Urea Cycle Nitrogen Atom Sources
The urea cycle is the primary pathway for removing excess nitrogen from the body, converting ammonia into urea. Urea contains two nitrogen atoms, and understanding their origin is crucial.
Nitrogen Atom Origins Explained
The two nitrogen atoms incorporated into the urea molecule are sourced from:
- First Nitrogen Atom ($N_1$): This nitrogen comes directly from a free ammonium group ($NH_4^+$). Ammonia is a byproduct of amino acid metabolism and is transported to the liver, where it enters the urea cycle by reacting with bicarbonate to form carbamoyl phosphate.
- Second Nitrogen Atom ($N_2$): This nitrogen is supplied by the amino acid aspartate. Aspartate donates its amino group ($-\text{NH}_2$) during the reaction with citrulline to form argininosuccinate, a key intermediate in the cycle.
Urea Cycle Nitrogen Flow
The sequence of nitrogen incorporation is:
- Ammonia ($NH_4^+$) provides the first nitrogen ($N_1$) via carbamoyl phosphate synthesis.
- Aspartate provides the second nitrogen ($N_2$) during the formation of argininosuccinate from citrulline.
- Urea is ultimately synthesized from arginine, containing both $N_1$ and $N_2$.
Thus, the two nitrogen atoms in urea originate from a free ammonium group and aspartate.
Important Questions from Metabolism Glycolysis Nucleotide
- The oxidative phase of the hexose monophosphate pathway generates ________ moles of NADPH per mole of glucose 6-phosphate (answer in integer).
- Which one of the following enzymes is NOT inhibited by glucose-6-phosphate?
- Assume that in the oxidative branch of the pentose phosphate pathway, each glucose-6-phosphate molecule generates the following products: 2 molecules of NADPH, 1 molecule of $\text{CO}_2$ and 1 molecule of ribulose-5-phosphate.
If 30 molecules of glucose-6-phosphate enter this pathway, and you randomly draw one molecule from the products of the pathway, the probability that this molecule is NADPH is __________ (round off to one decimal place). - Match the hormones in Group I with their metabolic precursor in Group II
Group I Group II P. 17-$\beta$ estradiol 1. Arachidonic acid Q. Thromboxane A2 2. Tyrosine R. Epinephrine 3. $\beta$-carotene S. Retinoic acid 4. Cholesterol Which of the following statements are TRUE for respiration?
P. The conversion of one molecule of pyruvate to three molecules of $CO_2$ generates four molecules of NADH
Q. Fructose 6-phospate is the principal substrate for glycolysis
R. The oxidation of glucose 6-phosphate to 6-phosphogluconate is the first step in the oxidative pentose phosphate pathway
S. The mitochondrial 'alternative oxidase' provides an alternative pathway for transfer of electrons from ubiquinone to oxygen utilizing proton pumping complex of the respiratory chain
