Infantile cardiomyopathy is a devastating disorder that is fatal during the first year of life due to defects in the function of heart muscles resulting from mitochondrial dysfunction. A study, performed by Götz et al. [(2011). Am. J. Hum. Genet. 88:635–642), identified two different causative mutations in the gene for mitochondrial alanyl-tRNA synthetase (mtAlaRS). One mutation changes a leucine residue at amino acid position 155 to arginine (p.Leu155Arg). The other mutation changes arginine at position 592 to tryptophan (p.Arg592Trp). The mtAlaRS enzyme has an N-terminal domain (amino acids 36–481) that catalyzes tRNA aminoacylation and an internal editing domain (amino acids 484–782) that catalyzes deacylation in the case that the tRNA is charged with the wrong amino acid.
Which mutation would you predict has a more severe impairment of translation in mitochondria, and why?

Verified step by step guidance

1. Understand the structure and function of the mtAlaRS enzyme: The mitochondrial alanyl-tRNA synthetase (mtAlaRS) enzyme has two key domains. The N-terminal domain (amino acids 36–481) is responsible for catalyzing tRNA aminoacylation, which is the process of attaching the correct amino acid (alanine) to its corresponding tRNA. The internal editing domain (amino acids 484–782) ensures fidelity by catalyzing deacylation if the tRNA is charged with the wrong amino acid.
2. Analyze the location of the mutations: The p.Leu155Arg mutation occurs within the N-terminal domain, which is directly involved in the aminoacylation process. The p.Arg592Trp mutation occurs within the internal editing domain, which is responsible for correcting errors in amino acid attachment.
3. Predict the functional impact of the p.Leu155Arg mutation: Since this mutation is located in the N-terminal domain, it is likely to impair the enzyme's ability to catalyze the aminoacylation of tRNA. This would directly disrupt the attachment of alanine to its tRNA, leading to a severe impairment of mitochondrial translation.
4. Predict the functional impact of the p.Arg592Trp mutation: This mutation is located in the internal editing domain, which is responsible for error correction. While this mutation may reduce the enzyme's ability to correct errors, it does not directly affect the aminoacylation process. Therefore, its impact on mitochondrial translation may be less severe compared to the p.Leu155Arg mutation.
5. Conclude which mutation has a more severe impact: Based on the functional roles of the two domains, the p.Leu155Arg mutation in the N-terminal domain is predicted to have a more severe impairment of mitochondrial translation because it directly disrupts the aminoacylation process, which is essential for protein synthesis in mitochondria.

Key Concepts

Mitochondrial Function and Translation

Mitochondria are essential organelles responsible for energy production and play a critical role in protein synthesis. They contain their own genetic material and machinery for translating mitochondrial genes. The process of translation in mitochondria involves the charging of tRNA with specific amino acids, which is crucial for synthesizing mitochondrial proteins. Any dysfunction in this process can lead to severe cellular consequences, particularly in energy-demanding tissues like the heart.

Aminoacyl-tRNA Synthetase (AlaRS) Function

Aminoacyl-tRNA synthetases are enzymes that attach the correct amino acid to its corresponding tRNA, a process vital for accurate protein synthesis. The mitochondrial alanyl-tRNA synthetase (mtAlaRS) specifically charges tRNA with alanine, ensuring that proteins are synthesized correctly. Mutations in this enzyme can disrupt its function, leading to mischarged tRNAs and impaired translation, which can have severe effects on mitochondrial protein production and overall cellular function.

Impact of Specific Mutations on Protein Function

Mutations in genes can lead to changes in the amino acid sequence of proteins, potentially altering their structure and function. The severity of a mutation's impact often depends on its location and the nature of the amino acid change. For instance, a mutation that changes a hydrophobic amino acid to a charged one may disrupt protein folding or function more severely than a conservative substitution. Understanding the specific mutations in mtAlaRS and their effects on enzyme activity is crucial for predicting the severity of translation impairment.