Each protein has what characteristic that defines it?

Each protein is defined by its unique sequence of amino acids, which determines its specific structure and function. Proteins are polymers made up of amino acids linked by peptide bonds, and the order in which these amino acids are arranged ultimately influences how the protein will fold and what role it will play in biological processes.

The distinct sequences allow for a vast diversity of proteins, each with unique properties and functionalities. This is why, for example, hemoglobin and antibodies, though both are proteins, perform entirely different functions in the body. Variability in the amino acid sequence is fundamental to the diversity of protein function across different organisms and within different tissues of the same organism.

Other options, such as a common structure across all proteins, high energy content, or identical composition across species, do not accurately represent the defining characteristic of proteins. While proteins may share certain structural features (like the primary, secondary, tertiary, and quaternary structures), it is their unique sequence of amino acids that truly distinguishes one protein from another. Similarly, proteins do not have identical compositions across species; they exhibit variability that is critical to their functionality.