In the urea cycle, the second nitrogen of urea enters the cycle in the form of which of the following metabolites? a) alanine b) glutamine c) omithine d) aspartate e) arginine

In the urea cycle, the second nitrogen of urea enters the cycle in the form of D) Aspartate .

Explanation:

Urea cycle -

Also referred as the Ornitine cycle ,

This is biochemical pathway that can produce urea from ammonia . The urea cycle majorly takes place in the liver and few steps in the kidneys .

Steps involved in Urea Cycle are:

1. The formation of Carbamoyl Phosphate:

The process of the condensation of the bicarbonate ions along with ammonium ions that results in the formation of the carbomoyl phosphate , by the help of carbomoyl phosphate synthase -I enzyme that is present in the mitochondria of the liver .With the requirement of dicarboxylic acid  which is the N - acetyl glutamate and Magnesium ion , and requires 2 ATP .

2. Synthesis of Citrulline:

From the first step , the carbomyl phosphate formed now combines with the ornithine and synthesis cirtulline by using citrulline synthase as the enzyme .And the Citrulline can easily diffuse into the cytosol from the mitochondria .

3. Synthesis of Argininosuccinate:

Now in the cytosol , the Citrulline combines to amino acid aspartate and results in the formation of argininosuccinate , and the step is catalyzed by the enzyme argininosuccinate synthase . The step need AMP and Magnesium ion as the cofactor.  

4. Cleavage of Argininosuccinate:

Now, Argininosuccinase enzyme reversibly acts in order to cleave argininosuccinate to Fumarate and Arginine .

5. Cleavage of Arginine:

Under the influence of Arginase enzyme , Arginine is lysed to urea and ornithine .Therefore , arginine is referred as the semi - essential amino acid    

And lastly the Ornithine is regenerated and completes the Urea Cycle by the formation of Urea.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.