Question

Illustrate and explain the importance of the urea cycle as it relates to nitrogen disposal. Explain the role of NAG in the regulation of CPSI activity and how this affects urea cycle function. Discuss how the urea cycle would be affected during a short-term fast (< 8 hr), long-term fast (> 24 hr), in response to a low (but adequate) protein diet, and in response to a high protein diet.

Answer 1

When proteins are break down, they are deaminated (-NH₂ group is removed) and converted to Ammonia.
NH₃ is extremely toxic in small concentrations and so it has to be removed quickly
In the Urea cycle, the Ammonia is converted to Urea - CO(NH₂)₂ , in the Liver which is less toxic
and can be filtered and removed dissolved in water as Urine.
Aquatic creatures like fish can release it directly into their environment, as NH₃ readily dissolves.
This is not possible for animals on land , as we need to store water.
So through the Urea cycle it is possible

In vertebrates, it is an essential allosteric cofactor for CPSI, the first enzyme of the urea cycle.
NAG is the allosteric cofactor, by word it means that NAG alters the activity of enzyme by means
of a conformational changes which are induced by a non-protein compound called cofactor. This is how
NAG regulates CPSI activity. It helps by removing nitrogen from amino acids.

Urea cycle will be regulated up when there is high protein intake , since more protein requires more nitrogen.
Short term regulation of the urea cycle is through mitochondrial CPSI activity which is regulated by NAG