December 7, 2021
Proteins are large groups of biomolecules formed from one or more long chains of amino acids. Proteins have many functions in living things, including speeding up metabolic reactions, replicating DNA, responding to stimuli, giving cells and bodies shape, and moving molecules from one location to another. The main difference between one protein and another is its amino acid sequence, which is determined by the nucleotide sequence of its genes, and usually causes the protein to fold into a special three-dimensional structure according to its function. A number of amino acids form a straight chain called a polypeptide. A protein consists of at least one long polypeptide. Short polypeptides (with fewer than 20–30 amino acids) are not usually considered proteins, but are called peptide molecules or oligopeptides. Each amino acid in a protein is bound to a nearby amino acid by a peptide bond. The order of the amino acids in a protein is determined by the sequence of the genes encoded in the genetic code. In general, the genetic code produces 20 standard amino acids, although some organisms have additional amino acids. Shortly after or even during synthesis, residues in proteins are often chemically modified through posttranslational modification processes that alter the physical and chemical properties, folding, stability, activity, and function of the protein. Some proteins have nonpeptide groups (not amino acids), which can be called cofactors and prosthetic groups. Several proteins can also work together to carry out certain functions, and such groups often form stable protein complexes. Once formed, proteins only exist for a certain period of time and then are degraded and recycled in the cell through the process of protein turnover. Protein life is measured by its half-life and covers a long range. Proteins can live from a few minutes to several years with an average lifespan of 1–2 days in mammalian cells. Abnormal or misfolded proteins degrade more rapidly, either because they are targeted for destruction or because they are unstable. Along with other giant biomolecules such as polysaccharides and nucleic acids, proteins are an essential part of organisms and are involved in almost all processes in cells. Some proteins are enzymes that function as catalysts in biochemical reactions and are vital for metabolism. Some proteins have a building or reinforcing function, for example the proteins actin and myosin in muscle and proteins in the cytoskeleton. Other proteins have important roles in cell signaling, immune response, cell adhesion, and the cell cycle. Animals need protein in their diet to obtain essential amino acids that cannot be synthesized in the body. The digestive system breaks down protein from food to be used in metabolism. Proteins can be purified from other cellular components using various techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography. Genetic engineering allows a number of methods to facilitate this purification. The methods commonly used to study protein structure and function are immunohistochemistry, location-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance, and mass spectrometry.