A peptide is the name assigned to short polymers of amino acids. They are classified by the number of amino acid units in the chain, called amino acid residues. Tripeptides have three amino acid residues while tetrapeptides have four. A polypeptide is formed when the chain of amino acid residues exceeds several dozen in length. A protein is a molecule composed of one or more polypeptide chains.
Proteins are unbranched polymers of amino acids linked head to tail from carboxyl group to amino group, through formation of covalent peptide bonds. The peptide backbone of a protein consists of the repeated sequence.
-N-Ca– C, where N represents the amide nitrogen, Ca represents the a-carbon atom of an amino acid in the polymer chain, and the final C is the carbonyl carbon of the amino acid. This C is in turn linked to the amide N of the next amino acid, and so on down the line. The unbranched polypeptide chain has two ends, an amino-terminal or N-terminal end and a carboxyl-terminal or C-terminal end.