Biochemistry: following Carbons in TCA cycle

[lovelyrwwr]

Hello all,

 

this is a question I have on a practice exam.

 

 

If the glucose is labeled with

C-14 at the 1-position, what fraction of the isotope will not have

been released after each two-carbon fragment has gone through four complete turns of the cycle?

 

 

The answer: Glucose labeled in the 1-position will produce methyl-labeled acetyl CoA. After four complete

turns, one fourth of the input-labeled carbon will be present in oxaloacetate.

 

I have read the Lehninger "Principles of BIochemistry" textbook many times on how to go about reaching this answer but I still do not understand. please help!