Proteins are complex molecule composed of amino acids and necessary for the chemical processes that occur in living organisms.
Proteins are basic constituents in all living organisms. Their central role in biological structures and functioning was recognized by chemists in the early 19th century when they coined the name for these substances from the Greek word proteios, meaning "holding first place." Proteins constitute about 80 percent of the dry weight of muscle, 70 percent of that of skin, and 90 percent of that of blood. The interior substance of plant cells is also composed partly of proteins. The importance of proteins is related more to their function than to their amount in an organism or tissue. All known enzymes, for example, are proteins and may occur in very minute amounts; nevertheless, these substances catalyze all metabolic reactions, enabling organisms to build up the chemical substances--other proteins, nucleic acids, carbohydrates, and lipids--that are necessary for life.
Proteins are sometimes referred to as macromolecular polypeptides because they are very large molecules and because the amino acids of which they are composed are joined by peptide bonds. (A peptide bond is a link between the amino group [-NH2] of one amino acid and the carboxyl group [-COOH] of the next amino acid in the protein chain.) Although amino acids may have other formulas, those in protein invariably have the general formula RCH(NH2)COOH, where C is carbon, H is hydrogen, N is nitrogen, O is oxygen, and R is a group, varying in composition and structure, called a side chain. Amino acids are joined together to form long chains; most of the common proteins contain more than 100 amino acids.
The vast majority of the proteins found in living organisms are composed of only 20 different kinds of amino acids, repeated many times and strung together in a particular order. Each type of protein has its own unique sequence of amino acids; this sequence, known as its primary structure, actually determines the shape and function of the protein.
Interactions among the amino acids cause the protein chain to assume a characteristic secondary structure and, under some circumstances, a tertiary structure. The secondary structure is a function of the angles formed by the peptide bonds that link together the amino acids. These bond angles are held in position by the development of hydrogen bonds between the nitrogen-bound hydrogen atom of one amino acid unit and the oxygen atom of another. Commonly, these hydrogen bonds cause the chain to assume a helical secondary structure--i.e., the backbone of the chain resembles a rope spirally wound along an imaginary tube.
The tertiary structure refers to the looping and folding of the protein chain back upon itself. Such a structure characterizes the globular proteins (i.e., those with a more or less spherical shape). Tertiary structure is determined largely by the side chains of the amino acids. Some side chains are so large that they disrupt the regular helical secondary structure of the chain, causing it to have kinks and bends. Furthermore, side chains that carry opposite electrical charges attract one another and form ionic bonds; those with like electrical charges repel one another. Hydrophobic side chains--i.e., those that are insoluble in water--cluster together at the centre of the folded protein, avoiding exposure to the aqueous environment. Hydrophilic side chains, which readily form hydrogen bonds with water molecules, are left on the outside of the protein structure.
Some proteins, such as hemoglobin, are composed of more than one protein subunit (polypeptide chain). The spatial conformation of these chains is known as the quaternary structure. Quaternary structure is maintained by the same kinds of forces that determine tertiary structure.
Excerpt from the Encyclopedia Britannica without permission.