Explanation:
Describe the steps in the urea cycle.
The urea cycle has four enzymatic steps. In the first one, carbamoyl phosphate donates its carbamoyl group to ornithine, forming citrulline and releasing phosphate. This reaction occurs by a nucleophilic acyl substitution mechanism, where ornithine carboxylic oxygen acts as a nucleophile attacking carbamoyl phosphate carbonyl, and phosphate is the leaving group. This reaction is catalyzed by the enzyme ornithine transcarbamoylase, and the produced citrulline migrates from mitochondria to cytosol.
In the second stage, Aspartate is derived from citric acid cycle oxaloacetate. Oxaloacetate undergoes transamination with glutamate with the aid of PLP, forming aspartate and α-ketoglutarate. Citrulline is then activated by attacking ATP-α-phosphorus to form citrulline-AMP and pyrophosphate. The aspartate amino group then attacks the carbon of the citrulyl-AMP imine, with AMP being the leaving group and forming arginine succinate.
In the third stage of the urea cycle, the arginine succinate enzyme breaks down the arginine succinate molecule, forming arginine and fumarate. The fumarate returns to the citric acid cycle in mitochondria.
In the fourth stage of the urea cycle, arginine is hydrolyzed by the enzyme arginase, producing urea and ornithine. Ornithine rebounds to mitochondria to restart the cycle while urea is excreted. All α-keto acids will be metabolized, some enter the glycolytic pathway, others undergo β-oxidation, and others directly enter the citric acid cycle.
Is it linked to the citric acid cycle? Yes.
