What is a defining feature of proteins?

Proteins are defined by their composition, which consists of long chains of amino acids linked together through peptide bonds. This structure allows proteins to fold into complex shapes, leading to their diverse roles in biological systems, including enzymes, structural components, and transport molecules. Amino acids are the building blocks of proteins, and their specific sequences determine the protein's unique properties and functions.

The other choices do not accurately define proteins. While some compounds may consist solely of carbon and hydrogen, proteins specifically contain carbon, hydrogen, nitrogen, oxygen, and sometimes sulfur due to the presence of amino acids. Additionally, proteins can form various types of bonds, including ionic and hydrogen bonds, not just covalent bonds. Moreover, proteins serve a range of functions beyond energy storage, including catalyzing biochemical reactions and facilitating cellular processes. Thus, the defining characteristic of proteins is their composition as polymers of amino acids.