Protein

• a long chain of amino acids joined together by peptide bonds (a polypeptide chain).
  Source: Anthromorphemics
• compound of carbon, hydrogen, oxygen, and traces of phosphorous or sulfur
  Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby
• either a single polypeptide chain or a complex of polypeptide chains
  Source: Jenkins, John B. 1990. Human Genetics, 2nd Edition. New York: Harper & Row
• A polymer of amino acids linked via peptide bonds and which may be composed of two or more polypeptide chains.
• Proteins are an important class of molecules found in all living cells. A protein is composed of one or more long chains of amino acids, the sequence of which corresponds to the DNA sequence of the gene that encodes it. Proteins play a variety of roles in the cell, including structural (cytoskeleton), mechanical (muscle), biochemical (enzymes), and cell signaling (hormones). Proteins are also an essential part of diet.