Question

Since the \(K_{m}\) of aminotransferases for amino acids is much higher than that of aminoacyl-tRNA synthetases, A. at low amino acid concentrations, protein synthesis takes precedence over amino acid catabolism. B. liver cannot accumulate amino acids. C. amino acids will undergo transamination as rapidly as they are delivered to the liver. D. any amino acids in excess of immediate needs for energy must be converted to protein. E. amino acids can be catabolized only if they are present in the diet.

Short answer

A. At low amino acid concentrations, protein synthesis takes precedence over amino acid catabolism.

Step-by-step solution

Understanding \(K_{m}\)

\(K_{m}\) stands for the Michaelis-Menten constant, which is a measure of the substrate concentration required for an enzyme to reach half of its maximum reaction velocity. A higher \(K_{m}\) indicates that a higher concentration of the substrate is needed to achieve the same catalytic efficiency as a lower \(K_{m}\) enzyme. In this problem, we know that aminotransferases have a higher \(K_{m}\) for amino acids than aminoacyl-tRNA synthetases.

Evaluate Statement A

A. At low amino acid concentrations, protein synthesis takes precedence over amino acid catabolism. This statement is true because the lower \(K_{m}\) of aminoacyl-tRNA synthetases allows them to bind amino acids more efficiently and catalyze protein synthesis even at low amino acid concentrations. Aminotransferases, with higher \(K_{m}s\), need higher amino acid concentrations to effectively catalyze amino acid catabolism.

Evaluate Statement B

B. Liver cannot accumulate amino acids. This statement is not necessarily true. The liver can accumulate amino acids, just not as efficiently as aminoacyl-tRNA synthetases. Moreover, liver function is not solely determined by the \(K_{m}\) values of these enzymes.

Evaluate Statement C

C. Amino acids will undergo transamination as rapidly as they are delivered to the liver. This statement is not true. Given the higher \(K_{m}\) of aminotransferases, they need higher concentrations of amino acids to be active and effectively catalyze transamination. At lower concentrations, aminoacyl-tRNA synthetases will react more readily with amino acids.

Evaluate Statement D

D. Any amino acids in excess of immediate needs for energy must be converted to protein. This statement is also not true. Although protein synthesis does require amino acids, it is not the only process that utilizes them. Amino acids can also be catabolized for energy or converted to other metabolic intermediates.

Evaluate Statement E

E. Amino acids can be catabolized only if they are present in the diet. This statement is not true. Amino acids can be synthesized in the body and are not exclusively dependent on the dietary intake. Moreover, this statement doesn't relate to the \(K_{m}\) information given.

Conclusion

Based on analyzing each statement and comparing them to the information given about the \(K_{m}\) values of aminotransferases and aminoacyl-tRNA synthetases, the correct answer is statement A: at low amino acid concentrations, protein synthesis takes precedence over amino acid catabolism.

Key concepts

Aminotransferases

Aminotransferases are vital enzymes in the metabolism of amino acids; they facilitate the transfer of an amino group from an amino acid to an \(\alpha\)-keto acid, a process known as transamination. This biochemical reaction is crucial in both the synthesis and the degradation of amino acids.

Considering their function, aminotransferases often have a relatively high Michaelis-Menten constant (\(K_m\)) for their amino acid substrates. A higher \(K_m\) value implies these enzymes require greater concentrations of substrate to be effective. This characteristic influences how the body prioritizes the use of amino acids: at lower concentrations, the body will tend to use amino acids for processes facilitated by enzymes with a lower \(K_m\), such as protein synthesis, rather than for catabolism, which relies on aminotransferases.

Aminoacyl-tRNA Synthetases

Aminoacyl-tRNA synthetases are a class of enzymes that play a pivotal role in protein synthesis. They catalyze the attachment of amino acids to their respective transfer RNA (tRNA) molecules - a process necessary for translating the genetic code into functional proteins. Each amino acid has a specific synthetase that recognizes both it and its corresponding tRNA.

These enzymes generally have a lower \(K_m\), meaning they can bind and react with amino acids even when the latter are in low concentration. In a cellular milieu where amino acids are scarce, aminoacyl-tRNA synthetases effectively secure the amino acids needed for protein synthesis, asserting the cellular preference for building proteins over catabolizing amino acids when their levels are low.

Protein Synthesis

Protein synthesis is the process by which cells build proteins, the workhorses of the cell that perform a multitude of functions, from catalyzing metabolic reactions to providing structural support. It involves the translation of nucleotide sequences from messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins.

The lower \(K_m\) of aminoacyl-tRNA synthetases facilitates this process, as they are able to bind amino acids efficiently even at low concentrations, prioritizing the synthesis of proteins over their breakdown for energy—a confirmation of the organism's tendency to conserve resources and ensure essential proteins are produced.

Amino Acid Catabolism

Amino acid catabolism refers to the breakdown of amino acids for energy production and the synthesis of other compounds. This process typically occurs when there's an excess of amino acids or when the body needs to generate energy during fasting or strenuous exercise.

Aminotransferases, with their higher \(K_m\), come into play under these conditions, operating efficiently when amino acid levels are high. The catabolism of amino acids is not contingent upon dietary intake alone; amino acids can also be synthesized de novo or through the breakdown of existing proteins within the body. Due to this flexibility, the body can balance between conserving amino acids for protein synthesis and exploiting them for energy and other metabolic needs, depending on the organism's state and available resources.