In protein structure, which of the following levels of structure is defined by the sequence of amino acids?

The primary structure of a protein is defined by the unique sequence of amino acids that make up the polypeptide chain. This sequence is determined by the genetic code and dictates how the protein will fold and function. Each amino acid is linked together through peptide bonds, and the specific order of these amino acids influences the higher levels of protein structure and ultimately the protein’s shape and function.

Understanding primary structure is essential because any alterations, such as mutations in the amino acid sequence, can lead to significant changes in the protein's properties and roles in biological processes. The other levels of structure, such as secondary, tertiary, and quaternary, relate to how this primary sequence folds and interacts with itself or other polypeptide chains, but they do not define the linear arrangement of amino acids.