Question about Vit B12 on a practice exam

[qman123]

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Hey, there was a question on an exam in which the patient obviously had B12 deficiency and asks what this impairs:
There were two choices that for some reason I couldn't decide:
a) DNA synthesis
b) Fatty acid oxidation

Since b12 traps folate --> Dna syth should be messed up
but b12 is also used to in the odd chain fatty acid pathway.

can any clear this up?

 

[Sol Rosenberg]

Hey, there was a question on an exam in which the patient obviously had B12 deficiency and asks what this impairs:
There were two choices that for some reason I couldn't decide:
a) DNA synthesis
b) Fatty acid oxidation

Since b12 traps folate --> Dna syth should be messed up
but b12 is also used to in the odd chain fatty acid pathway.

can any clear this up?

I remember this question and thinking the same thing.

Ultimately, I think the better (and correct) answer is DNA synthesis, because B12 isn't involved in the actual oxidation of fatty acids (Oxidation is breakdown into Acetyl-CoA and, in the case of odd-chain fatty acids, 1 propionyl-CoA.) The reaction in that pathway that uses B12 as a cofactor is the methylmalonyl-coA MUTASE reaction, so I think all the fatty acid oxidation is done by the time B12 comes into the picture.

But, that is a bit of a stretch, since the entire pathway is part of odd-chain fatty acid oxidation, so I think that was a bit of a crappy question.

Also, B12 deficiency will obviously have no effect on even-chain fatty acid oxidation, of course, so I can kind of see how that is a "wrong" answer for this question.....

 

[hoqhuuep]

You're right, it does both. What I believe the question writers were getting at, wasn't the biochemical mechanisms of B12, but the pathology of megaloblastic anemia (as in the RBC precursors can't synthesize DNA).