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Nonessential in normal health but essential amino acid under conditions of enhanced growth or surgical recovery- Case discussion
Case details
An 8-year-old child requires surgical intervention and substantial recovery time in the hospital. Therefore, a nutritionist recommends a specific dietary plan. The plan includes a protein-rich diet with the special supplementation of an amino acid that is typically considered nonessential but becomes an essential amino acid under enhanced growth or surgical recovery conditions. Which of the following amino acids should be considered in this dietary plan?
- Alanine
- Arginine
- Glycine
- Serine
- Tyrosine
The correct answer is B. Arginine. Arginine is a semi-essential amino acid. This is because it can be synthesized in the human body. Still, during growth, reproduction, or recovery from illness, its requirement is increased. Under those conditions, an imbalance is set between demand and supply; thus, it has to be supplied in the diet, making it essential during those conditions. Arginine is synthesized in the urea cycle, from argininosuccinic acid, by the action of the Argininosuccinase enzyme (Figure). However, the urea cycle operates only in the liver, thus limiting the synthesis of Arginine through this pathway. Nevertheless, the Arginine produced through this pathway is sufficient for healthy adults. Still, it may not be adequate in times of growth or repair, where increased protein synthesis demands an increased supply of amino acids. Thus, in growing children and in adults following surgery, Arginine becomes an essential amino acid.
Figure- Synthesis of Arginine. Enzymes- CPS I- Carbamoyl synthetase-1, OTC- Ornithine transcarbamoylase, ASS- Argininosuccinic acid synthetase, ASL- Argininosuccinic acid lyase, and ARG- Arginase.
As regards other options
Option A-Alanine is a nonessential amino acid. It can be synthesized from pyruvate by transamination. Option C- Glycine is a nonessential amino acid; it can be synthesized in sufficient amounts even during increased requirements. Furthermore, it can be synthesized de novo or from serine. Option D-Serine can be synthesized from Glycine, phosphoglycerate, or hydroxy pyruvate. Its production is also not limited during conditions of increased demand. Option E-Tyrosine can be synthesized from phenylalanine, but it becomes an essential amino acid in Phenylketonuria, which occurs due to a deficiency of phenylalanine hydroxylase. This enzyme catalyzes the conversion of phenylalanine to tyrosine. Hence the most suited option out of all is Arginine.