# Vitamin B12 trafficking and selectivity in gut bacteria

> **NIH NIH K99** · UNIVERSITY OF MICHIGAN AT ANN ARBOR · 2022 · $100,000

## Abstract

Project Summary
The human gut microbiome is inhabited by trillions of bacteria that encode over 150-fold more genes than
the human genome itself. The inter-individual differences in microbial composition can be significant, and
the factors contributing to this diversity are not well understood. Metagenomic studies suggest that the
microbiome might play an important role in determining an individual’s predisposition to disease and
responses to treatments. The paucity of understanding how cofactors and other factors influence microbial
composition limit strategies to rationally alter it for therapeutic purposes. Much of our current understanding
of the factors that shape the gut microbial flora composition derives form studies on how bacteria generate
energy, maintain redox balance and acquire carbon and nitrogen. The enzymatic reactions that support
these metabolic processes often rely on cofactors that are in short supply. Vitamin B12 is an example of
one such cofactor that is essential for many bacteria that are unable to biosynthesize it and lack parallel
B12-independent metabolic pathways to circumvent its absence. So, one approach to the targeted
manipulation of the gut microbiome is via altering the levels of available corrinoids. In this proposal, I seek
to elucidate the corrinoid selectivity of transport systems to provide needed insights into how gut bacteria
compete with each other and their hosts for a critical resource in a complex ecosystem. My studies will
focus on Bacteroidetes thetaiotaomicron, a common gut bacterium, which lacks the genes required for de
novo synthesis of vitamin B12 but encodes multiple B12-dependent enzymes. 5’-Deoxyadenosylcobalamin
is the active cofactor form that is utilized by some B12 dependent enzymes and is synthesized by BtuR in
B. thetaiotaomicron. The chaperone and catalytic activities are uncharacterized and will be addressed in
Aim 1. It also encodes three copies of the outer membrane B12-transporter BtuB with each system
displaying a different preference for corrinoid derivatives. The bacterium also possesses additional
transport machinery that is not observed in E. coli, a model organism in which studies on B12 transport in
gram-negative bacteria have been focused. Using a combination of biochemical and biophysical
approaches, I propose to elucidate the mechanism of B12 transport by the B12-uptake (Btu) system in Aim
2. The kinetic and thermodynamic studies in Aims 1 and 2 will define the selectivity of the Btu proteins for
cobamides and provide insights into protein-protein interactions. Combined with the structures determined
in Aims 1 and 2, my studies will furnish mechanistic insights into how a precious and rare cofactor is relayed
from the environment across two layers of bacterial membranes to support the metabolic needs of a
common gut bacterium.

## Key facts

- **NIH application ID:** 10447917
- **Project number:** 1K99GM143482-01A1
- **Recipient organization:** UNIVERSITY OF MICHIGAN AT ANN ARBOR
- **Principal Investigator:** Romila Nina Mascarenhas
- **Activity code:** K99 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $100,000
- **Award type:** 1
- **Project period:** 2022-07-06 → 2024-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10447917

## Citation

> US National Institutes of Health, RePORTER application 10447917, Vitamin B12 trafficking and selectivity in gut bacteria (1K99GM143482-01A1). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10447917. Licensed CC0.

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