Proteins are important to all living organisms; they're the major structural and functional components of cells. Peptides are very similar to proteins; technically, all proteins are peptides, but not all peptides are proteins. The difference between a peptide and a protein is that a protein is typically larger, folded, and has biological significance.
Peptides
A peptide is a short or medium-length chain of building block molecules called amino acids. An amino acid consists of a central carbon with four chemical groups attached: a hydrogen, a nitrogen-containing group, an oxygen-containing carboxylic acid group, and a sidechain. The sidechain varies depending upon the identity of the amino acid -- there are 20 "typical" amino acids, and 20 different sidechains. Amino acids can bond together in chains of various lengths; the bonds between them are called peptide bonds.
Protein Structure
A protein has more structural complexity than a peptide; protein chains are also typically longer. In addition to being a chain of amino acids, proteins consist of secondary, tertiary, and sometimes quaternary structure. Secondary structure is local folding that involves part, but not all, of the chain. Tertiary structure is the overall folding pattern of the protein. Quaternary structure is association of multiple individual folded peptide chains.
Peptide Function
Peptides, though they're not folded like proteins, still have biological significance. For instance, there are many human hormones that are peptides; insulin, glucagon, and oxytocin are examples. Insulin is the hormone that causes cells to take up glucose -- blood sugar -- from the bloodstream. Glucagon causes the liver to release stored glucose back into the bloodstream if your blood sugar is running low. Oxytocin is involved in delivery of a baby, milk ejection, and bonding emotions.
Protein Roles
Proteins have a variety of roles in the body, both structural and functional. The majority of each cell, apart from water, consists of protein. Further, proteins act as receptors to aid in cell-to-cell communication; they have roles in the immune system -- antibodies are proteins -- and some of them are enzymes. An enzyme is an important class of molecules that help speed and regulate the chemical reactions taking place in the body.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
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