The name protein comes from the Greek proteios, meaning primary. Although many other important biomolecules exist, the emphasis on protein as fundamental is appropriate. Proteins serve as important structural components of cells. More importantly, almost all the catalysts, or enzymes, in biological systems are composed of proteins. Proteins are linear chains of amino acids joined by peptide bonds. Twenty amino acids are incorporated into a protein by translation. In some proteins, the amino acids are modified by subsequent post ‐translational events. The sequence of amino acids of a protein is termed its primary structure.
The amino acid chain, or backbone, forms one of a few secondary structures, based on the interactions of the peptide bond with nearby neighbors. The secondary structure that a chain forms is determined by the primary structure of the chain. Some amino acids favor one type of secondary structure, others prefer another, and still others are likely to form no particular secondary structure at all. Secondary structures are based on the interactions of closely neighboring amino acids.
The 20 amino acids differ in the nature of their side chains, the groups other than the repeating peptide unit. Interactions among the amino acid side chains within a single protein molecule determine the protein's tertiary structure. Tertiary structure is the most important of the structural levels in determining, for example, the enzymatic activity of a protein. Folding a protein into the correct tertiary structure is an important consideration in biotechnology. The usefulness of a cloned gene is often limited by the ability of biochemists to induce the translated protein product to assume the proper tertiary structure. (In the cell, specialized proteins, called chaperonins, help some proteins acheive their final structure.)
Finally, protein chains interact with each other as subunits associate to make a functional species. For example, hemoglobin, the mammalian oxygen carrier, contains two each of two different subunits. The ability of hemoglobin to deliver oxygen to the tissues is dependent on the association of these subunits. Interaction of proteins to form a multimer composed of several subunits is termed the protein's quaternary structure. Quaternary structure is often very important in determining the regulatory properties of a protein.