Proteins are responsible for nearly all bodily and cellular functions from structural proteins in bones to contractile proteins in muscles that transport proteins in blood plasma to hormones antibodies, cell receptors, ion channels, and enzymes that catalyze almost every chemical reaction in biological systems. Proteins are polymers of amino acids linked together by peptide bonds. An amino acid consists of an amino group, a carboxyl group, and a unique side chain, connected to a central carbon, the α-carbon. Instead of being an extended chain of amino acids, a protein usually folds into a three-dimensional conformation that is critical for its functions. The structure forms as a result of interactions between the side chains of amino acids and are thus dictated by the amino-acid sequence. Of the 20 amino acids that make up proteins, nearly half are essential, meaning the body cannot synthesize them and must get them from the di...
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