A cofactor-promiscuous HMGR from the Lyme disease pathogen illuminates diversity in bacterial isoprenoid biosynthesis

Technical Summary

Problem Statement
Borrelia burgdorferi, the causative agent of Lyme disease, possesses a highly reduced genome that lacks numerous primary metabolic pathways. Despite this reduction, the pathogen retains the mevalonate pathway, which is essential for synthesizing isopentenyl pyrophosphate (IPP), a precursor required for the production of the peptidoglycan carrier lipid. While the mevalonate pathway and its rate-limiting enzyme, 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMGR), are well-characterized in vertebrates, the specific mechanisms, enzymatic properties, and physiological roles of HMGR in B. burgdorferi and many other pathogenic bacteria remain poorly understood.

Methodology
The authors employed a multi-faceted approach combining genetic manipulation, biochemical characterization, and structural biology:

• Genetic and Physiological Analysis: The study involved the loss-of-function analysis of HMGR in B. burgdorferi to observe phenotypic consequences. The researchers assessed morphological defects and the rate of de novo peptidoglycan synthesis. To confirm pathway specificity, they tested whether these defects could be rescued by the exogenous addition of mevalonate and IPP.
• Biochemical Characterization: The enzyme was purified and subjected to kinetic assays to determine its cofactor specificity, specifically testing its ability to utilize different nicotinamide cofactors.
• Bioinformatics and Structural Analysis: The authors conducted bioinformatic surveys to identify similar HMGRs in other organisms and performed structural studies (likely X-ray crystallography or cryo-EM, implied by "structures of the enzyme") to visualize the active site architecture.

Key Results

• Physiological Necessity: The study demonstrates that HMGR is a critical metabolic enzyme in B. burgdorferi. Its loss results in distinct morphological defects and a muted capacity for de novo peptidoglycan synthesis. These phenotypes were successfully ameliorated by supplementing the culture with exogenous mevalonate and IPP, confirming the enzyme's role in this specific pathway.
• Cofactor Promiscuity: Biochemical characterization revealed that the B. burgdorferi HMGR is a highly unusual, cofactor-promiscuous oxidoreductase. Unlike the strict cofactor specificity often seen in other HMGRs, this enzyme functions effectively with both major nicotinamide cofactors.
• Evolutionary and Structural Insights: Bioinformatics analysis uncovered other examples of similarly promiscuous HMGRs, suggesting a previously unrecognized evolutionary link regarding cofactor choice. Furthermore, structural analysis revealed that the enzyme possesses a highly divergent active site architecture compared to known HMGRs.

Significance and Claims
The paper establishes HMGR as a validated and promising target for developing antibacterials specifically against diderm pathogens like B. burgdorferi. Beyond its immediate application to Lyme disease treatment, the work highlights cofactor promiscuity as an evolutionary acquired feature within the HMGR family. By illuminating the diversity in bacterial isoprenoid biosynthesis and the unique structural and functional properties of this enzyme, the study provides a foundation for understanding how reduced-genome pathogens maintain essential metabolic functions.