# B12 Trafficking and Inherited Defects

> **NIH NIH R01** · UNIVERSITY OF MICHIGAN AT ANN ARBOR · 2021 · $446,160

## Abstract

The intracelluar passage of B12 (or cobalamin), a rare and reactive organometallic cofactor, is fraught with
perils as it navigates its way to only two enzymes in humans that rely on it: cytoplasmic methionine synthase
(MS) and mitochondrial methylmalonyl-CoA mutase (MCM). A squad of dedicated chaperones guards against
inadvertant loss of B12 via its dilution into solution. The existence of B12 chaperones was first hinted at by
clinical genetics studies on patients with inborn errors of B12 metabolism, which lead to isolated or combined
homocystinuria and methylmalonic aciduria, and can present early (birth to a few months) or late (years),
depending on the severity of the biochemical deficit. The P.I's laboratory has been at the forefront of
deciphering functions as the genes encoding B12 chaperones have been identified, and has furnished a wealth
of kinetic, spectroscopic and structural insights that are consistently interwoven with clinical data and
characterization of patient mutations. In the next cycle, we propose to elucidate how redox-linked coordination
chemistry is used as an exquisite and recurring strategy for controlling the chemical reactivity of B12 as it is
processed, and for enabling its mobility, as it is transferred between active sites. Specifically, we will address
the following aims. (i) Elucidate the cytoplasmic pathway comprising CblC, that displays a “jack of all trades”
style chemical versatility as it processes varied incoming B12 derivatives to a common cob(II)alamin
intermediate, and CblD, an elusive protein bearing strong structural resemblance to CblC, but lacking its ability
to bind B12. The cytoplasmic pathway terminates in MS and we will address how B12 is loaded from a novel
CblD-thiolato-Co(II)-CblC intermediate that we have discovered, and assess the dependence of this transfer
process on yet another chaperone, MS reductase. (ii) Elucidate the mitochondrial pathway comprising the
enzyme, adenosyltransferase, which doubles as an escort, synthesizing the active 5´-deoxyadenosylcobalamin
form of the cofactor and transferring it to MCM in a process that is gated by CblA, a GTPase. We will build on
our discovery of an unprecedented strategy for cofactor retention involving sacrificial cobalt-carbon bond
homolysis that is triggered when MCM acceptor sites are unavailable, and elucidate the rationale for
equilibrating MCM-G-protein oligomeric complexes that are nucleotide sensitive. (iii) Elucidate the intersections
between B12 and itaconate, an immunomodulatory molecule that is linked to the recently de-orphaned
citramalyl-CoA lyase. Itaconyl-CoA (the dehydrated form of citramalyl-CoA) is a potent inhibitor of human
MCM. We will determine the underlying mechanism of B12 deficiency when citramalyl-CoA lyase is missing (as
in ~3-6% of some populations) and whether mycobacterial MCM represents an additional target of itaconate
for shutting down pathogenic cholesterol-dependent energy metabolism.

## Key facts

- **NIH application ID:** 10086462
- **Project number:** 5R01DK045776-30
- **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:** 2021
- **Award amount:** $446,160
- **Award type:** 5
- **Project period:** 1998-02-01 → 2023-01-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10086462

## Citation

> US National Institutes of Health, RePORTER application 10086462, B12 Trafficking and Inherited Defects (5R01DK045776-30). Retrieved via AI Analytics 2026-05-25 from https://api.ai-analytics.org/grant/nih/10086462. Licensed CC0.

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