# B12 Trafficking and Inherited Defects

> **NIH NIH R01** · UNIVERSITY OF MICHIGAN AT ANN ARBOR · 2024 · $517,454

## Abstract

Vitamin B12 (or cobalamin) supports just two enzymes in the human proteome, but its absence is incompatible
with life. This high value dietary commodity is escorted by an elaborate system of chaperones to its two client
enzymes: methionine synthase (MS) in the cytoplasm and methylmalonyl-CoA mutase (MCM) in the
mitochondrion. Clues to the complexity and multicompartment geography of the B12 trafficking pathway had
emerged from clinical genetics studies on patients with inborn errors of cobalamin metabolism, which led to their
classification into nine complementation groups (cblA-G, cblJ and mut). Immodest in size, translocation of this
portly and complex organometallic cofactor, complete with a tail that is absent in other tetrapyrroles, presents
challenges, both chemical and steric. With the discovery of the disease-associated cbl genes, the PI’s group has
led the effort to decipher function, illuminating structural, spectroscopic, and kinetic details of the elegant redox-
linked coordination chemistry that is integral to the trafficking pathway. Exciting advances in the past cycle
combined with a wealth of provocative leads, position us to address major gaps in our understanding of B12
trafficking, which are fundamentally important and clinically relevant to isolated homocystinuria (due to a
functional deficiency of MS) or methylmalonic aciduria (due to a functional deficiency of MCM) or both (due to
mutations in the early steps in the trafficking pathway). Clinical data on patient mutations in the cbl and mut
(MCM) genes, will continue to be integrated into our studies as we address the following specific aims in the next
cycle. (i) Elucidate the mechanism by which chaperones in the shared cytoplasmic pathway direct B12 traffic at
a branchpoint. Specifically, we will test our hypothesis that alternative interprotein Co-S complexes between
CblC, a chemically versatile b-ligand transferase/eliminase, and CblD, which we discovered has two B12 bindings
sites, govern partitioning between the cytoplasmic versus mitochondrial branches. We will also test the intriguing
hypothesis that B12 piggybacks on CblD to enter the mitochondrion. (ii) Elucidate the mechanism of B12 transfer
in the cytoplasmic branch from CblD to MS following our discovery that vicinal thiols are important for B12 delivery.
We hypothesize that B12 tethered via a Co-S bond to CblD is loaded onto MS, and that the complex is freed via
displacement of the thiolate ligand to form holo-MS and an intramolecular disulfide on CblD. (iii) Elucidate the
molecular traffic lights in the mitochondrial branch that permit 5´-deoxyadenosylcobalamin loading from ATR
(adenosyltransferase) to MCM and cob(II)alamin off-loading from MCM to ATR (for repair) in the presence of the
G-protein chaperone, CblA. We will also assess the potential for rhodibalamin to form a rhodium-carbon bond
(catalyzed by ATR) but resist its cleavage (by MCM). A small gene cluster, including ATR and MCM is essential
for Mycob...

## Key facts

- **NIH application ID:** 10766845
- **Project number:** 5R01DK045776-33
- **Recipient organization:** UNIVERSITY OF MICHIGAN AT ANN ARBOR
- **Principal Investigator:** RUMA V BANERJEE
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $517,454
- **Award type:** 5
- **Project period:** 1998-02-01 → 2028-01-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10766845

## Citation

> US National Institutes of Health, RePORTER application 10766845, B12 Trafficking and Inherited Defects (5R01DK045776-33). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10766845. Licensed CC0.

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