https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2995210/

D. Sean Froese1 and Roy A. Gravel2,*
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Abstract
Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Vitamin B12, also known as cobalamin (Cbl), is a micronutrient that is synthesised only by microorganisms, yet is essential to human health. Cobalamin was first isolated by Smith (Ref. 1) and Rickes (Ref. 2), after Minot and Murphy (Ref. 3) showed that pernicious anaemia could be treated with oral liver extract. Later, vitamin B12 deficiency as a result of genetic disease was described despite adequate vitamin intake (Ref. 4). Some patients responded successfully to very high doses of vitamin B12, suggesting blocks in vitamin processing. These patients had homocystinuria and/or methylmalonic aciduria, implicating dysfunctional methionine synthase (MS) and/or methylmalonyl-CoA mutase (MUT or MCM). We now know that blocks in the intracellular processing of cobalamin into cofactor forms, methylcobalamin (MeCbl) for MS and adenosylcobalamin (AdoCbl) for MCM, or in the functional activity of MS or MCM result in inborn errors. These genetic blocks may be devastating in newborns or in early childhood. Understanding the genes, gene products and subcellular transport of vitamin B12 is important for minimising the disease burden from these disorders. This review outlines the present knowledge of cobalamin metabolism, with a focus on steps related to the intracellular human pathway and the initial cataloguing of cobalamin-utilisation disorders into complementation groups and biochemically distinct classes. Key to these discoveries have been the hundreds of patients who have been the source of cell cultures and DNA samples that have given us our current understanding of vitamin B12 utilisation in humans.

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Vitamin B12 structure

The structure of cobalamin was first solved by Hodgkin (Ref. 5) using x-ray crystallography. It is a large organometallic molecule, ~1300–1500 Da in size, and is the most chemically complex vitamin known. The focal point of vitamin B12 is the central cobalt atom, which has up to six ligands bound to it. Four of the ligands are the nitrogen atoms of the planar corrin ring that surround the cobalt atom (Fig. 1). The α-axial ligand, extending below the corrin ring, is a nitrogen of the 5,6-dimethylbenzimidazole (DMB) phosphoribosyl moiety that also attaches back to the corrin ring through one of its propionamide side chains. The upper or β-axial ligand varies, depending on the modification state of cobalamin (R-group in Fig. 1a). Functional β-axial ligands are methyl (MeCbl) or 5′-deoxyadenosyl (AdoCbl) groups. Additionally, a hydroxyl group (OHCbl) or a cyano group (CNCbl) can be bound as physiologically relevant β-axial ligands.

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