Cobalamin metabolism in mammalian cells.The pathways by which vitamin B₁₂ (cobalamin) is taken up by cells and converted to its active coenzyme derivatives are shown. The steps affected by inborn errors of cobalamin metabolism (cblA-cblH, mut), as well as MTHFR deficiency are shown. The methionine synthase reaction (cblG) is shown twice in order to separate folate and homocysteine metabolism.

Abbreviations: AdoCbl, Adenosylcobalamin; AdoMet, S-adenosylmethionine; 5-Methyl-THF, methyltetrahydrofolate; MTHFR, methylenetetrahydrofolate reductase; TCII, transcobalamin II.