Which level of protein structure refers to the amino acid code?

The level of protein structure that refers to the amino acid code is known as the primary structure. This primary structure is essentially a linear sequence of amino acids bonded together by peptide bonds, which dictates the unique characteristics and functionalities of the protein. The specific order and composition of these amino acids determine how the protein will fold and ultimately what role it will serve in biological processes.

In understanding protein structures, it's essential to recognize that higher levels of organization, such as secondary, tertiary, and quaternary structures, involve the folding and interaction of the primary structure. These levels explain how proteins achieve their final three-dimensional shapes and how they interact with one another, but they rely on the sequence defined in the primary structure to do so. Thus, focusing on the amino acid sequence, which is the defining feature of primary structure, is crucial in comprehending protein function and behavior.