Question

Draw the pathway from ammonia and bicarbonate to urca as it occurs in the urea cycle. Show the structures and names of all reactants and products, as well as the names of the enzymes. You do not need to draw the structures of ATP, NAD, COA, etc., but do show them as reactants or products in the appropriate places. You do not need to show the mechanisms of the enzymes.

Answer 1

The urea cycle converts ammonia to urea in five steps catalyzed by different enzymes, starting with the formation of carbamoyl phosphate and ending with the production of urea and the regeneration of L-ornithine. The first two steps take place in the mitochondria, and the last three occur in the cytosol.

Step-by-step explanation:

The urea cycle is a biochemical process that converts ammonia into urea to prevent toxic levels of ammonia in the body. This cycle predominantly takes place in the liver and, to a lesser extent, in the kidneys. Below is a summary of the urea cycle starting from ammonia and bicarbonate:

1. Carbamoyl phosphate synthesis: Ammonia combines with bicarbonate to form carbamoyl phosphate in the mitochondrial matrix. This reaction is catalyzed by the enzyme carbamoyl phosphate synthase-I and requires two ATP molecules, Mg2+, and N-acetyl glutamate as an activator.
2. The carbamoyl phosphate then reacts with the amino acid L-ornithine to produce citrulline. The enzyme that catalyzes this reaction is ornithine transcarbamylase. This step occurs in the mitochondria as well.
3. Citrulline then moves to the cytosol, where it is combined with aspartate to form argininosuccinate, mediated by the enzyme argininosuccinate synthase. This step utilizes ATP.
4. Argininosuccinate is then cleaved into arginine and fumarate by argininosuccinase.
5. The last step in the urea cycle involves the enzyme arginase, which hydrolyzes arginine into urea and L-ornithine, the latter of which re-enters the cycle.

Ammonium ions are formed from the breakdown of amino acids before entering the urea cycle through transamination reactions necessary for the Krebs cycle. The urea produced is then eliminated via the kidneys.

The blood urea nitrogen (BUN) is a clinical measure indicative of kidney function, reflecting the effectiveness of the urea cycle in processing ammonia.