Question

1. If the keto carbon atom (#2) of pyruvate were to be labeled with ¹⁴C , which of the carbon atom(s) of oxaloacetate would be labeled at the end of one turn of the citric acid cycle? (numbering similar to sugars with #1 the carboxylic acid carbon, #2 the keto group, #3 the methyl group) [Ogston’s symmetry argument was that citric acid exhibited chirality when it bound to enzyme explaining why neither carbon of acetylCoA came off as CO₂ in the first turn of the cycle—the same chirality argument does not hold for succinate because it is symmetrical about a bond]

a) None. The label will be lost in CO₂.

b) #1 carbon

c) #2 carbon

d) #3 carbon

e) #4 carbon

f) the #2 and #3 carbons will be labeled

g) the #1 and #4 carbons will be labeled

2. If acetyl-CoA is labeled at the methyl carbon (#2) with ¹⁴C, how many turns of the citric acid cycle (starting at the formation of Citrate) will be required to first release some labeled CO₂ ?

a) less than one complete turn

b) one complete turn (back to citrate)

c) more than one, but less than two complete turns

d) only after two complete turns

3. How many ATP or equivalents would be generated from the Succinyl-CoA step after the equivalent of all its four carbons (don’t worry about position) were converted to CO₂? State your assumptions, e.g. ample AcCoA is present.