Proteins are large organic compounds made of amino acids
arranged in a linear chain and joined together between the
carboxyl atom of one amino acid and the amine nitrogen of
another. This bond is called a peptide bond. The sequence of
amino acids in a protein is defined by a gene and encoded in the
genetic code. Although this genetic code specifies 20 "standard"
amino acids, the residues in a protein are often chemically
altered in post-translational modification: either before the
protein can function in the cell, or as part of control
mechanisms. Proteins can also work together to achieve a
particular function, and they often associate to form stable
complexes. Like other biological macromolecules such as
polysaccharides and nucleic acids, proteins are essential parts
of all living organisms and participate in every process within
cells. Many proteins are enzymes that catalyze biochemical
reactions, and are vital to metabolism. Other proteins have
structural or mechanical functions, such as the proteins in