Proteins are chemical compounds that contain the same atoms as carbohydrates and lip- ids—carbon (C), hydrogen (H), and oxygen (O)—but proteins are different in that they also contain nitrogen (N) atoms. These nitrogen atoms give the name amino (nitrogen containing) to the amino acids that form the links in the chains we call proteins.
The Structure of Proteins
About 20 different amino acids may appear in proteins. All amino acids share a common chemical “backbone,” and it is these backbones that are linked together to form proteins. Each amino acid also carries a side group, which varies from one amino acid to another. The side group makes the amino acids differ in size, shape, and electrical charge. The side groups on amino acids are what make proteins so varied in comparison with either carbohydrates or lipids.
The 20 amino acids can be linked end to end in a virtually infinite variety of sequences to form proteins. When two amino acids bond together, the resulting structure is known as a dipeptide. Three amino acids bonded together form a tripeptide. As additional amino acids join the chain, the structure becomes a polypeptide. Most proteins are a few dozen to several hundred amino acids long.
Polypeptide chains twist into complex shapes. Each amino acid has special characteristics that attract it to, or repel it from, the surrounding fluids and other amino acids. Because of these interactions, polypeptide chains fold and intertwine into intricate coils and other shapes. The amino acid sequence of a protein determines the specific way the chain will fold.
The dramatically different shapes of proteins enable them to perform different tasks in the body. Some, such as hemoglobin in the blood, are globular in shape; some are hollow balls that can carry and store materials within them; and some, such as those that form tendons, are more than 10 times as long as they are wide, forming stiff, sturdy, rodlike structures.
Nonessential and Essential Amino Acids
More than half of the amino acids are nonessential amino acids, meaning that the body can make them for itself. Proteins in foods usually deliver these amino acids, but it is not essential that they do so. There are other amino acids that the body cannot make at all, however, and some that it cannot make fast enough to meet its needs. The proteins in foods must supply these nine amino acids to the body; they are therefore called essential amino acids. (Some researchers refer to essential amino acids as indispensable and to nonessential amino acids as dispensable.)
Under special circumstances, a nonessential amino acid can become essential. For example, the body normally makes tyrosine (a nonessential amino acid) from the essential amino acid phenylalanine. If the diet fails to supply enough phenylalanine or if the body cannot make the conversion for some reason (as happens in the inherited disease phenylketonuria), then tyrosine becomes a conditionally essential amino acid.
Chemically speaking, proteins are more complex than carbohydrates or lipids; proteins are made of some 20 different nitrogen-containing amino acids, 9 of which the body cannot make (they are essential).
Each amino acid contains a central carbon atom with an amino group, an acid group, a hydrogen atom, and a unique side group attached to it.
The distinctive sequence of amino acids in each protein determines its shape and function.