Discovery of BilV reveals a multienzymatic basis for bilirubin reduction across vertebrate gut microbiomes

Technical Summary: Discovery of BilV and the Multienzymatic Basis for Bilirubin Reduction

Problem Statement
The reduction of bilirubin to urobilinogen by gut bacteria is a critical step in the excretion of bile pigments via feces and urine. While the enzyme bilirubin reductase (BilR) has long been known to facilitate this process, historical observations of heterogeneous mixtures of partially reduced bilirubin-derived intermediates suggested that a single enzyme might be insufficient for complete conversion. This heterogeneity implied the existence of additional enzymatic activities capable of reducing specific structural features of bilirubin that BilR alone could not fully process.

Methodology
To address the mechanism behind this incomplete reduction, the authors employed a multi-faceted approach combining genomic analysis, heterologous protein expression, and mass spectrometry:

• Genomic Analysis: The researchers examined the genomic neighborhoods of known bilR genes to identify co-occurring genes. They specifically focused on the Old Yellow Enzyme (OYE) family, identifying a novel gene, bilV, located adjacent to bilR.
• Heterologous Expression: The bilV gene was expressed in a heterologous host system alongside bilR to test for functional complementation.
• LC-MS/MS Analysis: Liquid chromatography-tandem mass spectrometry was utilized to characterize the reaction products. This allowed for the precise identification of intermediates and final products generated by BilR alone versus the co-expression of BilR and BilV.
• Metagenomic Survey: The study analyzed 1,197 gut metagenomes spanning 14 vertebrate species to map the distribution and co-occurrence patterns of bilR and bilV across different host lineages.

Key Contributions and Results

• Identification of BilV: The study identifies Bilirubin Vinyl Reductase (BilV) as a novel OYE-family reductase.
• Mechanistic Elucidation: Experimental data demonstrates that BilR acts specifically on the methine bridges of bilirubin. However, BilR alone cannot reduce the vinyl groups, leading to the accumulation of partially reduced intermediates. Co-expression of BilV enables the reduction of these vinyl groups, facilitating the complete conversion of bilirubin to urobilinogen.
• Genomic Co-occurrence Patterns: The analysis reveals a distinct genomic association: bilV co-occurs primarily with the bilR-insertion subtype of the bilirubin reductase gene but is largely absent in genomes containing the bilR-short subtype.
• Host-Specific Distribution: This differential co-occurrence shapes the metabolic potential of gut microbiomes across vertebrates. Carnivores and omnivores typically possess a balanced presence of both bilR and bilV, enabling complete bilirubin reduction. In contrast, avian microbiomes, which are dominated by the bilR-short subtype, are depleted for bilV, suggesting a limitation in their capacity for complete vinyl-group reduction.

Significance
The paper establishes that the reduction of bilirubin to urobilinogen is not a unimolecular process but a multienzymatic pathway requiring two enzymes with complementary regioselectivity: BilR for methine bridge reduction and BilV for vinyl-group reduction. Furthermore, the study demonstrates that the distribution of these enzymatic capabilities across vertebrate gut microbiomes is not uniform; rather, it varies in concert with host-specific bile pigment chemistry. These findings resolve the long-standing observation of heterogeneous intermediates by defining the specific enzymatic roles required for complete bilirubin metabolism in the gut.