Proteins are the primary structural and functional polymers in living systems. They have a broad range of activities, including catalysis of metabolic reactions and transport of vitamins, minerals, oxygen, and fuels. Some proteins make up the structure of tissues, while others function in nerve transmission, muscle contraction, and cell motility, and still others in blood clotting and immunologic defenses, and as hormones and regulatory molecules. Proteins are synthesized as a sequence of amino acids linked together in a linear polyamide (polypeptide) structure, but they assume complex three-dimensional shapes in performing their function. There are approximately 300 amino acids present in various animal, plant and microbial systems, but only 20 amino acids are coded by DNA to appear in proteins. Many proteins also contain modified amino acids and accessory components, termed prosthetic groups. A range of chemical techniques is used to isolate and characterize proteins by a variety of criteria, including mass, charge, and three-dimensional structure. Proteomics is an emerging field which studies the full range of expression of protein in a cell or organism, and changes in protein expression in response to growth, hormones, stress, and aging.
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