Final answer:
A protein signature is a unique pattern of proteins that can be associated with a specific disease or condition and relates closely to changes in the protein's structural levels, particularly beyond its primary structure.
Step-by-step explanation:
A protein signature can refer to a unique pattern of proteins associated with a disease or condition, specifically answers (a) and (d) from the provided references. This is notable in the context of biomarkers where a distinct set of proteins is present in a diseased state. Understanding the structure of proteins is crucial in this context, where proteins have different structural levels including the primary, secondary, tertiary, and quaternary structures. The primary structure is the sequence of amino acids, the secondary structure involves folds due to hydrogen bonds, the tertiary structure is the overall shape of the protein, and the quaternary structure involves multiple protein units associating. The secondary, tertiary, and quaternary structures are essential for the biological function of proteins. Changes, such as protein misfolding, in these structures can lead to diseases.