Since we're in a musical mood...
WE’RE HERE BECAUSE UREA
(Tune: “The Bold Gendarmes’”)
The endogenous repletion of water in the sea
Lets nitrogenous excretion proceed quite easily
For ammonia though toxic is quickly washed away
So fish excrete (4 times) the ammonotelic way (repeat)
But new terrestrial creatures to survive where it was dry
Developed metabolic features in order to detoxify
Since urea is quite soluble and its doesn’t make you sick
We each excrete (4 times) as a ureotelic (repeat)
When protein breakdown is induced (to make new glucose, say)
Amino acids thus produced give nitrogen away
Keto acids are acceptors; and oxaloacetate
Transaminates …….. giving rise to aspartate
Glutamate too may be produced from oxoglutarate
And now that it’s been introduced deamination is its fate
For inside each mitochondrion of every liver cell
Is GDH ….. (reducing NAD as well)
Ammonia that’s thus set free combined with CO2
Utilising ATP (and an extra squiggle too)
The effector of the synthetase is acetyl glutamate
The product formed …… carbamoylphosphate
Two amino acid oddities now enter on the scene
The essential commodities, ornithine and citrulline
Ornithine starts in the cytosol, citrulline in mitos, free
Then they exchange …… electrogenically
Carbamoylphosphate carbamylates the ornithine
So we get a kind of steady state generating citrulline
Citrulline now is exported, then combined with aspartate
And generates ….. argininosuccinate
That Schiff base condensation utilises ATP
But there is now elimination and fumarate’s set free
Fumarate through citric cycle yields oxaloacetate
That then in turn ….. gives another aspartate
That cleavage mentioned just before also yielded arginine
And what this pathway’s called a cycle for can now readily be seen
For arginine is hydrolysed regenerating ornithine
Which can exchange ….. for another citrulline
That arginase reaction then also yielded urea
(Aspartate gave one nitrogen, one from ammonia)
And we thus complete the cycle that let us leave the sea
Sing urea …… which set the people free
"Scientists are people too. So when they go to the bar, they sing these songs"