically BL21 or derivatives). Induce expression using IPTG or similar inducers, with optimal conditions typically involving lower temperatures (16-25°C) to enhance proper folding of the flavoprotein.

• Purification Strategy:

  • Lyse cells in buffer containing mild detergents to solubilize the protein

  • Perform immobilized metal affinity chromatography (IMAC) using the His-tag

  • Consider a second purification step such as ion-exchange or size-exclusion chromatography

  • Include FMN in purification buffers to maintain protein stability

• Quality Control: Verify the purified protein by SDS-PAGE under both reducing and non-reducing conditions. The expected molecular weight for K. lactis URA9 with His-tag is approximately 48-50 kDa .

For researchers requiring specialized modifications or higher purity, commercial recombinant proteins are available (Cat# RFL21208KF), specifying "Full Length of Mature Protein (17-445)" with His-tag .

What methods are effective for measuring URA9 enzymatic activity?

URA9 activity can be measured through several established techniques:

• Spectrophotometric Assays:

  • Monitor the reduction of artificial electron acceptors like 2,6-dichloroindophenol (DCIP) during oxidation of dihydroorotate

  • Follow absorbance changes at specific wavelengths (typically 300nm for dihydroorotate disappearance or 610nm for DCIP reduction)

  • Include appropriate controls for non-enzymatic reactions

• Coupled Enzyme Assays:

  • Connect DHODH activity to the reduction of a colorimetric indicator through secondary enzymes

  • Calculate activity rates from the linear portion of the reaction progress curve

• Direct Product Quantification:

  • Analyze orotate production using HPLC or LC-MS methods

  • Incorporate isotope-labeled substrates for more sensitive detection

The standard assay conditions typically include:

• Buffer: 50 mM Tris-HCl or phosphate buffer, pH 7.5-8.0

• Substrate: 0.1-0.5 mM dihydroorotate

• Electron acceptor: 0.1 mM DCIP or appropriate concentration of natural acceptor

• Temperature: 30°C for yeast enzymes, 37°C for human comparisons

• Cofactor: Ensure sufficient FMN is present (typically pre-bound to the enzyme)

Activity is typically reported as μmol substrate converted per minute per mg protein under standard conditions .

How can yeast complementation assays be used to study URA9 function?

Yeast complementation assays provide a powerful method for studying URA9 function in a cellular context:

• Strain Construction:

  • Use a S. cerevisiae ura1Δ strain (deficient in endogenous DHODH activity)

  • The strain should be derived from a suitable background (e.g., BY4741 or S288C)

• Vector Design:

  • Construct a shuttle plasmid containing the K. lactis URA9 gene

  • Include selection markers (e.g., LEU2) and appropriate promoters (e.g., copper-inducible CUP1 promoter)

  • Add a 6xHis tag if protein purification is required later

• Transformation and Selection:

  • Transform the construct into the ura1Δ strain

  • Select transformants on synthetic medium lacking leucine (SD-leu)

  • Induce expression with copper sulfate (typically 50 μM CuSO₄)

• Functional Analysis:

  • Test complementation by growing cells on medium lacking uracil (SD-leu-ura)

  • Perform serial dilution assays starting with 1×10⁵ cells/ml

  • Incubate plates for three days at 30°C and evaluate growth

• Variations for Mechanistic Studies:

  • Create site-directed mutants to study specific residues

  • Test growth under various conditions (aerobic, hypoxic)

  • Add specific inhibitors to test drug susceptibility

This approach has been successfully used to characterize DHODHs from various fungal species, confirming their functional identity and providing insights into their biochemical properties .

What are the optimal conditions for storing and handling recombinant URA9?

To maintain the stability and activity of recombinant K. lactis URA9:

• Short-term Storage:

  • Store at 4°C in buffer containing 20-50 mM phosphate or Tris, pH 7.5-8.0

  • Include 100-150 mM NaCl to maintain ionic strength

  • Add stabilizers such as 10% glycerol

  • Consider including reducing agents (1-5 mM DTT or β-mercaptoethanol)

• Long-term Storage:

  • Aliquot to avoid freeze-thaw cycles

  • Store at -80°C in buffer containing 20-50% glycerol

  • Flash-freeze in liquid nitrogen for optimal preservation

• Working Conditions:

  • Maintain protein at 4°C when handling

  • Avoid multiple freeze-thaw cycles

  • Consider adding FMN (1-10 μM) to maintain cofactor saturation

  • Protect from light when possible as FMN is light-sensitive

• Stability Considerations:

  • The protein typically remains stable for 1-2 weeks at 4°C

  • Freezer storage (-80°C) can maintain activity for months to years

  • Monitor activity periodically using standard assays to confirm functionality

Following these guidelines will help ensure that your recombinant URA9 remains active and stable throughout your experimental procedures .

Why is K. lactis preferred over S. cerevisiae for certain DHODH studies?

K. lactis offers several advantages over S. cerevisiae for DHODH studies, particularly when modeling human systems:

• Metabolic Similarities to Human Cells:

  • K. lactis has a predominantly respiratory metabolism similar to human tissues, whereas S. cerevisiae is primarily fermentative

  • This makes K. lactis a better model for studying oxidative metabolism-related processes

• Redox Metabolism Differences:

  • K. lactis has higher glucose flow through the pentose phosphate pathway than through glycolysis

  • This results in higher cytosolic NADPH production compared to S. cerevisiae

  • The reoxidation mechanisms involve mitochondrial external alternative dehydrogenases

• Oxygen Response:

  • Unlike S. cerevisiae, K. lactis does not have duplicate genes specialized for aerobic/hypoxic conditions

  • Instead, the single copies of respiratory genes are regulated by oxygen availability

  • This simplifies interpretation of oxygen response experiments

• Evolutionary Considerations:

  • K. lactis diverged from S. cerevisiae before the whole genome duplication (WGD) event

  • This provides insights into evolutionary adaptations of DHODH function

These characteristics make K. lactis particularly valuable for studying mitochondrial proteins like URA9 in a context more relevant to human biology, especially for research on respiratory metabolism and hypoxia response mechanisms .

How does oxygen availability affect URA9 expression and function in K. lactis?

Oxygen availability significantly impacts URA9 expression and function in K. lactis through several mechanisms:

• Transcriptional Regulation:

  • Unlike S. cerevisiae, which has duplicate genes differentially expressed under aerobic and hypoxic conditions, K. lactis has single gene copies regulated by oxygen availability

  • The unique copy of URA9 is regulated in response to changing oxygen levels

• Metabolic Adaptations:

  • K. lactis can grow in hypoxic conditions (below 1% of fully aerobic oxygen levels) but not in strictly anoxic environments

  • Under hypoxic conditions, K. lactis shifts to more fermentative metabolism, which affects the cellular redox balance and consequently URA9 function

• Electron Transport Chain Interactions:

  • As a mitochondrial protein, URA9 interacts with the electron transport chain

  • Under hypoxic conditions, alterations in the electron transport chain activity affect URA9 function

  • The enzyme's ability to transfer electrons to ubiquinone may be compromised in low-oxygen environments

• Redox Balancing:

  • Hypoxia affects the NAD(P)H-redox reactions in K. lactis

  • The mitochondrial external alternative dehydrogenases (NDEs) used for NADPH reoxidation are affected by oxygen availability

  • This indirectly impacts URA9 function through changes in cellular redox state

These oxygen-dependent effects make K. lactis URA9 an interesting model for studying how mitochondrial enzymes adapt to changing oxygen environments, with potential implications for understanding human cellular responses to hypoxia .

How can I establish a K. lactis expression system for URA9 research?

Establishing a K. lactis expression system for URA9 research involves several key steps:

• Strain Selection:

  • Choose appropriate K. lactis strains (e.g., GG799, ATCC 8585, or CBS 2359)

  • Consider using URA9-deficient strains for complementation studies

  • Verify the genetic background is suitable for your specific research questions

• Vector Construction:

  • Utilize specialized K. lactis vectors containing:

    • K. lactis origin of replication

    • Selection markers (often the K. lactis LAC4 promoter with β-galactosidase selection)

    • Appropriate promoters (e.g., LAC4 promoter for induction with galactose)

  • Include tags for detection and purification if needed

• Transformation Protocol:

  • Use lithium acetate or electroporation methods adapted for K. lactis

  • Select transformants on appropriate media

  • Verify integration or maintenance of your construct

• Expression Optimization:

  • Determine optimal media composition (often YPD or minimal media with specific carbon sources)

  • Establish ideal growth conditions (temperature typically 28-30°C)

  • Test various induction methods if using inducible promoters

• Phenotypic Analysis:

  • Compare growth under different conditions (aerobic vs. hypoxic)

  • Measure URA9 activity in cell extracts

  • Assess complementation of URA9 deficiency

• Controls and Validation:

  • Include appropriate control strains (wild-type and URA9-deficient)

  • Validate URA9 expression by Western blot or activity assays

  • Confirm the expected phenotypes under various conditions

This system allows for studying URA9 in its native context, providing insights that may not be apparent in heterologous expression systems like E. coli or S. cerevisiae.

How can URA9 be used in drug discovery and development research?

K. lactis URA9 offers valuable applications in drug discovery and development:

• Antifungal Drug Development:

  • As DHODHs are essential enzymes in many fungi, inhibitors can be screened against K. lactis URA9

  • The yeast complementation system allows testing of compound specificity in a cellular context

  • Differences between fungal and human DHODHs can be exploited for selective targeting

• Model for Human DHODH Inhibitors:

  • K. lactis URA9's mitochondrial localization makes it a better model for human DHODH than S. cerevisiae URA1

  • Researchers can test cross-reactivity of compounds developed for human conditions

  • This is particularly relevant for compounds targeting autoimmune diseases, cancer, and viral infections

• Structure-Based Drug Design:

  • The structural similarities between K. lactis URA9 and human DHODH can inform structure-activity relationships

  • Recombinant K. lactis URA9 can be used for co-crystallization with inhibitors

  • This approach has successfully identified selective inhibitors for various DHODH enzymes

• High-Throughput Screening Platform:

  • The yeast complementation system can be adapted for high-throughput screening

  • Growth inhibition in URA9-dependent strains provides a straightforward readout

  • Hits can be validated using purified enzyme assays

For example, compounds like olorofim have been tested against various fungal DHODHs to understand susceptibility patterns and resistance mechanisms, demonstrating the value of comparative DHODH studies in antifungal development .

What approaches can identify critical residues and domains in URA9?

Researchers can employ several complementary approaches to identify critical residues and domains in URA9:

• Site-Directed Mutagenesis:

  • Target conserved residues identified through sequence alignment

  • Create alanine scanning mutants across putative functional domains

  • Use fusion PCR or commercial mutagenesis kits to introduce specific mutations

  • Assess the impact on enzyme activity and protein stability

• Chimeric Enzyme Construction:

  • Create fusion proteins swapping domains between K. lactis URA9 and other DHODHs

  • This approach can identify domains responsible for specific properties (substrate specificity, electron acceptor preference)

  • Test functionality through complementation assays and biochemical characterization

• Structural Analysis Techniques:

  • X-ray crystallography of recombinant K. lactis URA9

  • Molecular docking of substrates and inhibitors

  • Hydrogen-deuterium exchange mass spectrometry to identify flexible regions

• Functional Complementation:

  • Test mutated versions of URA9 in the yeast complementation system

  • Growth on media lacking uracil provides a clear readout of functional impact

  • Compare growth rates and enzyme activities to quantify effects

• Inhibitor Binding Studies:

  • Use differential scanning fluorimetry to assess thermal stability changes

  • Surface plasmon resonance or isothermal titration calorimetry for binding kinetics

  • Chemical cross-linking coupled with mass spectrometry to identify binding sites

These approaches have successfully identified catalytic bases, substrate binding residues, and electron acceptor interaction domains in various DHODH enzymes, with implications for understanding URA9 function .

How do mutations in URA9 affect pyrimidine metabolism and cellular function?

Mutations in URA9 can have profound effects on pyrimidine metabolism and broader cellular functions:

• Impact on Pyrimidine Biosynthesis:

  • Mutations in catalytic residues can reduce or eliminate enzyme activity

  • This creates a uracil auxotrophy, requiring external pyrimidine supplementation

  • The severity depends on the specific mutation and its effect on enzyme kinetics

  • Quantitative measurements show variable growth rates on media lacking uracil depending on the specific mutation

• Redox Balance Disruption:

  • URA9 participates in cellular electron transfer chains

  • Mutations can disrupt mitochondrial redox balance

  • This may trigger compensatory mechanisms involving alternative dehydrogenases

  • The effect is particularly significant in K. lactis due to its respiratory metabolism

• Respiratory vs. Fermentative Metabolism:

  • URA9 mutations may force metabolic shifts

  • K. lactis normally prioritizes respiratory metabolism

  • Compromised URA9 function may increase reliance on fermentation

  • This shift can alter growth characteristics and stress responses

• Oxygen Response Alterations:

  • Mutations affecting electron transfer efficiency may alter the cell's response to hypoxia

  • This can disrupt normal adaptation mechanisms to oxygen limitation

  • The single-copy nature of K. lactis URA9 (compared to duplicated genes in S. cerevisiae) makes these effects more pronounced

• Developmental and Morphological Effects:

  • In some fungi, DHODH mutations affect cellular morphology and development

  • This demonstrates the integration of pyrimidine metabolism with broader cellular processes

  • Such effects highlight the importance of URA9 beyond its immediate enzymatic function

Understanding these effects provides insights into both the specific role of URA9 and the broader implications of pyrimidine metabolism for cellular physiology in eukaryotes .

How does K. lactis URA9 compare to DHODH enzymes from pathogenic fungi?

Comparing K. lactis URA9 to DHODHs from pathogenic fungi reveals important differences with clinical and research implications:

• Class Distribution:

  • K. lactis URA9 belongs to Class 2 DHODH (mitochondrial), similar to human DHODH

  • Some pathogenic fungi possess Class 1A DHODHs (cytosolic, fumarate-utilizing)

  • Others have Class 2 enzymes similar to K. lactis

  • Certain fungi, particularly Mucorales species, have only Class 1A DHODH

• Inhibitor Susceptibility:

  • The class of DHODH determines susceptibility to specific inhibitors

  • Olorofim, a fungal-specific DHODH inhibitor, shows differential effectiveness

  • K. lactis URA9 susceptibility patterns may provide insights into inhibitor mechanisms

  • Testing through yeast complementation assays reveals class-specific inhibition profiles

• Subcellular Localization:

  • Unlike cytosolic Class 1A DHODHs in some pathogenic fungi, K. lactis URA9 is mitochondrial

  • This affects the enzyme's integration with cellular metabolism

  • Localization differences impact potential drug targeting strategies

  • Respiratory vs. fermentative metabolism influences the criticality of DHODH function

• Evolutionary Relationships:

  • Phylogenetic analysis places K. lactis URA9 closer to human DHODH than to Class 1A fungal enzymes

  • This evolutionary relationship provides context for structural differences

  • Understanding these relationships helps in developing selective inhibitors

  • Some fungi appear to have acquired DHODH genes through horizontal gene transfer

This comparative understanding is essential for researchers working on antifungal drug development and for those using K. lactis as a model system for studying DHODH biology across species .

What experimental approaches can differentiate between DHODH classes?

Researchers can employ several complementary approaches to differentiate between DHODH classes:

These approaches have successfully distinguished between predicted Class 1A-like genes that were actually dihydrouracil oxidases and true DHODHs in various fungal species, highlighting the importance of functional verification beyond sequence analysis .

What can K. lactis URA9 research tell us about human DHODH-related diseases?

Research on K. lactis URA9 provides valuable insights into human DHODH-related diseases:

• Model for Miller Syndrome:

  • Mutations in human DHODH cause Miller syndrome (postaxial acrofacial dystosis syndrome)

  • K. lactis URA9, as a Class 2 DHODH with mitochondrial localization, serves as a model for studying these mutations

  • Equivalent mutations can be introduced into URA9 to study effects on enzyme function

  • The yeast system allows isolation of specific DHODH effects from complex developmental contexts

• Understanding Drug Mechanisms:

  • DHODH inhibitors are used to treat autoimmune diseases like rheumatoid arthritis and multiple sclerosis

  • K. lactis URA9 can model the effects of these drugs at the molecular level

  • Structure-activity relationships derived from URA9 studies inform human DHODH drug development

  • The effects of inhibitors on cellular metabolism in K. lactis may parallel effects in human cells

• Cancer Research Applications:

  • DHODH is targeted in cancer therapy due to its importance in rapidly proliferating cells

  • K. lactis provides a eukaryotic model for studying metabolic effects of DHODH inhibition

  • The respiratory metabolism of K. lactis better mimics human cancer cells than fermentative yeasts

  • Effects on redox balance and mitochondrial function have parallels in cancer cell metabolism

• Viral Infection Insights:

  • DHODH inhibitors show promise against viral infections

  • K. lactis URA9 can help understand how pyrimidine depletion affects viral replication

  • The conserved function between K. lactis and human DHODHs allows testing of broad-spectrum effects

  • Structure-based drug design can utilize insights from the K. lactis enzyme

By studying the fundamental biology of K. lactis URA9, researchers gain insights applicable to understanding human DHODH-related diseases and developing targeted therapeutic approaches .