Final answer:
Steric hindrance in the editing domain of isoleucyl-tRNA synthetase ensures that valine is rarely incorporated in place of isoleucine, thus maintaining the accuracy of protein translation.
Step-by-step explanation:
The aminoacyl-tRNA synthetase for isoleucine rarely incorporates valine despite their similar structures because of steric hindrance. This steric hindrance arises in the editing domain of the synthetase enzyme, which has evolved to differentiate between the side chains of these two amino acids with high precision. Although valine and isoleucine are similar, the active site of the isoleucyl-tRNA synthetase has a specific three-dimensional structure that can only accommodate isoleucine correctly. If valine is mistakenly attached, the editing mechanism of the synthetase will recognize and remove it before the tRNA is used in protein synthesis. This specificity is crucial because it prevents the misincorporation of amino acids, maintaining the fidelity of protein translation.