TBR Biology Chapter 8 Passage 13 #81

[Jengreef]

The question asks what is the overall balanced reaction for the urea cycle?

The stated answer is 2NH4+ + HCO3- + 3ATP + H2O -> Urea + 2ADP + AMP + PPi + 2Pi

I chose NH4+ +HCO3- + 3ATP + Aspartate + H2O -> Urea + Fumarate + 2 ADP + AMP + PPi + 2Pi

The reason the explanation said my choice was wrong was because the Nitrogen count was not balanced. But if we count NH4 as one N and Aspartate as the source for the second N, it would balance out with Urea wouldn't it? Can anyone tell me anything else that is wrong with my choice over the stated chosen answer?

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[Captain Sisko]

The question asks what is the overall balanced reaction for the urea cycle?

The stated answer is 2NH4+ + HCO3- + 3ATP + H2O -> Urea + 2ADP + AMP + PPi + 2Pi

I chose NH4+ +HCO3- + 3ATP + Aspartate + H2O -> Urea + Fumarate + 2 ADP + AMP + PPi + 2Pi

The reason the explanation said my choice was wrong was because the Nitrogen count was not balanced. But if we count NH4 as one N and Aspartate as the source for the second N, it would balance out with Urea wouldn't it? Can anyone tell me anything else that is wrong with my choice over the stated chosen answer?

The only thing that I can think of is that it asks for the overall reaction, and aspartate is created by transaminating an oxaloacetate, which comes from malate which in turn comes from fumarate. So essentially your asp comes from fumarate plus nh4. Cancel fumarate on both sides and you have your answer.