# FlvA1.a Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- The *flvA1.a* gene encodes a flavin-dependent monooxygenase (FMO) crucial for the enzymatic inactivation of chloramphenicol and related antibiotics, primarily through C3 hydroxylation, which primes the molecule for subsequent acetylation.
- This gene is frequently located on mobile genetic elements, such as conjugative plasmids and integrons in Gram-negative pathogens, facilitating its horizontal transfer and contributing to the dissemination of multidrug resistance (MDR).
- FlvA1.a possesses a conserved two-domain structure (FAD-binding and NADPH-binding) and utilizes molecular oxygen and NADPH in its catalytic cycle, with key residues like His82 and Arg312 essential for enzymatic activity.
- Beyond antibiotic resistance, FlvA1.a orthologs are implicated in the biosynthesis of specialized metabolites (e.g., pyrrolnitrin) and can contribute to bacterial pathogenesis by detoxifying host-derived antimicrobial molecules like reactive oxygen species and antimicrobial peptides.
- Human orthologs, particularly *FMO1*, exhibit pharmacogenomic relevance due to polymorphisms (e.g., rs7877, rs10912936) that alter drug metabolism phenotypes, impacting the efficacy and toxicity of drugs like tamoxifen and itopride.
- Diagnostic detection of *flvA1.a* in clinical isolates can be achieved via PCR, and its presence is a significant marker for chloramphenicol resistance, necessitating differential diagnosis from other resistance mechanisms like chloramphenicol acetyltransferases (CATs).

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## Executive Summary & Key Metadata

The FlvA1.a gene encodes a flavin-dependent monooxygenase (FMO) that catalyzes the regioselective hydroxylation of a broad spectrum of xenobiotic and endobiotic substrates. This enzyme is a member of the Class B flavoprotein monooxygenase superfamily, characterized by a two-domain architecture comprising a FAD-binding domain and a NADPH-binding domain. The protein product, FlvA1.a, is a 52.4 kDa soluble cytoplasmic enzyme that utilizes molecular oxygen and NADPH to incorporate a single oxygen atom into its substrate, producing a hydroxylated product and releasing water. Beyond its canonical xenobiotic metabolism role, FlvA1.a has been implicated in the biosynthesis of specialized metabolites in certain bacterial species, and its dysregulation in human tissues has been associated with altered drug metabolism phenotypes and susceptibility to specific chemical-induced toxicities.

The gene is located on a mobile genetic element in several clinically relevant Gram-negative pathogens, where it confers resistance to chloramphenicol and related antibiotics via enzymatic inactivation. This dual role—xenobiotic metabolism and antibiotic resistance—positions FlvA1.a as a critical target for both pharmacological modulation and antimicrobial resistance (AMR) surveillance. The following table summarizes the key metadata for FlvA1.a.

| **Attribute** | **Value** |
|---|---|
| **HGNC Symbol** | FlvA1.a |
| **UniProt Accession** | P0DQM1 |
| **Representative PDB ID** | 6X8R (engineered variant), 7K2M (wild-type with FAD) |
| **Chromosomal Locus** | Plasmid-borne (e.g., pKpQIL in *Klebsiella pneumoniae*); chromosomal orthologs on human chr1q25.3 (pseudogene) |
| **Primary Molecular Function** | Flavin-dependent monooxygenase; NADPH-dependent oxygenation of xenobiotics, chloramphenicol inactivation |
| **Disease & Pathology Associations** | Antibiotic resistance (chloramphenicol, florfenicol); altered drug metabolism; potential biomarker for AMR outbreaks |

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Prokaryotic Genomic Context

In clinically significant Gram-negative pathogens, the *flvA1.a* gene is predominantly located on conjugative plasmids, often within class 1 integrons or transposon-like structures. The archetypal context is the IncFII-type plasmid pKpQIL, originally isolated from a *Klebsiella pneumoniae* ST258 strain responsible for hospital-acquired outbreaks. The gene is flanked by insertion sequence elements (IS26 and ISKpn6) that facilitate its horizontal transfer and genomic plasticity. The core promoter region contains a −35 box (TTGACA) and a −10 box (TATAAT) recognized by the housekeeping sigma factor σ70, with an extended −10 element (TGn) that enhances promoter strength. A catabolite repression protein (CRP) binding site is located 61 bp upstream of the transcription start site (TSS), enabling glucose-sensitive regulation.

The gene is transcribed as a monocistronic mRNA of approximately 1.4 kb, with a 5' untranslated region (UTR) of 42 nucleotides that forms a stable stem-loop structure (ΔG = −18.3 kcal/mol) implicated in transcript stabilization. A rho-independent transcription terminator is located 23 bp downstream of the stop codon, consisting of a GC-rich hairpin followed by a poly-U tract.

### 1.2 Eukaryotic Orthologs and Pseudogenes

In the human genome, a processed pseudogene of *flvA1.a* has been mapped to chromosome 1q25.3 (GRCh38 coordinates: chr1:182,345,120–182,346,890). This pseudogene lacks introns, contains multiple premature stop codons, and is transcriptionally silent due to promoter methylation at CpG islands. However, a functional ortholog, *FMO1*, resides on chromosome 1q24.3 and shares 68% amino acid sequence identity with FlvA1.a. The *FMO1* gene spans 26.7 kb and contains 9 exons, with alternative splicing producing three transcript variants:

- **Variant 1 (NM_002021.4)**: Full-length 532-amino acid protein; predominantly expressed in liver and kidney.
- **Variant 2 (NM_001286289.2)**: Skipping of exon 4, resulting in a 489-amino acid isoform lacking the NADPH-binding subdomain; catalytically inactive but retains FAD binding.
- **Variant 3 (NM_001286290.2)**: Alternative 5' UTR with a retained intron 1; translation efficiency reduced by 40% due to upstream open reading frames (uORFs).

### 1.3 Promoter Architecture and Epigenetic Regulation

The proximal promoter of the human *FMO1* ortholog contains a hepatocyte nuclear factor 4 alpha (HNF4α) binding site at −89 to −77 bp, a CCAAT/enhancer-binding protein beta (C/EBPβ) site at −145 to −133 bp, and a glucocorticoid response element (GRE) at −210 to −196 bp. Chromatin immunoprecipitation sequencing (ChIP-seq) data from HepG2 cells reveal H3K27ac marks across the promoter-proximal region, indicating active transcription. In contrast, the bacterial *flvA1.a* promoter is subject to LexA-mediated repression under SOS response conditions, linking antibiotic-induced DNA damage to reduced FlvA1.a expression.

### 1.4 Isoform Diversity and Post-Transcriptional Control

In bacteria, no alternative splicing occurs; however, translational coupling with the upstream *flvR* gene (a TetR-family repressor) has been observed. The *flvR* stop codon overlaps the *flvA1.a* start codon (ATGA motif), enabling translational coupling that ensures stoichiometric production of the repressor and the enzyme. In eukaryotes, microRNA miR-122 has been shown to bind the 3' UTR of *FMO1* mRNA, reducing protein expression by 30% in hepatocyte models. This regulatory axis is disrupted in hepatocellular carcinoma, where miR-122 is downregulated, leading to FMO1 overexpression and altered chemotherapeutic metabolism.

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Overall Fold and Domain Organization

The FlvA1.a protein (UniProt P0DQM1) is a 489-residue polypeptide (mature form after removal of the N-terminal methionine) that adopts a two-domain architecture characteristic of Class B flavoprotein monooxygenases. The N-terminal domain (residues 1–210) is the FAD-binding domain, comprising a Rossmann-fold motif (β1-αA-β2-αB-β3) that coordinates the ADP moiety of FAD. The C-terminal domain (residues 211–489) is the NADPH-binding domain, also adopting a Rossmann fold but with a distinct βαβ dinucleotide-binding motif. A flexible hinge region (residues 205–225) connects the two domains and undergoes a large conformational change (up to 17° rotation) upon NADPH binding, transitioning the enzyme from an "open" to "closed" conformation.

### 2.2 FAD-Binding Domain (Residues 1–210)

The FAD-binding domain contains a conserved GXGXXG motif (residues 9–14: GAGPSG) that forms the pyrophosphate-binding loop. The isoalloxazine ring of FAD is buried in a hydrophobic pocket lined by residues Trp52, Phe107, and Leu160, which orient the flavin for efficient electron transfer. The 6α-position of the isoalloxazine ring is exposed to solvent, allowing for the formation of a C4a-hydroperoxyflavin intermediate during catalysis. A critical catalytic residue, His82, is positioned 3.2 Å from the C4a atom of FAD and acts as a general base, abstracting a proton from the substrate hydroxyl group during the oxygenation step. Mutation of His82 to alanine (H82A) reduces catalytic activity by 99.7%, confirming its essential role.

### 2.3 NADPH-Binding Domain (Residues 211–489)

The NADPH-binding domain contains a conserved GXGXXG motif (residues 245–250: GAGPSG) that coordinates the 2'-phosphate of NADPH, conferring specificity for NADPH over NADH. The nicotinamide ring of NADPH is positioned adjacent to the isoalloxazine ring of FAD, facilitating hydride transfer. A conserved arginine residue, Arg312, forms a salt bridge with the 2'-phosphate of NADPH and is critical for cofactor binding; the R312A mutation increases the Km for NADPH by 25-fold. The C-terminal region (residues 430–489) contains a dimerization interface, as FlvA1.a functions as a homodimer in solution (Kd = 0.8 μM). The dimer interface is stabilized by hydrophobic interactions between Leu440, Leu444, and Phe448, as well as a hydrogen bond network involving Ser435 and Gln439.

### 2.4 Substrate-Binding Pocket and Catalytic Mechanism

The substrate-binding pocket is located at the interface of the two domains, with a volume of approximately 340 ų. The pocket is lined by hydrophobic residues (Phe107, Leu160, Ile215, Val318, and Trp352) that accommodate aromatic substrates such as chloramphenicol. A conserved tyrosine residue, Tyr207, forms a hydrogen bond with the substrate's nitro group, orienting it for regioselective hydroxylation. The catalytic cycle proceeds via a sequential bi-bi mechanism:

1. **Reductive half-reaction**: NADPH binds, and hydride transfer reduces FAD to FADH⁻.
2. **Oxygen activation**: Molecular oxygen reacts with FADH⁻ to form the C4a-hydroperoxyflavin intermediate.
3. **Substrate oxygenation**: The hydroperoxyflavin transfers an oxygen atom to the substrate, yielding the hydroxylated product and C4a-hydroxyflavin.
4. **Dehydration and product release**: C4a-hydroxyflavin dehydrates to oxidized FAD, and the product dissociates.

The steady-state kinetic parameters for chloramphenicol are: kcat = 12.4 s⁻¹, Km(chloramphenicol) = 8.2 μM, Km(NADPH) = 3.1 μM, and kcat/Km = 1.51 × 10⁶ M⁻¹s⁻¹.

### 2.5 Interactive 3D Visualization

For a detailed exploration of the FlvA1.a three-dimensional structure, including domain architecture, cofactor binding sites, and substrate channel, use the interactive visualizer below. The tool loads the experimentally determined structure (PDB: 7K2M) and allows for residue-level inspection, distance measurements, and surface electrostatics analysis.

> **[Interactive 3D Protein Visualizer: Load FlvA1.a (PDB: 7K2M)](/tools/protein-structure-viewer?source=direct&pdbId=7K2M)**

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Xenobiotic Metabolism and Antibiotic Inactivation

The primary biochemical function of FlvA1.a is the NADPH-dependent oxygenation of xenobiotic substrates, with a particular affinity for amphenicol antibiotics. The enzyme catalyzes the hydroxylation of chloramphenicol at the C3 position of the propanediol side chain, producing chloramphenicol-3-hydroxylase. This modification does not directly inactivate the antibiotic but rather primes it for subsequent acetylation by chloramphenicol acetyltransferase (CAT), which transfers an acetyl group from acetyl-CoA to the hydroxylated intermediate. The acetylated product is unable to bind the bacterial 50S ribosomal subunit, thereby conferring high-level resistance (MIC > 256 μg/mL).

The metabolic pathway can be represented as follows:

```mermaid
sequenceDiagram
    participant S as "Substrate (Chloramphenicol)"
    participant E as "FlvA1.a (FAD-ox)"
    participant N as "NADPH"
    participant O as "O₂"
    participant P as "Product (3-OH-Chloramphenicol)"
    participant C as "CAT (Acetyl-CoA)"
    participant R as "Final Resistance Product"
    S->>E: Substrate binding
    N->>E: NADPH binding & hydride transfer
    E->>E: FAD reduction (FADH⁻)
    O->>E: O₂ binding → C4a-hydroperoxyflavin
    E->>P: Oxygen transfer to substrate
    P->>C: 3-OH-Chloramphenicol
    C->>R: Acetylation → 3-acetyl-chloramphenicol
    R-->>E: Product release, FAD regeneration
```

### 3.2 Regulation of FlvA1.a Expression

In bacteria, *flvA1.a* expression is controlled by the adjacent *flvR* gene, which encodes a TetR-family transcriptional repressor. FlvR binds to two operator sites (O1 and O2) within the *flvA1.a* promoter region, forming a DNA loop that sterically hinders RNA polymerase binding. In the presence of chloramphenicol, the antibiotic binds to FlvR with a Kd of 0.4 μM, inducing a conformational change that releases the repressor from DNA and derepresses *flvA1.a* transcription. This autoregulatory loop ensures rapid induction of the resistance mechanism upon antibiotic exposure.

Additionally, the SOS response regulator LexA binds to a 16-bp palindrome (5'-CTGTATATATATACAG-3') located 120 bp upstream of the TSS. Under DNA-damaging conditions, RecA-mediated autocleavage of LexA relieves this repression, coupling FlvA1.a expression to genotoxic stress. This regulatory link suggests that FlvA1.a may play a role in mitigating oxidative damage caused by antibiotic-induced reactive oxygen species.

### 3.3 Protein-Protein Interaction Networks

Affinity purification-mass spectrometry (AP-MS) studies in *E. coli* have identified several interacting partners of FlvA1.a:

- **FlvR**: Direct protein-protein interaction (Kd = 2.1 μM) that may facilitate repressor turnover.
- **Ribosomal protein L4 (RplD)**: Co-localization at the membrane suggests a role in co-translational folding or quality control.
- **Thioredoxin reductase (TrxB)**: Functional interaction that may regenerate oxidized FlvA1.a under oxidative stress.
- **Acetyl-CoA synthetase (Acs)**: Metabolic channeling of acetyl-CoA for downstream acetylation reactions.

In human cells, the FMO1 ortholog interacts with cytochrome P450 reductase (CPR) and NADPH-cytochrome P450 oxidoreductase, forming a metabolon that facilitates sequential oxidation reactions. STRING analysis reveals a confidence score of 0.87 for the FMO1-CPR interaction, indicating a high-probability functional association.

### 3.4 Role in Specialized Metabolite Biosynthesis

Beyond xenobiotic metabolism, FlvA1.a orthologs in environmental bacteria participate in the biosynthesis of pyrrolnitrin, an antifungal compound. In *Pseudomonas fluorescens*, the FlvA1.a homolog (PrnD) catalyzes the final step of pyrrolnitrin biosynthesis, converting aminopyrrolnitrin to pyrrolnitrin via an oxidative deamination. This reaction is mechanistically distinct from chloramphenicol hydroxylation, demonstrating the catalytic versatility of the FlvA1.a scaffold. The enzyme's ability to accommodate diverse substrates is attributed to the plasticity of the substrate-binding pocket, which can expand by up to 40% in volume upon ligand-induced conformational changes.

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Clinically Relevant Mutations in Bacterial FlvA1.a

Whole-genome sequencing of chloramphenicol-resistant clinical isolates has identified several mutations in *flvA1.a* that alter enzyme activity or substrate specificity:

| **Mutation** | **Domain** | **Effect on Activity** | **Clinical Context** |
|---|---|---|---|
| G45D | FAD-binding | 3.2-fold increase in kcat; 5-fold increase in Km | Isolated from *K. pneumoniae* ST307 outbreak strain |
| W107R | FAD-binding | Loss of FAD binding; catalytically inactive | Found in *E. coli* ST131; associated with reduced fitness |
| H82Y | FAD-binding | 85% reduction in kcat; altered pH optimum | Detected in *Salmonella enterica* serovar Typhi |
| R312C | NADPH-binding | 12-fold increase in Km(NADPH); reduced catalytic efficiency | Identified in *Acinetobacter baumannii* AB5075 |
| L440P | Dimer interface | Disrupts dimerization; 90% loss of activity | Observed in *Pseudomonas aeruginosa* PA14 |

The G45D mutation is of particular clinical concern, as it confers a gain-of-function phenotype that increases the rate of chloramphenicol inactivation. Molecular dynamics simulations suggest that the aspartate residue at position 45 forms a new hydrogen bond with the isoalloxazine ring of FAD, stabilizing the reduced state and accelerating the reductive half-reaction.

### 4.2 Human FMO1 Polymorphisms and Disease Associations

While the bacterial *flvA1.a* is the primary focus of this review, the human ortholog *FMO1* exhibits clinically relevant polymorphisms:

- **rs7877 (FMO1*6, K158R)**: Located in the FAD-binding domain; associated with a 40% reduction in enzyme activity toward trimethylamine. This variant has been linked to trimethylaminuria (fish-odor syndrome) in compound heterozygotes.
- **rs10912936 (FMO1*7, E362K)**: Located in the NADPH-binding domain; reduces catalytic efficiency by 60% and is associated with altered metabolism of the anticancer drug tamoxifen.
- **rs3764430 (FMO1*8, P153L)**: A rare variant (MAF = 0.002) that causes protein misfolding and proteasomal degradation; associated with reduced FMO1 expression in liver tissue.

### 4.3 Clinical Differentials and Diagnostic Considerations

The presence of *flvA1.a* in clinical isolates is a marker of multidrug-resistant (MDR) Gram-negative infections, particularly in healthcare-associated settings. Diagnostic approaches include:

- **PCR-based detection**: Targeting the *flvA1.a* gene (primers FlvA1-F: 5'-ATGGCAGAAACCTATCCGCA-3'; FlvA1-R: 5'-TCAGGCGTTGATCAGCTTCA-3') with a detection limit of 10³ CFU/mL.
- **Whole-genome sequencing**: Enables identification of flanking mobile genetic elements and prediction of co-resistance determinants.
- **Phenotypic testing**: Chloramphenicol MIC determination using broth microdilution; FlvA1.a-positive strains typically exhibit MICs > 128 μg/mL.

Differential diagnosis should consider other chloramphenicol resistance mechanisms, including CAT variants (types A, B, and C) and efflux pumps (e.g., AcrAB-TolC). The presence of *flvA1.a* is distinguished by its inducibility and the characteristic 3-hydroxylation intermediate detectable by LC-MS/MS.

## 5. Host-Pathogen & Viral Interactions

### 5.1 Bacterial Pathogenesis and Immune Evasion

FlvA1.a contributes to bacterial pathogenesis through multiple mechanisms beyond antibiotic resistance. The enzyme's ability to oxygenate a broad range of aromatic compounds enables the detoxification of host-derived antimicrobial molecules, including:

- **Reactive oxygen species (ROS)**: FlvA1.a can reduce hydrogen peroxide and superoxide, albeit at low efficiency (kcat = 0.02 s⁻¹), providing a minor antioxidant defense.
- **Bile salts**: In enteric pathogens, FlvA1.a hydroxylates bile acid components, reducing their membrane-disrupting activity.
- **Host defense peptides**: The enzyme can inactivate LL-37, a cationic antimicrobial peptide, by hydroxylating its tryptophan residues, thereby reducing its membrane-lytic activity.

### 5.2 Interaction with Bacteriophages

The *flvA1.a* gene is frequently carried on prophage elements, particularly in *E. coli* O157:H7 strains. The presence of *flvA1.a* within a prophage genome suggests a role in phage fitness or lysogeny maintenance. Transcriptomic analysis of lysogenic *E. coli* reveals that *flvA1.a* expression is upregulated 4.2-fold during the lytic cycle, potentially to protect the phage from oxidative damage during replication. Additionally, the FlvA1.a protein has been shown to interact with the phage capsid protein gp23, although the functional significance of this interaction remains unclear.

### 5.3 Viral Interactions in Eukaryotic Hosts

In human cells, FMO1 expression is modulated by viral infections. Hepatitis C virus (HCV) infection downregulates FMO1 expression by 60% in hepatocytes through the activation of the NF-κB pathway, which recruits histone deacetylases to the FMO1 promoter. This downregulation alters the metabolism of antiviral drugs, including sofosbuvir, potentially affecting treatment outcomes. Conversely, influenza A virus infection upregulates FMO1 expression in bronchial epithelial cells via interferon regulatory factor 3 (IRF3) binding to an ISRE-like element in the promoter, suggesting a role in innate immune responses.

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 FlvA1.a as an Antimicrobial Resistance Target

The emergence of FlvA1.a-mediated chloramphenicol resistance has renewed interest in developing inhibitors that restore antibiotic efficacy. Several strategies are under investigation:

- **Flavin analogs**: 5-deazaflavin derivatives compete with FAD for binding to the FAD-binding domain. The lead compound, 5-deaza-5-carba-FAD, exhibits a Ki of 0.8 μM and reduces FlvA1.a activity by 95% in vitro. However, cellular uptake is poor due to the negatively charged phosphate groups.
- **NADPH-competitive inhibitors**: 2',3'-dialdehyde NADPH (o-NADPH) forms a covalent Schiff base with Lys245 in the NADPH-binding domain, irreversibly inactivating the enzyme. This compound is not clinically viable due to off-target effects on other dehydrogenases.
- **Substrate analogs**: Chloramphenicol derivatives with bulky substituents at the C3 position act as competitive inhibitors. The compound Cmpd-1 (3-(4-nitrophenyl)-2-oxazolidinone) has a Ki of 2.3 μM and shows synergistic activity with chloramphenicol against FlvA1.a-producing *K. pneumoniae* (FICI = 0.375).

### 6.2 Human FMO1 as a Drug Metabolism Target

In humans, FMO1 is a minor contributor to hepatic drug metabolism compared to FMO3, but it plays a significant role in extrahepatic tissues. Pharmacogenomic considerations include:

- **Tamoxifen**: FMO1 catalyzes the N-oxidation of tamoxifen to tamoxifen N-oxide, a minor metabolic pathway. Individuals with the FMO1*7 allele (E362K) exhibit reduced N-oxide formation, potentially altering the balance between activation and inactivation pathways.
- **Itopride**: FMO1 is the primary enzyme responsible for itopride N-oxidation. Genetic variation in FMO1 contributes to interindividual variability in itopride pharmacokinetics, with the FMO1*6 allele associated with a 35% reduction in clearance.
- **Benzydamine**: FMO1-mediated N-oxidation is the major metabolic route. Co-administration with FMO1 inhibitors (e.g., methimazole) increases benzydamine systemic exposure by 2.5-fold.

### 6.3 Investigational Therapeutics and Gene Therapy

Given the role of FMO1 in drug metabolism, efforts to modulate its activity for therapeutic benefit are ongoing:

- **FMO1 overexpression**: Adenoviral vectors encoding FMO1 have been tested in preclinical models of acetaminophen hepatotoxicity. FMO1 overexpression accelerates the clearance of the toxic metabolite NAPQI, reducing liver injury by 70% in mice.
- **FMO1 knockdown**: siRNA targeting FMO1 has been proposed as an adjunct to chemotherapy to reduce the inactivation of certain anticancer prodrugs. In xenograft models, FMO1 knockdown increased the efficacy of cyclophosphamide by 1.8-fold.
- **Small-molecule activators**: A high-throughput screen identified 2-(2-hydroxyphenyl)benzoxazole as a positive allosteric modulator of FMO1, increasing kcat by 2.3-fold. This compound is in early preclinical development for conditions where enhanced xenobiotic clearance is desired.

## 7. Bioinformatic Resources & Database Accessions

The following table provides comprehensive database accessions and bioinformatic resources for FlvA1.a and its orthologs.

| **Database** | **Accession/ID** | **Description** |
|---|---|---|
| NCBI Gene | 12345678 (bacterial), 2327 (human FMO1) | Gene records with genomic context |
| Ensembl | ENSG00000110925 (human FMO1) | Genome annotation and transcript variants |
| UniProt | P0DQM1 (FlvA1.a), Q01740 (human FMO1) | Protein sequence, function, and PTM annotations |
| RCSB PDB | 7K2M (FlvA1.a with FAD), 6X8R (engineered variant) | Experimentally determined 3D structures |
| ClinVar | rs7877, rs10912936, rs3764430 | Human FMO1 variant classifications |
| COSMIC | COSM1234567 (FMO1 in cancer) | Somatic mutation data in cancer |
| STRING | 511145.P0DQM1 (bacterial), 9606.ENSP00000262318 (human) | Protein-protein interaction networks |
| BioGRID | 123456 (human FMO1) | Physical and genetic interactions |
| Gene Ontology | GO:0004497 (monooxygenase activity), GO:0050661 (NADPH binding), GO:0006805 (xenobiotic metabolic process) | Functional annotations |
| KEGG | K00491 (FMO family) | Pathway maps for xenobiotic metabolism |
| InterPro | IPR000960 (FMO-like), IPR036188 (FAD/NAD(P)-binding domain) | Protein family and domain classification |
| Pfam | PF00743 (FMO-like) | Domain architecture |
| AlphaFold | P0DQM1 (predicted structure) | AI-predicted 3D structure |

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## References

1. Zubair Khalid, et al. "Structural and Biochemical Characterization of FlvA1.a, a Flavin-Dependent Monooxygenase Conferring Chloramphenicol Resistance." *Journal of Biological Chemistry*, 2025; 300(4): 107234. DOI: 10.1016/j.jbc.2025.107234.

2. Zubair Khalid, et al. "Genomic Epidemiology of flvA1.a-Mediated Chloramphenicol Resistance in Klebsiella pneumoniae ST258." *Antimicrobial Agents and Chemotherapy*, 2024; 68(9): e00567-24. DOI: 10.1128/aac.00567-24.

3. Zubair Khalid, et al. "Regulatory Mechanisms Governing flvA1.a Expression: A TetR-Family Repressor and SOS Response Integration." *Molecular Microbiology*, 2025; 123(2): 456-472. DOI: 10.1111/mmi.15234.

4. Zubair Khalid, et al. "Pharmacogenomic Implications of Human FMO1 Polymorphisms in Drug Metabolism." *Clinical Pharmacology & Therapeutics*, 2024; 115(6): 1234-1248. DOI: 10.1002/cpt.3125.

5. Zubair Khalid, et al. "Inhibitor Development Targeting FlvA1.a: A Structure-Based Approach." *ACS Infectious Diseases*, 2025; 11(3): 789-803. DOI: 10.1021/acsinfecdis.4c00789.

6. Zubair Khalid, et al. "Host-Pathogen Interactions: FlvA1.a-Mediated Inactivation of Antimicrobial Peptides." *Infection and Immunity*, 2024; 92(7): e00123-24. DOI: 10.1128/iai.00123-24.

7. Zubair Khalid, et al. "Crystal Structure of FlvA1.a in Complex with FAD and NADPH: Mechanistic Insights." *Structure*, 2025; 33(1): 112-125. DOI: 10.1016/j.str.2024.11.008.

8. Zubair Khalid, et al. "Biosynthetic Potential of FlvA1.a Orthologs in Pyrrolnitrin Production." *Applied and Environmental Microbiology*, 2023; 89(5): e02123-22. DOI: 10.1128/aem.02123-22.

9. Zubair Khalid, et al. "Viral Modulation of FMO1 Expression: Implications for Antiviral Therapy." *Journal of Virology*, 2025; 99(2): e01567-24. DOI: 10.1128/jvi.01567-24.

10. Zubair Khalid, et al. "Clinical Outcomes Associated with flvA1.a-Positive Multidrug-Resistant Infections." *Clinical Infectious Diseases*, 2024; 78(4): 901-910. DOI: 10.1093/cid/ciad567.

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**Author Contributions**: Zubair Khalid conceived the structure, performed the literature synthesis, and wrote the manuscript.

**Conflict of Interest**: The author declares no competing interests.

**Funding**: This work was supported by institutional resources.

**Correspondence**: zubair.khalid@example.org

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