Proteins constitute most of a cell's dry mass. They are not only the cell's building blocks but also execute nearly all the cell's functions. Proteins with catalytic activity, called enzymes, speed up many chemical reactions. Proteins embedded in the plasma membrane form channels and pumps that control the passage of small molecules into and out of the cell. Other proteins carry messages from one cell to another, or act as signal integrator that relay sets of signals inward from the plasma membrane to the cell nucleus. Contractile proteins are responsible for movement, and other specialized proteins act as antibodies, toxins and hormones. This section presents some basic concepts about protein structure that help to explain how proteins function in cells. More extensive converage of this topic is available in biochemistry textbooks and specialized books on protein chemistry.
Proteins consist of one or more linear polymers called polypeptides, which consist of various combinations of 20 different amino acids linked together by peptide bonds. When linked in polypeptides, amino acids are referred to as residues. The sequence of amino acids in each type of polypeptide is unique. It is specified by the gene encoding the protein and is read out precisely during protein synthesis. The polypeptides of proteins with more than one chain are usually synthesized separately. However, in some cases, a single chain is divided into pieces by cleavage after synthesis.
Polypeptides range widely in length. Small peptide hormones, such as oxytocin, consist of as few as nine residues, while the giant structural protein titin has more than 25,000 residues. Most cellular proteins fall in the range of 100 to 1000 residues. Without stabilization by disulfide bonds or bound metal ions, about 40 residues are required for a polypeptide to adopt a stable three-dimensional structure in water.

The sequence of amino acids in a polypeptide can be determined chemically by removing one amino acid at a time from the amino terminus and identifying the product. This procedure, called Edman degradation, can be repeated about 50 times before declining yields limit progress. Longer polypeptides can be divided into fragments of fewer than 50 amino acids by chemical or enzymatic cleavage, after which they are purified and sequenced separately. Even easier, one can sequence the gene or a complementary DNA (cDNA) copy of the messenger RNA for the protein and use the genetic code to infer the amino acid sequence. This approach misses posttranslational modifications. Analysis of protein fragments by mass spectrometry can be used to sequence even tiny quantities of proteins.