Biochemistry at The Biology Project

B12/Folate Problem set

Question 1: "Folate trapping" in a vitamin B12 deficiency

Tutorial to help answer the question

Insufficient Vitamin B12 often creates an apparent folate deficiency by trapping which folate intermediate?

A. folic acid
B. dihydrofolate (DHF)
C. tetrahydrofolate (THF)
D. N5-methyl THF
E. N10-formyl THF


Accumulation of N5-methyl tetrahydrofolate in Vitamin B12 deficiency

Fig. 1 - Site of folate trap

Several forms of folate are shown here, all of which must pass thru tetrahydrofolate (THF), the central intermediate. The form trapped by a B12 deficiency is N5-methyl THF, for the following two reasons: i) without B12, N5-methyl THF cannot donate its methyl group to make methyl-B12; ii) this is the only way N5-methyl THF can be converted back to THF for recycling. So, sooner or later, a significant portion of the body's folate winds up in this dead end and becomes useless. The next question explores the N5-methyl THF-requiring reaction a bit further...

[Problem 1] [Answer] [Problem 2]

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