Biochemistry at The Biology Project

B12/Folate Problem set

Question 4: A second reaction requiring B12

Tutorial to help answer the question

Buildup of which metabolite in serum is a specific indicator of a vitamin B12 deficiency?

A. methylmalonic acid
B. methylmalonyl CoA
C. N5-methyl THF
D. methionine
E. succinyl CoA


The methylmalonic aciduria connection

Fig. 4 - Buildup of methylmalonate and propionyl CoA in B12 deficiency

Methylmalonyl CoA mutase catalyzes one of only two known B12-requiring reactions, and the only one to occur in mitochondria. B12 must first be converted to its adenosyl-B12 form before it can serve as a cofactor for this enzyme. Disruption of this reaction leads directly to methylmalonic aciduria, with associated neurological symptoms and learning deficiencies. A contributing factor may be a secondary backup of propionyl CoA which may substitute for acetyl CoA in the citrate synthase reaction to form methyl citrate, a TCA cycle poison. Presumably, this scenario is particularly serious for neural tissue due to its high oxidative demands. Now on to folate metabolism...

[Problem 4] [Answer] [Problem 5]

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