From which precursors that directly enter the urea cycle are the two nitrogen atoms found in...

From which precursors that directly enter the urea cycle are the two nitrogen atoms found in a molecule of urea derived from? How might the levels of these precursors be regulated relative to each other to optimize urea cycle cycling?

Expert Solution

In the urea cycle the two nitrogen atom comes from two different sources: Out of two one nitrogen atoms of the urea is transferred from aspartate amino acid while second . nitrogen atom is derived directly from free NH4+ions .

The free ammonium used in the urea cycle is largely released from glutamate by
glutamate dehydrogenase. This reaction occurs in the mitochondria. The carbamoyl
phosphate synthetase I and ornithine transcarbamoylase reactions also occur in the
mitochondria, allowing the ammonium to be handled under controlled conditions.
The remaining reactions of the urea cycle, however, occur in the cytoplasm.
The urea cycle is regulated by substrate availability and by the enzyme carbamoyl
phosphate synthetase I, which is regulated by N-acetylglutamate. Both N-acetyl-
glutamate and ornithine are synthesized from glutamate.

Increased urea synthesis is required when amino acid breakdown rate increase, generating excess nitrogen that must be excreted. Increase in these breakdown rates are signaled by increase in glutamatecconcentration through transamination reactions. This situation, in turn causes an increase in N-acetyl-glutamate synthesis, stimulating carbomyl phosphate synthetase and thus the entire urea cycle.

gladiator answered 2 years ago

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